An AFM study of the interactions between colloidal

Transcrição

An AFM study of the interactions
between colloidal particles
vorgelegt von
Liset A. C. Lüderitz M. Sc.
aus Havanna - Kuba
von der Fakultät II - Mathematik und Naturwissenschaften
der Technischen Universität Berlin
zur Erlangung des akademischen Grades
Doktor der Naturwissenschaften
Dr. rer. nat.
genehmigte Dissertation
Promotionsausschuss:
Vorsitzender: Prof. Dr. R. Schomäcker, TU Berlin
Berichter: Prof. Dr. R. von Klitzing, TU Berlin
Berichter: Prof. Dr. G. Papastavrou, Universität Bayreuth
Tag der wissenschaftlichen Aussprache: 14.09.2012
Berlin 2012
D 83
Abstract
This research project is focused on the study of the interaction forces between two colloidal particles. The interaction between colloidal particles may differ from the interaction
between macroscopic bodies. The interactions are measured across different electrolytes:
CsCl, KCl, NaCl and LiCl using Colloidal Probe Atomic Force Microscope (CP-AFM)
techniques. The resulting forces may be different depending on the electrolyte solution
used, which is known as ion specificity. In this study no ion specificity effect is observed
at long range for the adsorption of counterions to the silica surface but a slight tendency
for Cs+ to be more adsorbed at the surface than the other counterions is present at
10−3 M. Deviations from the DLVO theory at small separations (non-DLVO forces) are
reported in this work. Short range attractions at 10−4 M ionic strength were measured
whereas at 10−3 M short range repulsions are present. An explanation of the non-DLVO
forces is given based on the hydration forces. A further chapter studies the interaction
forces between silicon oxide surfaces in the presence of surfactant solutions. Based on
the qualitative and quantitative analysis of these interaction forces the correlation with
the structure of the aggregates on the surfaces is analysed. A colloidal probe atomic
force microscope (AFM) was used to measure the forces between two colloidal silica
particles and between a colloidal particle and a silicon wafer in the presence of hexadecyltrimethylammonium bromide (CTAB) at concentrations between 0.005 mM and 1.2
mM. Different interaction forces were obtained for the silica particle–silica particle system
when compared to those for the silica particle–silicon wafer system for the same studied
concentration. This indicates that the silica particles and the silicon wafer have different
aggregate morphologies on their surfaces. The point of zero charge (pzc) was obtained at
0.05 mM CTAB concentration for the silica particles and at 0.3 mM for the silica particle–
silicon wafer system. This indicates a higher charge at the silicon wafer than at the silica
particles. The observed long range attractions are explained by nanobubbles present at
the silicon oxide surfaces and/or by attractive electrostatic interactions between the surfaces, induced by oppositely charged patches at the opposing Si oxide surfaces. In order
to analyze the role of the nanobubbles on the hydrophobic interactions hydrophilic silicon
wafers were studied against aqueous solutions of CTAB at concentrations between 0.05
mM and 1 mM (CMC). AFM studies show that nanobubbles are formed at concentrations up to 0.4 mM. From 0.5 mM upward, no bubbles are detected. This is interpreted
as the formation of hydrophobic domains of surfactant aggregates, becoming hydrophilic
at about 0.5 mM. The high contact angle of the nanobubbles (140-150◦ through water) in
comparison to the macroscopic contact angle indicates that the nanobubbles are located
on the surfactant domains. A combined imaging and colloidal probe AFM study serves to
highlight the surfactant patches adsorbed at the surface via nanobubbles. The nanobubbles have a diameter between 30 and 60 nm (after tip deconvolution), depending on the
surfactant concentration. This corresponds to a Laplace pressure of about 30 atm. The
presence of the nanobubbles is correlated with force measurements between a silica probe
and a silicon wafer surface. The study is a contribution to a better understanding of the
short range attraction between hydrophilic surfaces exposed to a surfactant solution. The
substrate properties hydrophobicity and roughness influence the morphology and size of
the nanobubbles. Nanobubbles with a contact angle through water of 132◦ and a Laplace
pressure of 18 atm were visualized at the interface of a hydrophobically modified silicon
wafer exposed to water and surfactant solutions. An increase in surfactant concentration
has an impact on the morphology of the nanobubbles, they were flattened at the surface
with increasing surfactant concentration.
3
To my father and my grandparents
Contents
List of Figures
8
List of Tables
13
List of Symbols
14
Acknowledgments
16
1. Introduction and Literature Review
1.1. Colloidal Particles . . . . . . . .
1.2. Non-DLVO Forces . . . . . . . .
1.2.1. Hydration Forces . . . . .
1.2.2. Hydrophobic Interactions
1.2.3. Structural Forces . . . . .
1.3. Surfactants . . . . . . . . . . . .
1.3.1. Classification . . . . . . .
1.3.2. Surfactants at Interfaces .
1.4. Nanobubbles . . . . . . . . . . .
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Bibliography
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2. Techniques
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2.1. Atomic Force Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
2.2. Scanning Electron Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . 40
2.3. Zeta Potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Bibliography
45
3. Force Measurements between Colloidal Particles across Aqueous
3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2. Experimental Section . . . . . . . . . . . . . . . . . . . . . . .
3.2.1. Materials . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.2. Preparation and Methods . . . . . . . . . . . . . . . .
3.2.3. Simulations . . . . . . . . . . . . . . . . . . . . . . . .
3.3. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.1. Effect of Ionic Strength: 10−4 M and 10−3 M . . . . .
3.3.2. Effect of pH . . . . . . . . . . . . . . . . . . . . . . . .
5
Electrolytes
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Contents
3.3.3. Interactions through Water
3.4. Discussion . . . . . . . . . . . . . .
3.4.1. Effect of Ionic Strength . .
3.4.2. Effect of pH . . . . . . . . .
3.5. Conclusions . . . . . . . . . . . . .
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Bibliography
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4. Interaction Forces between Silica Surfaces in Cationic Surfactant Solutions
4.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2. Experimental Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.1. Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.2. Preparation and Methods . . . . . . . . . . . . . . . . . . . . . . .
4.2.3. Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.1. Interaction forces between two silica particles (system I) . . . . . .
4.3.2. Interaction forces between a silica particle and a silicon wafer (system II) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.3. Point of zero charge . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.1. Interaction between two silica particles (system I) . . . . . . . . . .
4.4.2. Interaction between a silica particle and a silicon wafer (system II)
4.4.3. Comparison between the system silica particle–silica particle (I)
and the system silica particle–silicon wafer (II) . . . . . . . . . . .
4.4.4. Non DLVO forces . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Bibliography
84
5. Scanning of Silicon Wafers in Contact with
the CMC
5.1. Introduction . . . . . . . . . . . . . . . .
5.2. Experimental Section . . . . . . . . . . .
5.2.1. Materials . . . . . . . . . . . . .
5.2.2. Preparation and Methods . . . .
5.2.3. Simulations . . . . . . . . . . . .
5.3. Results . . . . . . . . . . . . . . . . . . .
5.4. Discussion . . . . . . . . . . . . . . . . .
5.4.1. Nanobubbles . . . . . . . . . . .
5.4.2. Correlation with Force Curves . .
5.5. Conclusions . . . . . . . . . . . . . . . .
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Aqueous CTAB Solutions below
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Bibliography
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6. Nanobubbles at the Surface of a Divinyl Disilazan modified Silicon Wafer
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6
Contents
6.1. Introduction . . . . . . . . . . . .
6.2. Experimental Section . . . . . . .
6.2.1. Materials . . . . . . . . .
6.2.2. Preparation and Methods
6.2.3. Simulations . . . . . . . .
6.3. Results . . . . . . . . . . . . . . .
6.4. Discussion . . . . . . . . . . . . .
6.5. Conclusions . . . . . . . . . . . .
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Bibliography
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7. Conclusions and Future Work
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7.1. General Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
7.2. Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
Bibliography
121
A. Appendix
122
7
List of Figures
1.1. Scheme of the DLVO theory: (a) Surfaces repel strongly, small colloidal
particles remain stable; (b) Surfaces are at equilibrium at secondary minimum if it is deep enough, colloids remain kinetically stable; (c) Surfaces
come into secondary minimum, colloids coagulate slowly; (d) The critical
coagulation concentration, surfaces may remain in secondary minimum or
adhere, colloids coagulate rapidly; (e) Surfaces and colloids coalesce rapidly
[6]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2. Helmholtz double layer model . . . . . . . . . . . . . . . . . . . . . . . . .
1.3. Gouy-Chapmann double layer model . . . . . . . . . . . . . . . . . . . . .
1.4. Stern model of the double layer . . . . . . . . . . . . . . . . . . . . . . . .
1.5. Forces measured between mica surfaces in LiCl solutions at pH 5.4 [8] . . .
1.6. Force measured between mica surfaces in 1.4×10−3 M NaCl solution at pH
5.7. The full line corresponds to the charge regulation model, the dashed
line is the constant potential ψ = 138mV boundary condition [8]. . . . . .
1.7. Force measured between silica surfaces in 5×10−6 M CPC and 0.1M NaCl.
The interaction was measured in gassed (filled circles) and degassed (open
circles) solutions. The gassed solution was measured prior to the degassed
solution (A). The measured order was reversed (B) [32]. . . . . . . . . . .
1.8. Force measured in SDS micellar solution and microemulsion confined between two drops of perfluorooctane. The oil in water microemulsion consists of: 2 wt% oil phase (tetradecane), 5.5 wt% surfactant(SDS), 5.5 wt%
cosurfactant(pentanol) in water [36]. . . . . . . . . . . . . . . . . . . . . .
1.9. Aggregates formed by surfactants . . . . . . . . . . . . . . . . . . . . . . .
1.10. Models for the two step and the four region model [38] . . . . . . . . . . .
1.11. (a) Normalized Raman integrated intensities as a function of CTAB bulk
concentration (b) Adsorption isotherm obtained after subtraction of the
bulk contribution and conversion of the Raman integrated intensities into
adsorbed amounts [38] . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.12. AFM image of bubbles on mica surface in water in tapping mode, with
normal contact cantilever of spring constant equal to 0.38 N/m. Image
size 1 × 1 µm [43] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
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2.1. A representation of the MFP-3D used during experiments [5] . . . . . . . . 37
2.2. The raw data for InvOLS determination. The y axes represents the deflection of the cantilever in volts. The x axes (Zsnsr) is the piezo position. . . 38
2.3. Representation of a force measurement between two particles . . . . . . . . 40
8
List of Figures
2.4. Instrumentation of a scanning electron microscope [16] . . . . . . . . . . . 41
2.5. Double layer of a particle . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
2.6. Scheme of the Laser Doppler Velocimetry (LDV) [17] . . . . . . . . . . . . 43
3.1. Forces between a pair of colloidal silica particles across different aqueous
electrolyte solutions; LiCl, NaCl, KCl and CsCl, at a fixed electrolyte concentration of 10−4 M and pH=5.8; Hamaker constant A= 8.5 × 10−21 J.
The continuous lines correspond to constant charge and the discontinuous
ones to constant potential. . . . . . . . . . . . . . . . . . . . . . . . . . . .
electrolyte solutions; LiCl, NaCl, KCl and CsCl, at a fixed electrolyte concentration of 10−3 M and pH=5.8; Hamaker constant A= 8.5 × 10−21 J.
3.3. Monte Carlo simulation with an explicit surface charge description of the
experimental data at 1 mM ionic strength and pH=5.8. The calculations
were performed by Christophe Labbez. . . . . . . . . . . . . . . . . . . . . .
3.4. Monte Carlo simulation with an implicit surface charge description of the
were performed by Christophe Labbez. . . . . . . . . . . . . . . . . . . . . .
electrolyte solutions; LiCl, NaCl, KCl and CsCl, at a fixed electrolyte concentration of 10−4 M and pH=4. Hamaker constant A= 8.5 × 10−21 J.
3.6. Monte Carlo simulation with an explicit surface charge description for the
interaction curve between a pair of colloidal particles across NaCl aqueous
electrolyte solution at a fixed ionic strength of 1 mM and pH=4. The
calculations were performed by Christophe Labbez. . . . . . . . . . . . . . .
3.7. Monte Carlo simulation with an implicit surface charge description for the
electrolyte solution at a fixed ionic strength of 1 mM and pH=4. The
calculations were performed by Christophe Labbez. . . . . . . . . . . . . . .
3.8. Forces between a pair of colloidal silica particles in milli-Q water at pH=4.
Hamaker constant A= 8.5 × 10−21 J . . . . . . . . . . . . . . . . . . . . .
3.9. Sketch of the adsorption of the cation lithium at a) 10−4 M before and
after approaching and b) 10−3 M ionic strength before and after approaching
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4.1. Forces between a pair of colloidal silica particles (system I) across CTAB
surfactant solution, from 0 to 0.1 mM surfactant concentration. Hamaker
constant A= 8.5 × 10−21 J. DLVO_CC (constant charge) and DLVO_CP
(constant potential) fits are shown for 0.1 mM surfactant concentration . . 70
9
List of Figures
surfactant solutions from 0.1 to 0.5 mM surfactant concentration. Hamaker constant A= 8.5 × 10−21 J. Constant charge and constant potential fits
are shown for 0.2 mM (DLVO_CC), 0.3 mM (DLVO_CC - overlaps fit
at 0.2 mM), 0.4 (DLVO_CP) and 0.5 mM (DLVO_CC and DLVO_CP)
surfactant concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . .
surfactant solutions from 0.5 mM to 1.2 mM surfactant concentration.
Hamaker constant A= 8.5 × 10−21 J. Constant charge and constant potential fits are shown for 0.8 and 1 mM surfactant concentration. . . . . . . .
4.4. Forces between a colloidal silica particle and a silicon wafer (system II)
across CTAB surfactant solutions at 0.005 and 0.05 mM surfactant concentration. Forces between two colloidal silica particles (system I) at 0.05
mM surfactant concentration. Hamaker constant A= 8.5 × 10−21 J. . . . .
across CTAB surfactant solutions from 0.3 to 0.8 mM surfactant concentration. Hamaker constant A= 8.5×10−21 J. DLVO_CC (constant charge)
and DLVO_CP (constant potential) fits are shown for 0.4 mM surfactant
concentration. For the DLVO_CC fits shown at 0.5 mM and 0.8 mM the
plane of charge was set 4 nm away from each surface. . . . . . . . . . . . .
across 1 mM CTAB surfactant solution. Hamaker constant A= 8.5×10−21
J. The experimental curve was offset 8 nm under the assumption that micelles/patchy bilayers are adsorbed at the surface. DLVO_CC (constant
charge) and DLVO_CP (constant potential) fits are shown for the experimental curve and the shifted curve. Data taken from chapter 5. . . . . . .
4.7. Possible surfactant morphologies depending on the concentration. . . . . .
4.8. Interaction forces between two silica particles (system I) and between a particle and a silicon wafer (system II) at 0.4 mM surfactant concentration.
The Debye length is 15.2 nm for both cases. . . . . . . . . . . . . . . . . .
4.9. AFM tapping mode of a silicon wafer at 0.3 mM surfactant concentration.
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5.1. AFM tapping mode of a silicon oxide surface in water. AFM images taken
with a magnetic actuated cantilever; nominal spring constant: 0.09 N/m,
amplitude setpoint: 0.265 V, setpoint ratio: 0.26, scan rate: 0.5 Hz, drive
frequency: 6.91 KHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
5.2. AFM tapping mode of a silicon oxide surface at 0.05 mM CTAB concentration. AFM images taken with a magnetic actuated cantilever; nominal
spring constant: 0.09 N/m, amplitude setpoint: 0.430 V, setpoint ratio:
0.43, scan rate: 0.5 Hz, drive frequency: 6.34 KHz . . . . . . . . . . . . . 91
0.19, scan rate: 0.5 Hz, drive frequency: 6.16 KHz . . . . . . . . . . . . . 92
10
List of Figures
0.22, scan rate: 0.5 Hz, drive frequency: 6.03 KHz . . . . . . . . . . . .
0.23, scan rate: 0.5 Hz, drive frequency: 7.1 KHz . . . . . . . . . . . . .
0.21, scan rate: 0.5 Hz, drive frequency: 7.1 KHz . . . . . . . . . . . . .
5.7. Schematic picture of nanobubbles (not to scale) seated on the hydrophobic
tails of the surfactant molecules . . . . . . . . . . . . . . . . . . . . . . .
5.8. Force curves between a silica particle and a silicon oxide surface in the
presence of 0.4 and 0.5 mM CTAB concentration . . . . . . . . . . . . .
5.9. Force curves between a silica particle and a silicon oxide surface in the
presence of 1mM CTAB concentration . . . . . . . . . . . . . . . . . . .
. 93
. 94
. 95
. 96
. 99
. 100
6.1. AFM tapping mode of a hydrophobically modified silicon wafer in water.
AFM images taken with a magnetic actuated cantilever, nominal spring
constant 0.09 N/m, amplitude setpoint: 0.292 V, setpoint ratio: 0.29,
scan rate: 0.5 Hz, drive frequency: 6.27 KHz . . . . . . . . . . . . . . . . .
6.2. AFM tapping mode of a hydrophobically modified silicon wafer in 0.1 mM
CTAB solution. AFM images taken with a magnetic actuated cantilever,
nominal spring constant 0.09 N/m, amplitude setpoint: 0.239 V, setpoint
ratio: 0.24, scan rate: 0.5 Hz, drive frequency: 5.1 KHz . . . . . . . . . .
11
107
108
108
109
109
110
List of Figures
ratio: 0.24, scan rate: 0.5 Hz, drive frequency: 4.9 KHz . . . . . . . . . . 110
6.8. Forces between a pair of hydrophobically modified silica particles in water
and from 0.03 to 1.2 mM surfactant concentration at pH=5.8, Hamaker
constant A= 8.5 × 10−21 J. The continuous lines correspond to constant
charge. DLVO_CC (constant charge) fits are shown for 0.03 mM, 0.3
mM, 0.4 mM and 1 mM surfactant concentration . . . . . . . . . . . . . . 111
6.9. Interaction force between a tip and a bubble in 0.01 mM CTAB concentration113
7.1. Scanning electron microscopy of a modified cantilever . . . . . . . . . . . . 120
7.2. Scanning electron microscopy of a modified cantilever . . . . . . . . . . . . 120
A.1.
A.2.
A.3.
A.4.
A.5.
Scanning electron microscopy of silica particles . . . . . . . . . . . . . . .
Scanning electron microscopy of a magnetic actuated cantilever . . . . . .
Scanning electron microscopy of a magnetic actuated cantilever . . . . . .
Schematic cross section of a nanobubble . . . . . . . . . . . . . . . . . . .
Amplitude-distance data of nanobubbles on a silicon oxide surface at 0.05
mM CTAB concentration (see figure 5.2) . . . . . . . . . . . . . . . . . . .
A.6. Amplitude-distance data of nanobubbles on a silicon oxide surface at 0.3
A.9. Amplitude-distance data of nanobubbles on a modified silicon wafer immersed in water (see figure 6.1) . . . . . . . . . . . . . . . . . . . . . . . .
A.10.Amplitude-distance data of nanobubbles on a modified silicon wafer at 0.1
12
122
122
123
123
124
125
126
127
128
129
130
131
132
133
List of Tables
1.1. Geometrical relations of different aggregates; V, A, gmax and g refer to
the complete spherical aggregate, unit length of a cylinder or unit area of
a bilayer [39]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.1. Results of simulations of direct force measurements by the Poisson-Boltzmann theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
3.2. Size of the counterions, taken from [3] . . . . . . . . . . . . . . . . . . . . 62
4.1. Results for simulations of direct force measurements by the Poisson-Boltzmann theory. ∗ From fitting with the plane of origin of charge taken at 4
nm from each surface. ∗∗ Assuming that the surfaces were not in contact
(micelles/bilayer adsorption). . . . . . . . . . . . . . . . . . . . . . . . . . 71
5.1. Parameters of the nanobubbles obtained by fitting the cross section to
an arc of a circle. The small size of the nanobubbles at 0.4 mM CTAB
concentration introduces more errors in the fitting and in the obtained
parameters, but still the parameters of the nanobubbles are shown for
comparison. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
5.2. Parameters of the nanobubbles obtained after tip deconvolution, Rtip =15
nm±5 nm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
13
List of symbols
1. A: Hamaker constant
2. a: Effective area per head group
3. A2 : Second virial coefficient
4. c0 : Concentration
5. ci : Local ion density
6. D: Distance between the surfaces
7. E: Elastic modulus
8. F (D): Interaction force
9. f (ka): Henrys function
10. FH : Hydration force
11. g: Aggregation number
12. K: Optical constant
13. k: Cantilever spring constant
14. kB : Boltzmann constant
15. l: Cantilever length
16. lo : Length of the hydrocarbon chains
17. M : Molecular weight
18. Ns : Packing parameter
19. P (θ): Angular dependence of the sample scattering intensity
20. Rc : Curvature radius
21. Ref f : Effective radius
22. Rθ : Ratio scattered light to incident light of the sample
23. T : Temperature
24. t, h: Cantilever thickness
14
List of symbols
25. UE : Electrophoretic mobility
26. vo : Volume of the hydrophobic part
27. W : Work
28. W (D): Interaction free energy
29. W (D)str : Interaction energy of the structural force
30. w, b: Cantilever width
31. Wa (D): Van der Waals interaction free energy
32. WaHphb (D): Hydrophobic interaction energy
33. z: Zeta potential
34. zc : Cantilever deflection
35. ∆P : Laplace pressure
36. ∆V : Voltage measured by photodiode
37. : Dielectric constant
38. 0 : Permittivity in vacuum
39. η: Viscosity of the medium
40. θ: Contact angle
41. λD : Debye length
42. ν: Resonant frequency
43. ρe: Local electric charge density
44. σ: Surface tension
45. σp : The oscillatory period
46. ψ(x): Potential at a certain distance from the surface
47. ψ0 : Surface potential
48. ω: Angular frequency
15
Acknowledgments
The author wishes to thank the DFG via SPP 1273 "Kolloidverfahrenstechnik (KL1165-11)" for financial support. Thanks to Ulrich Gernert for the scanning electron
microscopy analysis of the spheres and the cantilevers. Prof. Schäfer and Mattias Böcker
are acknowledged for the donation of micropipettes to glue particles to the glass slides. I
would also like to thank the Stranski laboratory staff, workshop staff and staff secretary
for all the help over the years. My special thanks to my colleagues Yan Zeng and Cagri
Üzum who introduced me to the AFM measurements and provided valuable suggestions.
I would like to acknowledge Prof. Dr. Georg Papastavrou for helpful suggestions in the
first experiments with particles. Prof. Dr. Gerhard Findenegg and Prof. Dr. Vincent
Craig are acknowledged for helpful discussions. Dr. Christophe Labbez is acknowledged
for the debates and simulations of the experiments with silica particles. My thanks to
the members of the group of Prof. Dr. Regine v. Klitzing for the helpful discussions
and the pleasant moments. I would like to thank my colleagues Bhuvnesh Bharti, Heiko
Fauser and Adrian Carl for the pleasent atmosphere in the office. A special appreciation
goes to Prof. Dr. Regine v. Klitzing for all the help and fruitful discussions. My thanks
also to Prof. Dr. R. Schomäcker for his support in the final stage of my PhD.
I would like to thank my husband, Stephan, for his company and love, for giving me
the feeling that this is possible and for the care for the family during these years. My
thanks to my parents-in-law who helped with child care during my long hours in the lab.
Thanks to all my friends who cheered me up when it was necessary and for helping with
child care. Also thanks to my sweet children Laetitia and Leonardo for making me forget
my worries. I would like to thank my brother Reynel for all the emotional support and
above all I would like to thank a wonderful woman, my mother Gladys, for making me
who I am today.
1.1. Colloidal Particles
The definition of a colloid was given by Thomas Graham in 1861 and was based on the
size of the material. A colloid is a material with at least one dimension in the nanometer
scale, from 1 to 1000 nm [1]. When colloids are dispersed in a continuous medium,
colloidal dispersions are formed. Several types of colloidal dispersions are found in daily
life [2].
1. Colloidal sols: small solid particles are dispersed in a liquid (e.g. paint, ink, muddy
water)
2. Emulsions: small droplets dispersed in a liquid medium (e.g. drug delivery)
3. Foam: a gas is dispersed in a liquid medium (e.g. vacuoles, fire extinguishers)
4. Aerosol: small solid or liquid particles dispersed in a gas medium (e.g. volcanic
smoke, clouds, hair spray)
5. Solid suspension: Solid particles dispersed in a solid medium (e.g. wood)
6. Porous materials: liquid particles dispersed in a solid medium (e.g. oil reservoirs)
7. Solid foam: a gas dispersed in a solid medium (e.g. zeolites)
Colloidal pigments were used to write records of Egyptian Pharaohs [2]. Faraday prepared colloidal sols based on gold, which can still be seen in the British Museum in London [2]. There is an increased interest in the understanding of the interactions within
colloidal dispersions. Many of the biological molecules are in the colloidal range. These
molecules perform complex reactions inside our body [2], furthermore technical processes,
like mineral flotation, include colloidal particles. The stability of butter, milk, and other
emulsions are of great importance in the food industry. Interfacial effects dominate the
colloidal systems [1]. In 1945 Derjaguin-Landau and Verwey-Overbeek developed the
DLVO theory. The DLVO theory is based on some assumptions [3]:
1. Infinite flat solid surface
2. Uniform surface charge density
3. Constant surface electric potential
17
4. The concentration profile of both counter ions and surface charge determining ions
is constant
5. No chemical reaction between the particles and the solvent
The DLVO theory only takes two kinds of forces into account to explain the stability of
colloids in a suspension: van der Waals and electrostatic double layer forces. The van
der Waals forces are the sum of the interactions between atomic and molecular dipoles in
the particles or between different particles [4]. The van der Waals interaction free energy
between two flat surfaces can be defined as [4]:
Wa (D) = −
A 1
12π D2
(1.1)
(Wa (D): The van der Waals interaction free energy between two flat surfaces; A: Hamaker
constant; D: Distance between the surfaces). The interaction energy between to flat
surfaces can be related to the forces between two curved surfaces of radius R using the
Derjaguin approximation [2, 4]
F (D) = 2πRef f W (D)
1
Ref f
=
1
1
+
R1 R2
(1.2)
(1.3)
(F(D): Interaction force; Ref f : Effective radius; R1 and R2 : Curvature radius of the
sphere 1 and sphere 2 respectively; W(D): Interaction free energy)
then
F (D) = −
AR 1
12 D2
(1.4)
The Hamaker constant A depends on the polarisability, permanent dipole moment and
ionization energy of the interacting molecules. The values are in the order of (0.4 −
40) × 10−20 J [4] and can be negative or positive. For two colloidal particles of equal
sign in a medium the van der Waals interactions are always attractive; these systems will
have a positive Hamaker constant. While between different bodies in a medium it can
be attractive or repulsive (A negative). The Hamaker constant will be negative if the
Hamaker constant of the medium are intermediate between those of the two interacting
particles [4–6].
The other force described by the DLVO theory is the electrostatic double layer force,
which arises due to the overlap of the double layers. When a solid surface is placed in
a polar medium, charges will develop at the surface due to the dissociation of surface
groups present at the surface. The surface charges produce an electric field which will
18
attract the counterions. The layer of surface charge and counterions is called double layer
[1]. Figure 1.1 is a schematic representation of the DLVO theory [6]. The interactions
depend on the electrolyte concentration and surface charge density. A strong long range
repulsion is obtained for highly charged surfaces in dilute electrolyte (case a). At higher
electrolyte concentrations, also a secondary minimum is present, normally of about 3 nm
(see inset figure 1.1).
Figure 1.1.: Scheme of the DLVO theory: (a) Surfaces repel strongly, small colloidal particles remain stable; (b) Surfaces are at equilibrium at secondary minimum
if it is deep enough, colloids remain kinetically stable; (c) Surfaces come
into secondary minimum, colloids coagulate slowly; (d) The critical coagulation concentration, surfaces may remain in secondary minimum or adhere,
colloids coagulate rapidly; (e) Surfaces and colloids coalesce rapidly [6].
For the colloidal particles the thermodynamic equilibrium state may be with the particles
in contact in the primary minimum, but the energy barrier may be too high for the particles to overcome during a reasonable time period. When this happens, the particles will
either come to the weaker secondary minimum or remain totally dispersed in the solution
(kinetically stabilized colloid). For low surface charge surfaces the energy barrier is low
(case c) and slow aggregation (coagulation or flocculation) occurs. At concentrations
19
greater than the critical coagulation concentration, the particles will coagulate quickly
and the colloid will be unstable (case d). At zero surface charge only van der Waals
attraction will be present in the system (case e) [6].
Several models have been proposed to explain the electric double layer. Helmholtz proposed a model in 1879, which consists of a monolayer of fixed charges able to neutralize
the charges present at the surface [7] (see figure 1.2).
σ
ψ0
ψ=0
δ
x
Figure 1.2.: Helmholtz double layer model
ψ
0
Potential ψ(x)
σs
ψ=φ=0
x
Figure 1.3.: Gouy-Chapmann double layer model
Around 1910 Gouy and Chapman developed a model with a diffuse layer of counterions.
A high concentration of counterions at the surface is found under equilibrium conditions.
20
The counterions concentration decreases moving away from the surface. The potential
in the solution ψ(x) is also varying from the surface value ψ0 (x = 0) to zero far away
from the surface [7] (see figure 1.3).
In 1924 Stern improved the Gouy-Chapman model of the double layer with the addition
of an inner layer. In this new model, the double layer consists of an inner and an outer
layer. The inner layer is a monolayer of counterions, but in contrast to the Helmholtz
layer, this inner layer does not produce neutralization of the surface charge. The number
of ions in this layer is given by the Langmuir adsorption isotherm. The counterions are
not considered as point charges anymore, their proximity to the surface and to each other
depends on the hydrated radii. The ion specificity effect is also considered in this model
[7] (see figure 1.4).
ψ0
Stern layer
ψ
δ
ψ(x)
δ
δ
x
Figure 1.4.: Stern model of the double layer
The Poisson-Boltzmann theory makes several assumptions for the description of the
electric double layer [1, 6]:
• The ions are considered point charges
• The ions in solutions have a continuous charged distribution, the surface charge is
considered homogeneous and smeared out
• The non-coulombic interactions are not taken into account
• The solvent is taken as a continuous medium and the permittivity is assumed to
be constant
• The surfaces are taken as smooth in the molecular scale
• Image forces between the ions and the surfaces are not considered
The Poisson Boltzmann theory allows the determination of the electric potential ψ near
a planar surface [1]. The potential and the distribution of ions varies with the distance
normal to the surface x. The charge density and the electric potential are related by the
Poisson equation [1, 2] in the following way:
21
∇2 ψ =
ρe
∂2ψ ∂2ψ ∂2ψ
+
+
=−
2
2
2
∂x
∂y
∂z
0
(1.5)
(ρe: local electric charge density in C/m3 , : dielectric permittivity, 0 : permittivity in
vacuum, = 1). The local ion density can be calculated by the Boltzmann equation as
follows:
ci = c0i e−Wi /kB T
(1.6)
(Wi : work required to bring an ion from a distance far away from the surface to a
distance closer to the surface). Assuming that only electric work is done and that only
1:1 electrolyte is present in the system, we can define W + and W − as the electric work
required to bring a cation and anion respectively to a place with a potential ψ [1]
W + = eψ
(1.7)
W − = −eψ
(1.8)
for a cation and
for an anion
Using the Boltzmann equation (equation 1.6) we can rewrite the local cation and anion
density as follows:
c+ = c0 e−eψ/kB T
(1.9)
c− = c0 eeψ/kB T
(1.10)
for a cation and
(c0 : bulk salt concentration)
for an anion.
The local charge density ρe can be expressed as:
ρe = e(c+ − c− ) = c0 e(e
−eψ(x,y,z)
kB T
−e
eψ(x,y,z)
kB T
)
The charge density can be substituted in the Poisson equation 1.5
22
(1.11)
∇2 ψ =
−eψ(x,y,z)
c0 e eψ(x,y,z)
(e kB T − e kB T )
0
(1.12)
finally the Poisson-Boltzmann equation (see equation 1.12) is obtained. This equation
is solved numerically though for simple geometries like a planar surface it can be solved
analytically [1, 2]. For a planar surface of low potential eψ << kB T at room temperature
ψ ≤ kB T the linearized Poisson-Boltzmann equation can be used [1, 2]
ψ(x) = C1 e−κx + C2 eκx
(1.13)
where
s
κ=
2c0 e2
0 kB T
(1.14)
C1 and C2 are constant defined by the boundary conditions. At the surface, the potential
is equal to the surface potential ψ(x = 0) = ψ0 and at distances far away from the
potential tends to zero ψ(x → ∞) = 0. Now the potential can be expressed as
ψ = ψ0 e−κx
(1.15)
The Debye length is given by λD = κ−1 . Using the water parameters at 25◦ C, the Debye
length for a monovalent salt can be expressed as [1]:
3.04Å
λD = √
c0
(c0 : concentration in
(1.16)
mol
L ).
1.2. Non-DLVO Forces
1.2.1. Hydration Forces
Figure 1.5 shows the interaction between mica surfaces in a LiCl solution at pH 5.4
[8]. Pashley’s [8] experimental data show that for solutions up to 10−2 M of LiCl, the
mica surfaces always come into the primary minimum, but at a certain concentration
(6 × 10−2 M ) hydration forces are present in the system. For mica surfaces immersed in
NaCl solution, the hydration force appears at a similar concentration 10−2 M , whereas
for mica surfaces in solutions of KCl and CsCl, the hydration forces are already present
at 10−4 M and 4 × 10−5 M respectively. Between hydrophilic surfaces the hydration force
23
Figure 1.5.: Forces measured between mica surfaces in LiCl solutions at pH 5.4 [8]
is exponentially repulsive. The hydration forces are short range forces which avoid that
surfaces come into contact due to van der Waals attractions [6]. The hydration forces
observed in mica were related to the exchange of ions present in solution like Li+ , Na+ ,
Cs+ , H+ with the K+ ions at the mica surface. The concentration at which hydration
forces appeared at the mica surface was specific for each ion. The larger the hydration
shell of the cation the higher the concentration needed to replace the H+ ions from the
mica surface. The interaction of mica surfaces in water are DLVO like, no hydration
forces were observed [8].
The mechanism behind the hydration forces was related to the adsorbed layer of counterions. To produce hydration forces, the counterions have to be bound to the mica surface
in a specific way and should not to be desorbed upon the approach of the other surface
[8]. Pashley [8] also obtained an interesting phenomenon working with mica surfaces
at concentrations where the hydration force is still not present. The jump into contact
appears at a separation larger than that predicted by the DLVO theory. The jump-in distance increases with increasing concentration of hydrated counterions close to the mica
surface. The explanation given was the overlap of the counterions in the compressed
double layer and on the surfaces, which expels the cations and replaces them by H+ ions.
24
This causes a reduction in the surface potential and increases the jump-in distance. The
finite ion size was also given as explanation of the reduced repulsion observed (see figure
1.6).
Figure 1.6.: Force measured between mica surfaces in 1.4×10−3 M NaCl solution at pH
5.7. The full line corresponds to the charge regulation model, the dashed line
is the constant potential ψ = 138mV boundary condition [8].
Hydration forces have also been observed in other materials. Evidence for hydration
at the silica surface have been given by several authors [9–12], but the origin of the
hydration forces are still controversial. Traditionally, it was accepted that the hydration
forces at the silica surface were associated with the presence of a structured layer of
water molecules. The overlap of the hydrated layer upon the approach of the surfaces
will produce the short range repulsive hydration force [13]. A more recent interpretation
was given by Vigil et al. [14]. They reported that the short range repulsion is steric in
origin due to the overlap of polysilicic acid chains or silica gel layers present at the silica
surface in aqueous medium. The hydration forces for several surfaces, e.g. silica, mica,
lipid bilayer, can be fitted as in [13]:
D
)
λ
(1.17)
D
D
) + C2 exp(− )
λ1
λ2
(1.18)
FH (D) = CH exp(−
FH (D) = C1 exp(−
(FH : short range force, D: separation distance between the surfaces, λ: decay length,
CH : a hydration constant).
25
Repulsive hydration forces have been reported for mica and silica surface immersed in
1:1 electrolytes with decay lengths of about 1 nm. Their effective range is about 3-5 nm
[6] although hydration forces between a silica sphere and a silica plate in NaCl solutions
have been observed for distances up to 15 nm [15]. In equation 1.18 two decay lengths
are used for a better fit of the experimental data. Subramanian [16] cited values of λ1
and λ2 for mica surfaces in 1:1 electrolyte solutions in the range of 0.17-0.3 nm and 0.61.2 respectively. Valle-Delgado et al. [13] reported short range repulsion between silica
surfaces in 10−2 M NaCl at different pH values: 9, 7, 5 and 3. Dishon et al. [17] observed
short range repulsion between silica surfaces immersed in 10−3 M of different electrolyte
solutions: CsCl, NaCl and KCl. Other non-DLVO forces will be present at the surfaces
when hydrophobic surfaces, polymers, or surfactants are included in the interaction.
1.2.2. Hydrophobic Interactions
Water does not wet hydrophobic surfaces because these kind of surfaces cannot bind the
water by ionic or hydrogen bonds [18]. Hydrophobic interactions are attractive strong
interactions between hydrophobic objects or nonpolar molecules. These interactions play
an important role in biology, since they determine the conformation of proteins and
the structure of biological membranes [19]. The measured hydrophobic forces are in
some cases long range and decay exponentially with a decay length of 1-2 nm in the
range of 0-10 nm, and then more gradually further out [18]. Some contradictory results
are found in the literature. On one hand a decrease in magnitude of the hydrophobic
attraction is reported with decreasing hydrophobicity of surfaces modified by silanes [20].
On the other hand the opposite behaviour is found for Langmuir-Blodget monolayers, the
hydrophobic attraction increases with decreasing hydrophobicity [21]. Hato et al. [21]
argued that the hydrophobic forces between macroscopic hydrophobic surfaces in aqueous
solution have a range of 15-20 nm (short range interactions) and that the long range
attraction (for D ≥ 20 nm) observed between hydrophobic surfaces in some experiments
has no hydrophobic origin. Meyer et al. [22] reported that the long range hydrophobic
interaction observed in hydrophobic surfaces prepared by LB-deposition of DODAB is
due to the interaction between patchy bilayers.
Several theories have been proposed to explain the long range hydrophobic interactions:
electrostatic charges or correlated dipole-dipole [23–25], water structure [26, 27], phase
metastability [28], bridging nanobubbles at hydrophobic surfaces [29–31]. The hydrophobic interaction energy WaHph (D) can be defined as [4]
WaHphb (D) = −2γ1 e−D/λ1 − 2γ2 e−D/λ2
(1.19)
Where λ1 = 1-3 nm and γ1 = 10-50 mJm−2 describes the short range hydrophobic
interaction and λ2 and γ2 varies significantly. The interaction between silica surfaces in
5 × 10−6 M CPC and 0.1 M NaCl is represented in figure 1.7 [32]. Craig et al. [32]
obtained a decrease in the long range hydrophobic attraction when the solution was
26
degassed. The magnitude of the attraction at small distance is very similar for a gas
and degassed CPC solution. Although many models have been proposed to explain the
hydration and the hydrophobic forces, more experiments are still necessary to clarify the
mechanism behind these two important forces.
Figure 1.7.: Force measured between silica surfaces in 5×10−6 M CPC and 0.1M NaCl.
The interaction was measured in gassed (filled circles) and degassed (open
circles) solutions. The gassed solution was measured prior to the degassed
solution (A). The measured order was reversed (B) [32].
1.2.3. Structural Forces
Structural forces are important forces arising in confined liquids. They are oscillatory
changing from attraction to repulsion with distance and have a periodicity σp equal to
the diameter of the liquid molecule [33]. The confinement forces the liquid in the gap
27
to order in few layers, which are energetically or entropic favourable (energy minimum).
Upon approach of another surface the order is disrupted and the layers are squeezed out
successively from the gap with decreasing distance, giving rise to structural forces [33].
The structural forces can be defined as [33]:
W (D)str = W0 cos(2πD/σp )e−D/σ
(1.20)
(W (D)str : interaction energy of the structural force, W0 : interaction energy at distance,
D = 0, σp : oscillatory period) Structural forces are not restricted to spherical molecules,
these forces have been measured in colloidal dispersions confined in rigid and soft walls
[34, 35]. Recently, Tabor et al. [36] published measured structural forces in SDS micellar
solutions and microemulsions confined between two oil droplets (see figure 1.8)
Figure 1.8.: Force measured in SDS micellar solution and microemulsion confined between
two drops of perfluorooctane. The oil in water microemulsion consists of: 2
wt% oil phase (tetradecane), 5.5 wt% surfactant(SDS), 5.5 wt% cosurfactant(pentanol) in water [36].
Some other non-DLVO forces (not discussed here) may be present in colloidal systems,
e.g. depletion and steric forces.
1.3. Surfactants
1.3.1. Classification
Surfactants are used in many industries, e.g. chemical, cosmetic, food and pharmaceutical. They are also found in our body like the pulmonary surfactant. A surfactant is
an amphiphilic molecule composed of a hydrophilic and a hydrophobic part [37]. If such
molecules are dissolved in a polar solvent like water, its presence will disrupt the water
structure breaking the hydrogen bonds between the water molecules (the hydrophilic
28
group have strong attraction for the polar solvent, whereas the hydrophobic group has
little attraction for the polar solvent). The free energy of the system will increase and
the surfactant will be expelled to the air water interface. Its hydrophobic groups will
be facing the air (which is also hydrophobic) to minimize the contact with water, the
hydrophilic groups will stay in the aqueous phase. The air–water interface becomes covered with a monolayer of surfactant and the surface tension of water is decreased. The
surfactants are classified as anionic, cationic, amphoteric (zwitterionic), and non-ionic
surfactants [1]:
• Anionic surfactants have hydrophilic negative charged groups in aqueous solvents.
Sodiumdodecylsulfate and sodiumdodecanoate are examples of them.
• Cationic surfactants carry a positive charge in their hydrophilic groups. Ammonium
bromide surfactants like CTAB (C16 H33 N (CH3 Br)) belong to this category.
• Non-ionic surfactants do not carry any charge. They have polar groups which
can interact with water. Examples of these surfactants are alkylethylene oxide or
sugar surfactants. Alkylethylene oxide surfactants are represented as Cnc Ene ; nc
and ne indicate the number of carbon atoms in the alkyl chain and the number of
ethylene oxide units in the hydrophilic head respectively. C12 H25 (0CH2 CH2 )6 OH
will be written as C12 E6 . The sugar surfactants are also known as alkylglycosides
or alkylpolyglycosides. They consist of a hydrophilic head group (mono or oligosucrose, glucose or sorbitol) and a hydrophobic alkyl chain.
• Zwitterionic surfactants carry a positive and a negative charge. Phosphatidylcholine is an example of amphoteric surfactant.
As explained before, the addition of surfactants to water will decrease the surface tension
of water due to the tendency of surfactant to be adsorbed at the interface. At a certain
concentration the surface tension remains constant. This concentration is called the critical micelle concentration [1] and is a characteristic property of each surfactant. Above this
concentration the surfactant aggregates spontaneously building micelles. When negative
particles or solid surfaces are present in a solution of a cationic surfactant at concentrations well below the CMC, the surfactant will be adsorbed to the solid–liquid interface
due to electrostatic interaction with the surface rendering the surface hydrophobic. A further increase of the concentration will produce surface aggregates or so called admicelles
due to hydrophobic interactions. They resemble micelles formed in the bulk at a higher
concentration [38]. Above the CMC several structures may be found on the surface:
cylinders, micelles, bilayers, inverted micelles. Examples of the surfactant structures are
given in figure 1.9.
Israelachvili et al. [39] proposed the concept of molecular packing parameter. The type
of aggregates (spherical micelle, bilayer, cylinder) depends on the packing parameter in
the following way:
Ns = vo /alo
29
(1.21)
Spherical Micelle
Cylindrical or rod- like micelle
Vesicle or Liposome
Inverted micelles
Bilayer
Figure 1.9.: Aggregates formed by surfactants
(vo : volume of the hydrophobic part, lo : length of the hydrocarbon chains, a: effective
area per head group).
For spherical micelles with a core radius R, made up of g molecules, the volume of the
core can be expressed as [39]:
V = gvo = 4πR3 /3
(1.22)
the surface area of the core A can be calculated as follows:
A = ga = 4π/R2
(1.23)
R = 3vo /a
(1.24)
therefore
30
assuming that there is no empty space between the hydrocarbon chains forming the
micelle core. The core radius R will be equal to the length of the hydrocarbon chain lo ,
then
0 ≤ vo /alo ≤ 1/3
(1.25)
for spherical micelles. The packing parameter for the other aggregates is given in table
1.1
Variable
Volume of core
V = gvo
Surface area of core
A = ga
Area per molecule a
Packing parameter
vo /alo
Largest aggregation
number gmax
Aggregation
number g
Sphere
4πR3 /3
Cylinder
πR2
Bilayer
2R
4πR2
2πR
2
3vo /R
vo /alo ≤ 1/3
2vo /R
vo /alo ≤ 1/2
vo /R
vo /alo ≤ 1
4πlo3 /3vo
πlo2 /vo
2lo /vo
gmax (3v0 /al0 )3
gmax (2v0 /al0 )2
gmax (v0 /al0 )
Table 1.1.: Geometrical relations of different aggregates; V, A, gmax and g refer to the
complete spherical aggregate, unit length of a cylinder or unit area of a bilayer
[39].
1.3.2. Surfactants at Interfaces
The adsorption of a solute to the solid–liquid interface increases the surface concentration. When the interaction is favourable the concentration of the solute at the surface
will exceed the concentration of the solute in the bulk. This is known as surface excess
[40]. The adsorption of ionic surfactant to a hydrophilic surface can be described using
two principal models; the two step model and the four region model [38]. Electrostatic,
hydrophobic, or a combination of both interactions determine the structure of the aggregates at the surface (see figure 1.10). At the lowest concentrations (region I), only
electrostatic interactions between the positive surfactant head and the negative surface
will determine the adsorption. At the highest concentrations (around the CMC), aggregates are formed at the solid–liquid interface, due to hydrophobic interactions (region
IV). At intermediate concentrations (region II and III), the two models differ. In the two
step model only few isolated molecules are bound electrostatically to the surfaces, which
then nucleate the formation of aggregates(admicelles). In the four region model stronger
adsorption occurs at low concentration, which leads to the formation of hemimicelles
before a second layer is attached via hydrophobic interactions.
31
The CTAB adsorption isotherm at the silica surface is represented in figure 1.11. A
double plateau can be distinguished in the curve. The adsorption isotherm resembles the
two step model of figure 1.10 [38]. Velegol et al.[41] detected rod-like aggregates by AFM
at the silica surface with a peak to peak distance of 10 nm for concentration close to the
CMC and above. Spherical aggregates (full micelle or half micelle on monolayer) have also
been reported [42]. The low concentration region remains an area of investigation, since
little information about the adsorbed layer at low surfactant concentration is available.
Figure 1.10.: Models for the two step and the four region model [38]
Figure 1.11.: (a) Normalized Raman integrated intensities as a function of CTAB bulk
concentration (b) Adsorption isotherm obtained after subtraction of the bulk
contribution and conversion of the Raman integrated intensities into adsorbed amounts [38]
1.4. Nanobubbles
Lou et al. [43] obtained one of the first images of nanobubbles at a solid surface immersed
in a liquid (figure 1.12). Nanobubbles can be induced on a hydrophobic surface using
the solvent exchange method [43] or just immersing the hydrophobic surface in water
[44]. Other methods of nanobubbles production are electrolysis and temperature change
[45]. Nanobubbles are characterized by their high stability, several mechanisms have
been proposed to explain this property. Ducker [46] explained the high stability of the
nanobubbles based on contaminants, which will be present at the gas–liquid interface
even in pure solutions. The contaminants will be adsorbed at the gas–liquid interface
reducing the surface tension, and therefore the Laplace pressure. Another mechanism was
32
proposed by Brenner et al. [47]; the gas inside the nanobubbles diffuses out, but due to
the dynamic equilibrium, the gas molecules return to the nanobubbles and as a result the
nanobubbles will be stable. It is assumed that due to the small size of the nanobubbles,
the gas leaving the nanobubbles will not collide with the returning gas. This kind of gas
is called a Knudsen gas. There will be a circulating flow in the liquid near the gas–liquid
interface, which is responsible for the returning of the gas molecules to the substrate
and to the nanobubbles. The required energy is small and the substrate can supply the
necessary thermal energy to drive this flow over the time scale that nanobubbles are
observed [45].
Figure 1.12.: AFM image of bubbles on mica surface in water in tapping mode, with
normal contact cantilever of spring constant equal to 0.38 N/m. Image size
1 × 1 µm [43]
33
Bibliography
[1]
H-J. Butt, K. Graf, and M. Kappl. Physics and Chemistry of interface. Wiley-VCH,
2003.
[2]
D. F. Evans and H. Wennerström. The colloidal domain: Where Physics, Chemistry,
Biology and Technology meet. VCH publishers, 1994.
[3]
G. Cao. Nanostructures and Nanomaterials: Synthesis, Properties and Applications.
Imperial College Press, 2004.
[4]
W. Briscoe. Colloid Science: Principles, methods and applications. Ed. by T Cosgrove. John Wiley and Sons Ltd, 2010, p. 343.
[5]
H-J. Butt and M. Kappl. Surface and Interfacial forces. Wiley-VCH, 2010.
[6]
J. Israelachvili. Intermolecular and Surfaces Forces. 2nd. Edition. Academic Press,
1991.
[7]
C. J. Berg. An Introduction to Interfaces and Colloids: The Bridge to Nanoscience.
World Scientific Publishing Co. Ptc. Ltd, 2010.
[8]
R. M. Pashley. In: Journal of Colloid and Interface Sci. 83 (1981), pp. 531–546.
[9]
J.-P. Chapel. In: Langmuir 10 (1994), pp. 4237–4243.
[10]
A. Anderson and W. R. Ashurst. In: Langmuir 25 (2009), pp. 11549–11554.
[11]
V. V. Yaminski, B. W. Ninham, and R. M. Pashley. In: Langmuir 14 (1998),
pp. 3223–3235.
[12]
B. C. Donose, I. U. Vakarakelski, and K. Higashitani. In: Langmuir 21 (2005),
pp. 1834–1839.
[13]
J. J. Valle-Delgado et al. In: J. Chem. Phys. 123 (2005), pp. 034708–1.
[14]
G. Vigil et al. In: J. Colloid Interface Sci. 165 (1994), pp. 367–385.
[15]
W. A. Ducker, T. J. Senden, and R. M. Pashley. In: Nature 353 (1991), p. 239.
[16]
V. Subramanian. “Effects of Long-chain Surfactants, Short-chain Alcohols and Hydrolizable Cations on the Hydrophobic and Hydration Forces”. PhD thesis. 1998.
[17]
M. Dishon, O. Zohar, and U. Sivan. In: Langmuir 25 (2009), pp. 2831–2836.
[18]
M. Ruths and J. N. Israelachvili. “Nanotribology and Nanomechanics: An Introduction”. In: ed. by Bharat Bushan. Springer-Verlag, 2008. Chap. 9, pp. 417–497.
[19]
J. Israelachvili and R. Pashley. In: Nature 300 (1982), pp. 341–342.
[20]
Y. I. Rabinovich and R.-H. Yoon. In: Langmuir 10 (1994), pp. 1903–1909.
34
Bibliography
[21]
M. Hato. In: J. Phys. Chem. 100 (1996), pp. 18530–18538.
[22]
E. E. Meyer et al. In: PNAS 102 (2005), pp. 6839–6842.
[23]
Y-H. Tsao, D. F. Evans, and H. Wennerstroem. In: Langmuir 9 (1993), pp. 779–
785.
[24]
Y. I. Rabinovich, D. A. Guzonas, and R. H. Yoon. In: Langmuir 9 (1993), pp. 1168–
1170.
[25]
R. Podgornik. In: Chem. Phys. Lett. 156 (1989), pp. 71.–75.
[26]
L. R. Pratt and D. Chandler. In: J. Chem. Phys. 67 (1977), pp. 3683–3704.
[27]
J. Ch. Erikkson, S. Ljunggren, and P.M. Claesson. In: J. Chem. Soc. Faraday Trans.
2 85 (1989), pp. 163–176.
[28]
H. K. Christenson and Per M. Claesson. In: Science 239 (1988), p. 390.
[29]
V. V. Yaminski and B. Ninham. In: Langmuir 9 (1993), pp. 3618–3624.
[30]
A. Carambassis et al. In: Phys. Rev. Lett. 80 (1998), pp. 5367–5360.
[31]
J. W. G. Tyrrell and P. Attard. In: Phys. Rev. Lett. 87 (2001), pp. 176104–1–
176104–4.
[32]
V. S. J. Craig, B. W. Ninham, and R. M. Pashley. In: Langmuir 15 (1999), pp. 1562–
1569.
[33]
M. Ruths and J. N. Israelachvili. Handbook of Nanotechnology. Ed. by B. Bhushan.
2nd. Edition. Springer, 2007, p. 860.
[34]
S. Klapp et al. In: Phys. Rev. Lett. 100 (2008), p. 118303.
[35]
Y. Zeng and R. v. Klitzing. In: Soft Matter 7 (2011), p. 5329.
[36]
R. F. Tabor et al. In: J. Phys. Chem. 2 (2011), pp. 434–437.
[37]
M. J. Rosen. Surfactants and interfacial phenomena. Ed. by Inc Wiley & Sons. 3rd.
2004.
[38]
E. Tyrode, M. W. Rutland, and C. D. Bain. In: J. Am. Chem. Soc. 130 (2008),
pp. 17434–17445.
[39]
R. Nagarajan. In: Langmuir 18 (2002), pp. 31–38.
[40]
R. Atkin et al. In: Adv. Colloid Interface Sci. 103 (2003), pp. 219–304.
[41]
S. B. Velegol et al. In: Langmuir 16 (2000), pp. 2548–2556.
[42]
W. A. Ducker and E. J. Wanless. In: Langmuir 15 (1999), pp. 160–168.
[43]
Shi-Tao. Lou et al. In: J. Vac. Sci. Technol. B 18 (2000), pp. 2573–2575.
[44]
N. Ishida et al. In: Langmuir 16 (2000), pp. 6377–6380.
[45]
V S. J. Craig. In: Physics 70 (2011). doi: 10.1103/Physics.4.70.
[46]
W. A. Ducker. In: Langmuir 25 (2009), pp. 8907–8910.
[47]
M. P. Brenner and D. Lohse. In: Phys. Rev. Lett. 101 (2008), p. 214505.
35
2. Techniques
2.1. Atomic Force Microscopy
Atomic Force Microscopy (AFM) was developed by Binning, Quate and Gerber in 1985
to measure forces as small as 10−18 N [1]. An atomic force microscope can be used to
provide high resolution topographical analysis of conducting or non-conducting surfaces
[2]. Force measurements can also be performed with this equipment. The technique is
well described elsewhere [2–4]. The sample is placed on the scanner and a cantilever is
mounted on the AFM head. A piezoelectric positioner is used to bring the cantilever to
the surface. The deflection of the cantilever is held at a defined constant value by means
of feedback. The deflection of the cantilever due to the interactions with the surface is
monitored by a photosensitive detector. The MFP-3D Asylum Research AFM, mounted
in an inverted optical microscope (Olympus IX71), was used to perform the scanning
and the force measurements experiments. The MFP-3D provides an Igor Pro software
extension for the runs and analysis of the experiments. A more detailed description of
the apparatus is given below [5].
Figure 2.1 shows a MFP-3D atomic force microscope. A top view camera is included,
which allows to position the laser spot on the cantilever. This apparatus has x, y, z piezo
stages. The z piezo is seated on the head and is used to move and oscillate the cantilever.
The x and y piezos are seated on the base of the MFP-3D and move the sample in the
corresponding directions. A position sensitive detector is used to measure the deflection
of the cantilever due to the interaction with the surface. The detector consist of a
photodiode. The deflection of the cantilever produces changes in the position of the
laser beam reflected from the cantilever to the detector. The position of the reflected
laser beam on the detector is determined by the angle of the deflected cantilever. The
photodiode has 4 segments: A, B, C and D. The voltage generated in each segment
is proportional to the amount of light hitting the segment. The deflection signal can
be written as the difference between the two segments placed on the top minus the to
segments placed on the bottom (see photodetector in figure 2.3).
Def lection = Vtop − Vbottom = (VA + VB ) − (VC + VD )
(2.1)
The lateral signal can be calculated as the difference between the two left segment minus
the segments in the right side.
36
2. Techniques
Figure 2.1.: A representation of the MFP-3D used during experiments [5]
Lateral = Vlef t − Vright = (VA + VC ) − (VB + VD )
(2.2)
At the beginning of the experiment, the laser beam is aligned to hit the center of the
photodiode, so that the deflection and the lateral deflection is close to zero. If Vtop ≥
Vbottom , then the interactions between cantilever and surface are repulsive. When Vtop ≤
Vbottom , then the interactions between cantilever and surface are attractive. The inverse
of the optical level sensitivity (InvOLS) is used to convert the cantilever deflection from
volts to meter. The InvOLS is useful when force measurements are performed, it can be
determined from the constant compliance region performing a force curve against a hard
surface (see figure 2.2)
The InvOLS is assumed to be the "zero distance", but in some cases, when using highly
deformable surfaces or when layered structures cause strong repulsive forces, the constant
compliance region does not represent the "zero separation distance" [6]. The Hookes law
can be used to transform the measured deflection into force [6]:
F = k × zc
(2.3)
(k: spring constant of the cantilever, zc : cantilever deflection).
zc = InvOLS × ∆V
(∆V : voltage measured by the photodiode).
The distance between tip and surface D, can be calculated as follows:
37
(2.4)
2. Techniques
Constant
compliance
region
0
Deflection [V]
-1
-2
-3
-4V
-2
-4
-6
Zsnsr
-8
-10µm
Figure 2.2.: The raw data for InvOLS determination. The y axes represents the deflection
of the cantilever in volts. The x axes (Zsnsr) is the piezo position.
D = zc + zp
(2.5)
(zp : position of the piezo normal to the surface). There are several methods for the
determination of the spring constant [7, 8]. Cleveland et al. [9] proposed a method for
the determination of the spring constant of the cantilever based on the attachment of a
known mass to the end of the cantilever and measuring the change in resonant frequency.
The spring constant can be obtained from the geometry of the cantilever [9]:
k=
Et3 w
4l3
(2.6)
(E: elastic modulus; t, w, l: thickness, width and length of the cantilever respectively).
For a rectangular cantilever, when a mass M is added to the end, then the resonant
frequency can be calculated as:
ω
1
ν=
=
2π
2π
r
k
M + m∗
(2.7)
(m∗ ≈ 0.24mb ; mb : mass of the cantilever). The mass of the cantilever mb can be
obtained from the equation below:
38
2. Techniques
mb = ρωtl
(2.8)
(ρ: represents the mass of the material). When a mass is added then the resonant
frequency takes the form:
υ0 ≈
t E 1/2
( )
2πl2 ρ
(2.9)
and finally
M = k(2πν)−2 − m∗
(2.10)
The intercept gives the effective mass and from the slope of the equation 2.10 the spring
constant can be calculated. In the Sader method [10] the spring constant can be calculated from the dimensions of the cantilever. For rectangular cantilevers the expression
takes the form:
2
k = Me ρc bhLωvac
(2.11)
(ωvac : fundamental radial resonance frequency of the cantilever in vacuum; h, b, l: thickness, width, and length of the cantilever respectively; ρc : density of the cantilever;
Me = 0.2427 for L/b>5 - the normalized effective mass). The method used to measure the spring constant in this research was proposed by Butt et al. [11]. Using the
equipartition theorem:
1
1
kB T = kx2
2
2
(2.12)
(kB : Boltzmann constant), the solution of the equation 2.12 is:
k=
kB T
hx2 i
(2.13)
Then the power spectral density (PSD) of the distance x is fitted to the theoretical one
for a simple harmonic oscillator and from there the spring constant can be obtained [5].
The surface can be scanned in different modes: contact, tapping, and non-contact mode
[5]. The wavelength of the cantilever used is 860 nm. The forces are measured between
two colloidal particles. One particle is glued to the cantilever and the other one to a
glass slide using a micromanipulator with a mounted micropipette [12](see figure 2.3)
39
2. Techniques
Laser
Piezo
Photodetector
A
B
C
D
4.8 µm silica particle
Figure 2.3.: Representation of a force measurement between two particles
2.2. Scanning Electron Microscopy
A Hitachi S-4000 scanning electron microscope (SEM) was used with a cold field emitter
(resolution 2 nm) to obtain images of the silica particles. The accelerating voltage was 20
kV and the beam current 5.15 pA. The image mode was a secondary electron image. The
principles of a SEM are discussed below, see figure 2.4 for a representation of a scanning
electron microscope. The SEM can image and analyze bulk samples [13]. The electrons
coming from an electron gun have a typical energy of 2-40 kV. The electron beam is
demagnified into a probe of electrons [14]. The probe of electrons with a diameter of 110 nm carrying a current of 10−9 − 10−12 A is focused onto the surface and moved across
the surface in parallel lines [13, 15]. The interaction of the electrons with the surface
produces several phenomena, among them the emission of secondary electrons with an
energy of 2-5 eV, and high energy backscattered electrons. The limit between secondary
electrons and backscattered electrons is drawn at 50 eV. The secondary electrons are
emitted from the sample and generated by inelastic collisions to high energy levels, so
that the excited electrons can overcome the work function before a deceleration to the
Fermi level occurs [13]. The backscattered electrons are electrons from the incident beam,
which interact with atoms in the sample and are backscattered again. The intensity of
both emissions, secondary and backscattered electrons, is sensitive to the angle at which
the incident beam contact the surface. The emissions are collected by the detectors and
amplified. The resulting signal is used to control the brightness in a cathode ray tube
(CRT) [16]. The CRT scan is synchronized with the beam scan, which allows the signals
to be transferred point to point and a map of the scanned area can be displayed. The
scanning electron microscopy image is a magnification of the topography of the sample,
secondary or backscattered images can be obtained. The contrast of a backscattered
40
2. Techniques
SEM image depends on the intensity of the emitted backscattered electrons. When heavy
atoms are present in the sample, more backscattered electrons will be produced and a
brighter contrast is obtained. Therefore, local variations in average atomic number vary
the contrast of the image [15]. The interaction of the electrons with the sample produces
other emissions: X ray photons, Auger electrons, and perhaps light [14]. The spectrum
of the x-radiation can be used for quantitative chemical microanalysis. Auger electrons
are emitted from atomic layer close to the surface and give information about the surface
chemistry.
Figure 2.4.: Instrumentation of a scanning electron microscope [16]
2.3. Zeta Potential
A Malvern zetasizer nano ZS with a 633 nm red laser was used to measure the zeta
potentials. Other parameters like particle size (only for monodispersed samples) and
molecular weight can be measured with this instrument [17]. To determine the size of the
particles, it is necessary to measure the Brownian motion of the particles in the sample
using Dynamic Light Scattering (DLS), also known as Photocorrelation Spectroscopy
(PCS). Small particles will move quickly and bigger particles will move slower. The
particles are illuminated with a laser and the intensity fluctuations of the scattered light
are analyzed. If a small particle is hit by a light source the particle will scatter the
41
2. Techniques
--------------------------------------------------------------------------------------
Ions strongly
bound to particle
Slipping
plane
--------------------------------------------------------------------------------------
negative charged particle
--------------------------------------------------------------------------------------
Diffuse layer
potential
Zeta potential
Figure 2.5.: Double layer of a particle
light in all directions. If many particles are present in the system a speckle pattern will
be formed which consist of bright and dark areas. The bright areas are regions, where
the light scattered by the particles has the same phase and interferes constructively to
form a bright patch. The dark areas are regions, where the phase additions are mutually
destructive and cancel each other out. The Stokes-Einstein equation relates the size of the
particle with its speed due to Brownian motion. Since the particles move, the intensity
appears to fluctuate. The zetasizer measures the rate of the intensity fluctuation and
from there calculates the size of the particles [17]. When charged particles are present in
a medium, an electric double layer will be developed. The double layer consists of ions,
which are firmly bound to the surface (Stern layer) and ions, which are loosely bound
(diffuse layer) to the surface (see figure 2.5). The slipping plane describes a boundary
in the diffuse layer. Any ions within this boundary will move together with the particle.
The potential at the slipping plane is called zeta potential [17].
The zeta potential gives an indication of the stability of the sample. Low positive or
negative zeta potential values (below 30 mV) are considered as non-stable systems (steric
stabilization is not considered). The electrophoretic mobility of the particles is obtained
from an electrophoresis experiment performed on the sample and measuring the particles
velocity using Laser Doppler Velocimetry (LDV). Electrophoresis is the movement of
charged dispersed particles relative to the liquid (dispersant) under the influence of an
applied electric field [17]. When an electric field is applied to the dispersion, the charged
particles will move to the electrode of opposite charge. The velocity of the particles
depends on the following parameters [17]
• Strength of electric field
42
2. Techniques
Figure 2.6.: Scheme of the Laser Doppler Velocimetry (LDV) [17]
• Dielectric constant of the medium
• Viscosity of the medium
• Zeta potential
The electrophoretic mobility is defined as the velocity of a particle in an electric field.
From the Henry equation the zeta potential of the particle can be obtained [17]:
UE =
2zf (ka)
3η
(2.14)
(z: zeta potential; UE : electrophoretic mobility; : dielectric constant; η viscosity; f(ka):
Henrys function). The Henry function (f(ka)) takes the values 1.5 or 1.0, (f(ka)) is 1.5
for aqueous media and moderate electrolyte concentration and is also known as Smoluchowski approximation. To fit to the Smoluchowski model the particles have to be larger
than 0.2 µm dispersed in 10−3 M . For small particles in low dielectric constant medium
and for non-aqueous measurements, f(ka) is 1.0 and the Huckel approximation can be
used for the calculation of the zeta potential. The Laser Doppler Velocimetry measures
the velocity of the particles during the electrophoresis. The scattered light has an angle
of 17◦ and is combined with the reference beam, producing a fluctuating intensity signal.
The rate of fluctuation is proportional to the speed of the particles (see figure 2.6).
Electroosmosis can also occur in the measuring cell. The true electrophoretic mobility
is measured at the stationary layer. The stationary layer is a point in the cell where
the electroosmotic flow is zero [17]. The molecular weight of a sample can be obtained
from the static light scattering. The values obtained at one angle are not so accurate for
high molecular weight polymers, due to the non-isotropic scattering profiles; the intensity
depends on the angle of observation, but for small particles the sample scattering becomes
isotropic and the angle dependence is minimized. Therefore, the molecular weight of
small proteins and polymers can be measured using this method [17, 18]. The particles
43
2. Techniques
are illuminated by a light source. The particles scatter the light in all directions and
the time-averaged intensity of scattered light is measured. In that way the molecular
weight and the second virial coefficient A2 can be obtained. A2 describes the interaction
strength between the particles and the solvent. When A2 > 0 the dispersion is stable, for
A2 < 0 the particles aggregate, and for A2 = 0 the particle–solvent interaction strength
equals the molecule–molecule interaction strength, the solvent can then be defined as a
theta solvent. To determine the molecular weight, measurements of a sample at different
concentrations have to be performed. Then the Rayleigh equation can be applied [17]:
KC
1
=(
+ 2A2 C)P (θ)
Rθ
M
(2.15)
(Rθ : ratio of scattered light to incident light of the sample; M: molecular weight; A2 :
second virial coefficient; C: concentration; P (θ): angular dependence of the sample scattering intensity; K: optical constant).
When the dispersed particles are much smaller than the incident light then P (θ) is 1.
This type of scattering is known as Rayleigh scattering. Then equation 2.15 takes the
form:
KC
1
=(
+ 2A2 C)
Rθ
M
44
(2.16)
Bibliography
[1]
G. Binning, C. F. Quate, and C. Gerber. In: Phys. Rev. Lett. 56 (1986), pp. 930–
933.
[2]
J. Ralston et al. In: Pure Appl. Chem. 77 (2005), pp. 2149–2170.
[3]
P. West and A. Ross. An Introduction to Atomic Force Microscopy Modes. Santa
Clara, CA: Pacific Nanotechnology, Inc., 2006.
[4]
W. A. Ducker, T. J. Senden, and R. M. Pashley. In: Langmuir 8 (1992), pp. 1831–
1836.
[5] MFP-3D manual. url: https://support.asylumresearch.com/forum/content.
php?43-MFP-Manual-Version-04-08-Released.
[6]
H-J. Butt, B. Capella, and M. Kappl. In: Surface Science Reports 59 (2005), pp. 1–
152.
[7]
T. Senden and W. A. Ducker. In: Langmuir 10 (1994), pp. 1003–1004.
[8]
J. E. Sader et al. In: Rev. Sci. Instrum. 66 (1995), pp. 3789–3798.
[9]
J. P. Cleveland et al. In: Rev. Sci. Instr. 64 (1993), pp. 403–405.
[10]
J. E. Sader, J. W. M. Chon, and P. Mulvaney. In: Rev. Sci. Intr. 70 (1999), pp. 3967–
3969.
[11]
H-J. Butt and M. Jaschke. In: Nanotechnology 6 (1995), pp. 1–7.
[12]
G. Toikka, R. A. Hayes, and J. Ralston. In: Langmuir 12 (1996), pp. 3783–3788.
[13]
L. Reimer. Scanning Electron Microscopy; Physics of Image Formation and Microanalysis. 2nd. edition. Springer-Verlag, 1998.
[14]
K. D. Vernon-Parry. In: III-Vs Review 13 (2000), pp. 40–44.
[15]
A. Putnis. Introduction to Mineral Sciences. Cambridge University Press, 1992.
[16]
G. W. Kammlott. In: Surface Science 25 (1971), pp. 120–146.
[17] Zetasizer Nano Series User Manual. url: http://www.biophysics.bioc.cam.
ac.uk/files/Zetasizer_Nano_user_manual_Man0317-1.1.pdf.
[18] Molecular weight measurements with the Zetasizer Nano system. url: http://www.
malvern.com/common/downloads/campaign/MRK528-01.pdf.
45
3. Force Measurements between
Colloidal Particles across Aqueous
Electrolytes using CP-AFM
3.1. Introduction
Dispersions of colloidal particles find applications in many areas like pharmaceutics,
cosmetics, food industry and others. The DLVO theory remains the starting point to
describe the interaction between colloidal particles assuming only two types of forces,
repulsive electrostatic forces and attractive van der Waals forces [1]. At short separation
some deviations from the theory are reported [2–4]. These deviations are called nonDLVO forces and are present in water and electrolyte solutions. Several theories have
been developed to explain the origin of non-DLVO forces at small separation distances
between colloidal particles. Chapel [2] proposed that the hydration forces are caused by
creation of a hydrogen bonding network at the silanol level, and that this force produces
the short range repulsion. This repulsion is reduced in the presence of any salt, and the
more hydrated the ion is, the weaker this force will be. Some authors have proposed that
the short range repulsion is due to polysilicic acid chains protruding from the silica surface
[5–7]. Others related the short range repulsion in the presence of electrolytes to the
dehydration of counterions; well hydrated counterions will produce short range repulsion
of larger extent [8, 9]. According to the model of Torrie et al. [10], small counterions
with high affinity for water accommodate better on the structured water layer and large
ions like Cs+ prefer to reside outside the hydration layer producing short range repulsion
of longer extent. Higashitani’s model [11] explains the difference of the adsorbed layer at
the silica surface using highly and poorly hydrated counterions. The cation cesium is less
hydrated and can be closely packed at the silica surface. The adsorbed layer composed of
the poorly hydrated cesium cations will be thin but strong adsorbed to the silica surface.
The adsorbed layer composed of hydrated lithium cations will be thick. The larger the
hydration shell, the more unstable are the layers and they can be easily pressed out. The
Higashitani’s model contradicts the model proposed by Torrie but is in good agreement
with the mechanism explained by Pashley [12] for hydration forces. For hydration forces
to be present, the counterions have to be bound to the surface in a specific way and they
should not be desorbed upon the approach of the other surface [12].
Until now the interactions at short distances are still discussed controversially and not so
well understood. The interaction between colloidal particles may be different depending
46
3. Force Measurements between Colloidal Particles across Aqueous Electrolytes
on the electrolyte solution used, which is known as ion specificity. Usually the Hofmeister
series is observed for the adsorption of ions to the silica surface [13–15]. Their rheological
and zeta potential results at high ionic strength show that the binding of cations to the
silica surface is according to the sequence Cs+ >K+ >Na+ >Li+ , since with increasing
hydration shell, the ions prefer to stay in water.
The interaction between particles can be measured using several techniques. Franks
[14] investigated the interaction of suspension of two types of silica particles, amorphous
and quartz, in different electrolytes using rheology and zeta potential techniques. He
proved that less hydrated ions adsorb in larger amount to the silica surface than well
hydrated ions. Pashley and Israelachvili [16] demonstrated the presence of short range
repulsion between mica surfaces across K+ ions with the SFA. Chapel [2] investigated the
influences of ion size on hydration forces for silica surfaces with the SFA. The strongest
hydration force was obtained for silica surfaces immersed in pure water. The more
hydrated cation produces the weaker force. It might be that the cation Li+ competes
with the hydroxyl groups to order the water around the silica surface. Vakarelski et al.
[11] studied the adhesive force between a silica particle and a mica surface in electrolyte
solution using CP-AFM. A strong adhesion was found for highly hydrated ions (Li+ ,
Na+ ). Borkovec et al. [5] proved the validity of the Derjaguin approximation between
two colloidal particles by the colloidal probe technique across KCl electrolyte solution.
Dishon et al. [17] studied the effect of different salts on the force between a silica particle
and a silica surface using CP-AFM, but LiCl was not investigated. They obtained that
the tendency for the adsorption at the silica surface grew monotonically with the bare
ion size according to the sequence Cs+ >K+ >Na+ , which is in good correlation with
the Higashitani’s model. The addition of salt beyond neutralization led to excess cation
condensation and charge reversal in the presence of monovalent ions. To our knowledge,
there is no systematic study about the interaction between two silica particles (in the
colloidal range) across different electrolytes. The aim of the current work is to study
the role of the non-DLVO forces between two silica particles at low ionic strength across
different electrolytes, LiCl, NaCl, KCl, and CsCl and to clarify the contradictions between
different theoretical models (Chapel [2], Higashitani [11], Torrie [10]).
3.2. Experimental Section
3.2.1. Materials
Dry nonporous silica particles 4.74 µm mean diameter were purchased from Bangs Laboratories. The particles were resuspended in pure water (milli-Q water) and centrifuged
three times before use. The surface topography of the silica particles was investigated
with a MFP-3D by tapping mode in air. The roughness was calculated from images of 5
particles by flattening third order polynomials. Electrolytes solution of LiCl, NaCl, KCl,
and CsCl were prepared at different pH and ionic strength. The pH was adjusted by
47
addition of LiOH, NaOH, KOH, CsOH or HCl. All the chemicals used were of analytical
grade quality.
Preparation
A colloidal particle was glued to the end of a tip-less AFM cantilever (CSC12, µ-mach,
Lithonia) with a nominal spring constant of 0.03 N/m. Another particle was glued to a
glass slide (Menzel-Gläser, Germany) using an optical microscope and a micromanipulator. Colloidal probes and glass slides with attached particles were cleaned with ethanol
and water and placed 20 minutes in an air plasma cleaner (Diener electronic. Femto
timer).
Methods
The force measurements between the two silica particles [18] were performed using a
MFP-3D Asylum Research mounted in an inverted optical microscope (Olympus IX71).
This technique is well described elsewhere [19–21]. In brief, a cantilever with the colloidal
probe is mounted on the AFM head and the glass slide with the attached particle is placed
on the scanner [18]. The two opposing particles are optically aligned. A laser is pointed
at the end of the cantilever. The cantilever moves in the z-direction and the deflection of
the cantilever while approaching the surfaces is registered by a photosensitive detector.
The spring constant is determined using the thermal noise method; the typical value
is 0.03 N/m. During the measurements, an inverse microscope placed in the AFM was
used to check, if the particle was still attached to the cantilever. For the analysis, only
approach curves are shown. The velocity of the approach was 600 nm/s. All cantilevers
were plasma cleaned before use. The measurements with MPF-3D were obtained at room
temperature at 1 atm.
3.2.3. Simulations
The simulations are based on the DLVO theory (DLVO Fitting.ipf procedure written by
McKee [22] based on the algorithm proposed by Chan [23]). The DLVO theory only
takes two kinds of forces into account to explain the stability of colloids in a suspension: electrostatic double layer and van der Waals forces. The electrostatic interactions
are calculated solving numerically the non-linear Poisson-Boltzmann equation for two
identically charged solids in water using two boundary conditions, constant charge and
constant potential. The van der Waals forces are the sum of the interactions between
atomic and molecular dipoles in the particles or between different particles [24]. According to Borkovec [5], the Derjaguin approximation is still valid for the particle size
of 4.74 µm. A water dielectric constant of =78 is used and a non-retarded Hamaker
48
(si/water/si) constant=8.5×10−21 J [25] for the calculation of the van der Waals interaction is assumed.
3.3. Results
3.3.1. Effect of Ionic Strength: 10−4 M and 10−3 M
The roughness of the silica particles is RMS=2.0 nm which correlates well with the values
reported in literature. Figure 3.1 shows force curves against separation for different salts:
NaCl, KCl, LiCl, and CsCl at constant pH=5.8 measured between two silica particles of
4.74 µm in diameter.
0.1
NaCl, ö =-24mV, ë=30.03 nm
KCl,
ö=-20 mV, ë=30.3 nm
LiCl,
ö=-20mV,
0.1
ë=30.3 nm
]m/Nm[ Rð
8
7
6
5
CsCl, ö= -18mV, ë=30.3 nm
4
6
4
2
F/2
0.01
3
6
4
2
F/2
]m/Nm[ Rð
2
0.001
0
5
10
15
20
25
30
Separation, D [nm]
0.01
8
7
6
5
4
3
2
0.001
0
20
40
60
80
100
Separation, D [nm]
Figure 3.1.: Forces between a pair of colloidal silica particles across different aqueous
electrolyte solutions; LiCl, NaCl, KCl and CsCl, at a fixed electrolyte concentration of 10−4 M and pH=5.8; Hamaker constant A= 8.5 × 10−21 J. The
continuous lines correspond to constant charge and the discontinuous ones
to constant potential.
49
From zeta potential measurements it is known that at this pH the silica is negatively
charged [26]. The theoretical ionic strength was 10−4 M given by the added salt. The
DLVO fitting procedure for the experimental curves is described in 3.2.3. The respective
average potentials (based on different pairs of silica particles), ϕ and ionic strength, I
are shown in table 3.1.
At least 50 forces curves were taken during the AFM measurements for a pair of silica
spheres and reproducibility of the curves was observed. An 20% error due to spring
constant determination is assumed during the measurements. The slopes of the force
curves are the same for all the experimental curves (see figure 3.1) and corresponds to a
Debye length k−1 of 30 nm and an ionic strength I of 10−4 M. At distances larger than
50 nm the two boundary conditions constant charge and constant potential simulate
well the experimental curves. At smaller distances the experimental curves are located
between these two boundary conditions. Repulsion was observed at separation >10 nm
and is similar for all the studied electrolytes within the experimental errors (see table
3.1). At distances smaller than 10 nm attraction is observed. The attraction decreases
in the order of Li+ >Na+ >K+ >Cs+ .
Figure 3.2 represents force curves versus particle distance for the same salts at 10−3 M
ionic strength and constant pH=5.8 (not adjusted normal water pH). The Debye length
k−1 is 10.1 nm for NaCl and KCl and 9.59 nm for the other two salts which is close to the
ideal value of 10 nm for an ionic strength of 10−3 M. As for the lower ionic strength, the
experimental curves are well simulated at large distances (>20 nm) with the constant
charge and constant potential model using the same parameters as in figure 3.1 and only
repulsion is observed at larger separations. For a given distance the forces are similar at
long range, but a slightly lower repulsion is observed for Cs+ . That is related to a slight
decrease in the simulated surface potential Cs+ (15 mV) (see figure 3.2). A negative
sign is assumed for the simulated potentials. Interestingly no short range attraction is
observed at this ionic strength, only short range repulsion. The short range repulsion
decreases in the order Li+ >Na+ >K+ >Cs+ . Comparing the two ionic strengths, 10−4
M and 10−3 M, one can conclude that the surface potential decreases with increasing
salt concentration. No ion specific effect is observed for distances > 10 nm, but at
10−3 M ionic strength a slight tendency of Cs+ to be more adsorbed at the surface is
observed. Short range attraction is seen at 10−4 M, the attraction decreases in the order
of Li+ >Na+ >K+ >Cs+ for 10−4 M. In contrast to this, no short range attraction is
observed for 10−3 M.
The experimental curves are between the constant charge and constant regulation boundary conditions. Monte Carlo simulations were performed for two silica surfaces immersed
in 1 mM electrolyte solution. Two ion diameters were used for the calculation: 3.5 Åand
5 Å; and two different surface descriptions: smeared out surface charge (implicit) and
explicit sites (explicit). The parameters used for the calculations were taken from reference [27]. The pK of the sites was fixed at 7.7. The calculations were performed by
Christophe Labbez (see figures 3.3 and 3.4). The Monte Carlo simulations are in good
agreement with the experimental data. The best agreement is found with explicit sites.
50
0.1
NaCl, ö=-24 mV, ë=10.1 nm
KCl,
ö =-20mV, ë=10.1nm
LiCl,
ö=-16 mV, ë=9.59 nm
CsCl, ö=-15 mV, ë=9.59 nm
4
]m/Nm[ Rð
F/2
0.01
F/2
3
0.1
]m/Nm[ Rð
8
7
6
5
0.001
2
0
2
4
6
8
Separation, D [nm]
10
0.01
8
7
6
5
4
3
2
0.001
0
10
20
30
40
Separation, D [nm]
electrolyte solutions; LiCl, NaCl, KCl and CsCl, at a fixed electrolyte concentration of 10−3 M and pH=5.8; Hamaker constant A= 8.5 × 10−21 J. The
51
Salt
NaCl
LiCl
KCl
CsCl
Theoretical
Ionic Strength
I (mM)
0.20
0.10
1.00
1.00
0.20
0.10
1.00
1.00
0.20
0.10
1.00
1.00
0.20
0.10
1.00
1.00
pH
3.97
5.80
5.80
3.88
3.96
5.80
5.80
4.09
4.05
5.80
5.80
4.03
3.99
5.80
5.80
4.01
Avg. Diffuse layer
Layer Potential
ϕ (mV)
-24.00±0
-28.00±5.7
-28.00±10.0
0.00
Debye
Length
k−1 (nm)
21.40
30.03
10.10
-21.00±1.0
-17.00±1.4
-15.00±0
30.03
9.59
9.59
-21.00±2.0
-20.00±2.8
0.00
30.03
10.10
-19.00±1.4
-14.00±0.7
0.00
30.03
9.59
Zeta
Potential
ζ (mV)
-2.00
-70.00
-66.00
2.00
6.50
-70.00
-66.00
11.40
-2.00
-67.00
-66.00
0.00
-1.00
-67.00
-67.00
-1.58
Table 3.1.: Results of simulations of direct force measurements by the Poisson-Boltzmann
theory
52
0.25
GCMC_1mM_Exp_Dm_3.5
GCMC_1mM_Exp_Dm_5
LiCl
CsCl
KCl
NaCl
0.20
F/R [mN/m]
0.15
0.10
0.05
0.00
0
20
40
60
80
100
Separation, D[nm]
Figure 3.3.: Monte Carlo simulation with an explicit surface charge description of the
were performed by Christophe Labbez.
53
0.25
GCMC_1mM_Imp_Dm_3.5
GCMC_1mM_Imp_Dm_5
LiCl
CsCl
KCl
NaCl
0.20
F/R [mN/m]
0.15
0.10
0.05
0.00
-0.05
0
20
40
60
80
100
Separation, D[nm]
Figure 3.4.: Monte Carlo simulation with an implicit surface charge description of the
were performed by Christophe Labbez.
54
3.3.2. Effect of pH
Figure 3.5 shows interactions between two silica particles at an adjusted pH=4 across the
electrolytes solutions previously mentioned. The background electrolyte concentration
was 10−4 M.
0.1
0.2 mM NaCl pH=3.97,
0.2 mM CsCl
pH=3.99,
0.2 mM KCl
pH=4.05
0.2 mM LiCl
ë=21.4nm, ö=-24mV
pH=3.96
5
4
0.01
F/2
3
0.001
2
]m/Nm[ Rð
F/2
0.1
)m/Nm[ Rð
9
8
7
6
0
5
10
15
Separation, D [nm]
20
0.01
9
8
7
6
5
4
3
2
0.001
0
20
40
Separation, D [nm]
60
80
electrolyte solutions; LiCl, NaCl, KCl and CsCl, at a fixed electrolyte concentration of 10−4 M and pH=4. Hamaker constant A= 8.5 × 10−21 J. The
No repulsion between the silica particles in presence of CsCl, KCl, and LiCl could be
observed. The interactions in presence of NaCl electrolyte solution remain repulsive
even at short distances. The obtained potential and Debye length is 24 mV and 21.5
nm respectively. The potential is assumed to be still negative at this pH in presence of
NaCl, but a slight additional decrease of the pH causes the collapse of the double layer
and no repulsion is seen anymore (data not shown). The decrease of the pH to 4 at an
electrolyte concentration of 10−4 M causes a variation of the ionic strength from 1 × 10−4
M to 2 × 10−4 M due to the presence of more protons in the solution. Surprisingly, no
55
GCMC_1mM_Exp_Dm_3.5_pH=3.88
GCMC_1mM_Exp_Dm_5_pH=3.88'
NaCl_pH= 3.88'
0.2
F/R [mN/m]
0.1
0.0
-0.1
-0.2
0
20
40
60
80
100
Separation, D [nm]
Figure 3.6.: Monte Carlo simulation with an explicit surface charge description for the
electrolyte solution at a fixed ionic strength of 1 mM and pH=4. The calculations were performed by Christophe Labbez.
short range attraction is measured for NaCl at pH=4, instead short range repulsion is
observed. Is this measured short range repulsion due to hydrogen bonds at the silanol
groups? To answer this question, measurements through water were performed. At 10−3
M ionic strength and constant pH=4, attractive interactions were measured for all the
salts. A representation of the interaction in the presence of NaCl is given in figure 3.6.
The simulations performed with these conditions correlate well with the experimental
data (see figure 3.6, 3.7).
3.3.3. Interactions through Water
The interaction through water at constant pH=4 is shown in figure 3.8. The obtained
potential was 28 mV and the Debye length was 30.3 nm, which is in good agreement with
the Debye length for an ionic strength 10−4 M, i.e. pH=4. The simulated potential is
56
GCMC_1mM_Imp_Dm_3.5_pH=3.88
GCMC_1mM_Imp_Dm_5_pH=3.88
NaCl_pH=3.88
0.2
F/R[mN/m]
0.1
0.0
-0.1
-0.2
0
20
40
60
80
100
Separation, D [nm]
Figure 3.7.: Monte Carlo simulation with an implicit surface charge description for the
electrolyte solution at a fixed ionic strength of 1 mM and pH=4. The calculations were performed by Christophe Labbez.
57
assumed to be negative from zeta potential measurements. The interactions are repulsive
at larger distances, but for distances smaller than 20 nm only attraction is observed.
0.1
9
8
7
6
water, pH=4, ö= -28 mV, ë=30.3 nm
0.1
]m/Nm[ Rð
5
0.01
F/2
4
3
0.001
2
0
10
20
30
Separation, D [nm]
F/2
]m/Nm[ Rð
0.01
9
8
7
6
5
4
3
2
0.001
0
20
40
60
80
100
Separation, D [nm]
Figure 3.8.: Forces between a pair of colloidal silica particles in milli-Q water at pH=4.
Hamaker constant A= 8.5 × 10−21 J
As can be seen from table 3.1, the fitted diffuse layer potentials do not correlate with
the measured zeta potentials. At pH=5.8 the zeta potentials are always larger than the
fitted potentials for the studied ionic strengths.
3.4. Discussion
3.4.1. Effect of Ionic Strength
Long Range Interaction
At pH=5.8 and an ionic strength of 10−4 M, repulsion is observed for all salts at separations larger than 10 nm due to the overlap of the double layer and the resulting osmotic
58
pressure of the counterions (see figure 3.1). The obtained Debye length of 30.3 nm coincides with the expected Debye length of 30 nm for an ionic strength of 10−4 M. The fitted
potentials are similar for all the studied salts (see table 3.1). Some authors reported that
the adsorption to the silica surface follows the Hofmeister series [13–15] Cs+ >K+ >Na+ ,
since with increasing hydration shell the ions prefer to stay in water. No ion specificity
was observed in this experiments at long range, but at 10−3 M a slight tendency for Cs+
to be adsorbed at the silica surface is observed (figure 3.2). The obtained Debye length of
around 10 nm coincides with the expected theoretical value of 9.6 nm. The Debye length
at 10−3 M is lower than at 10−4 M meaning that the range of the interaction decreases
with increasing electrolyte concentration (see table 3.1). Repulsion is observed during
the whole range at this electrolyte concentration. The simulated potentials do not agree
with the zeta potentials. During the measurements the counterions are forced to condense or to bind to the surface as the distance decreases following a constant regulation
interaction between the two surfaces. During zeta potential measurements the ions build
a deformed ion cloud, which will result in less screening and a higher effective potential.
Another reason may be the difference in preparation methods. For force measurements,
the silica particles are subjected to plasma cleaning treatment for a couple of minutes.
That may change the surface chemistry. For zeta potential measurements a suspension of
silica particles was diluted in the desired electrolyte solution. Some authors [25] related
the discrepancy between zeta and fitted potentials to the roughness of the spheres. They
obtained larger values of zeta potentials compared to the values of fitted potentials at
10−3 M ionic strength, since under this condition the Debye length and the roughness
are similar in magnitude. We can exclude the roughness as a reason for the discrepancy
between zeta and diffuse layer potentials, because a disagreement between the two values
is also observed at 10−4 M.
Short Range Interaction
The reason for short range interaction is controversially discussed in the literature. While
many authors report that hydration forces or short range repulsive steric forces dominate
van der Waals forces [4, 5] others mention that van der Waals forces are measurable [2,
25, 28]. Figure 3.1 shows attractions at distances lower than 10 nm in the presence of
all salts for 10−4 M and pH=5.8. The short range attraction decreases in the sequence
Li+ >Na+ >K+ >Cs+ . The inverse Hofmeister series is observed with Li+ adsorbing
more to the negative silica surface than Cs+ . The results correlate well with rheological
studies performed by other authors [29]. They report the same adsorption sequence of
ions to the silica surface in concentrated solutions and interpret their results according to
the hypernetted chain model of Torrie et al. [10]. Chapel [2] cited a small van der Waals
jump for LiCl at low ionic strength. Higashitani et al. [11] reported a strong adhesive
force between a silica colloid and mica in electrolyte solutions of highly hydrated ions.
According to the model of Higashitani et al. [11] the cation cesium is less hydrated
and can be closely packed at the silica surface. The adsorbed layer composed of the
poorly hydrated cesium cations will be thin but strongly adsorbed to the silica surface.
59
The adsorbed layer composed of hydrated lithium cations will be thick. The larger the
hydration shell, the more unstable are the layers and they can be easily pressed out. The
observed short range attraction might have several possible explanations: van der Waals
forces, depletion forces, or steric forces. If the short range attractions are due to van der
Waals forces, stronger attraction will be observed with increasing ionic strength due to
electrostatic screening. The opposite occurs though and therefore it could be assumed
that hydration layer expulsion may dominate the interactions. It might be possible that
the Li+ ions are not closely packed to the silica surface due to its huge hydration shell
so that they can easily be desorbed from the surfaces and be replaced by the protons
from bulk solution; no hydration forces will then be observed. Less hydrated ions will
be strongly bound to the surface and will remain there under the approach of the other
surface giving rise to repulsive hydration forces. The statement was proposed by Pashley
[12] to explain the short range interaction of mica surfaces in the presence of electrolyte
solutions and is also consistent with Higashitani’s model [11](see figure 3.9 a).
The fact that the attraction increases with decreasing ion size (or increasing hydration
shell) may mean that at smaller distances the Li+ ions are partially expelled from the slit
pore, which leads to the largest attraction due to depletion forces. With ionic strength
increasing up to 10−3 M the attraction observed for Li+ and Na+ is reduced (see figure
3.2), meaning that the hydration layer is more stable and repulsive hydration forces may
dominate the interactions. Interestingly, the largest short range repulsion occurs for Li+
at this ionic strength. Maybe the ions were already partially dehydrated and more closely
packed at the surface with increasing ionic strength. Now the adsorbed layer cannot be
squeezed out from the silica surface (see figure 3.9 b). Dishon et al. [17] also observed
additional short range repulsion at small distance in the force curves measured between
a silica sphere and a silicon wafer at 10−3 M. Higashitani et al. [11] proved that the
adhesion force decreases with increasing electrolyte concentration. A slight attraction
was detected for Cs+ at 10−3 M ionic strength, which could be interpreted as van der
Waals forces. Pashley [12] demonstrated that repulsive hydration forces are not present
when the surface is rich in protons as counterions. The same result was obtained for the
interaction between a pair of silica particles in milli-Q water at pH=4 (see figure 3.8),
where attraction is only observed at short range. The results shown in figure 3.5 diverge
from Pashley’s statement [12]. Although the force curve in the presence of NaCl was
performed at pH=4, where the silica surface has to be covered with protons, the short
range attraction (compared to the force curve of NaCl at pH=5.8; figure 3.1) disappears.
Instead short range repulsion is observed. This repulsion may be due to a synergetic
effect between protons and cations. A gel layer composed of polysilicic acid tails may
be present at the silica surface [7] and influence the interactions. The thickness of this
gel layer was estimated to be in the range of 1 nm to 4.4 nm [7, 30]. The counterions
can penetrate the gel layer depending on their hydration shell [31] (see also table 3.2)
and produce a collapse of the gel layer due to electrostatic screening [30]. Tadros et
al. [31] argued that the diffuse layer potential of the silica surface will not be high if
penetration of counterions inside the pores of the gel occurs. The last statement may
also explain the difference in zeta and diffuse layer potential observed. It may be that
60
Silica
a)
Silica
Silica
Silica
b)
Silica
Silica
Silica
Silica
Li
Water
Figure 3.9.: Sketch of the adsorption of the cation lithium at a) 10−4 M before and after
approaching and b) 10−3 M ionic strength before and after approaching
61
Cation
Li+
Na+
K+
C+
TMA+
Bare radius(Å)
0.60
0.95
1.33
1.69
2.56-3.47
Hydrated radius(Å)
3.82
3.58
3.31
3.29
N/A
Table 3.2.: Size of the counterions, taken from [3]
during the force measurements the ions are forced to penetrate the gel layer. Another
argument supporting the theory of the gel layer is the soft contact wall obtained in some
experimental curves (see figure 3.2). The aging of silica in water may increase the grow
of the hair layer [32].
3.4.2. Effect of pH
Long Range Interaction
Figure 3.5 shows the interaction curves between the silica surfaces across different electrolytes at pH=4. Repulsion is still present for NaCl which gave a negative surface
potential of the silica surface at pH=5.8. It is assumed that the silica surface is still
negatively charged. In case of Li+ , K+ and Cs+ the silica surface is already neutralized
and no repulsion occurs anymore. At pH=4 the silica surface is close to its isoelectric
point, which leads to a reduction of long range electrostatic repulsion. The obtained
Debye length of 21.4 nm coincides with the ideal one of 21.4 nm for 2 × 10−4 M ionic
strength and is smaller than at pH=5.8 (see figure 3.1) due to the addition of protons
to the solution. If the electrolyte concentration is increased to 10−3 M, the double layer
collapse remains at pH=4 for all the electrolyte solutions (figure 3.6). Zeta potential
experiments performed under the same conditions confirm the last statement.
Short Range Interaction
The pronounced attraction seen for NaCl at pH=5.8 (figure 3.1) vanishes at low pH (see
figure 3.5). The repulsion may be due to the protons and cations adsorbed at the silica
surface. If polysilicic acid chains are present at the silica surface, the presence of more
protons will collapse the gel layer at the silica surface and no steric stabilization will
be present. A slight decrease in pH causes attraction during the whole range due to
the neutralization of the silica particles and the possible collapse of the polysilicic acid
chains. Measurements in water (figure 3.8) show a large attraction at smaller distances.
The attraction may be related to the collapse of the gel layer due to electrostatic screening
and van der Waals forces. By increasing the electrolyte concentration to 10−3 M ionic
62
strength, only attractive interactions are observed, due to the neutralization of the silica
surfaces (see figure 3.6).
3.5. Conclusions
The discussed experiments show that ion specific effects are still present even at low ionic
strengths. The long range interactions are similar for all the studied salts, but at 10−3
M ionic strength a slight tendency for Cs+ to be adsorbed at the surface is observed.
The surface potentials are slightly lower for 10−3 M ionic strength indicating additional
adsorption. The Debye length is different for 10−4 M and 10−3 M ionic strength and
coincides with the ideal values. The same qualitative long range interaction behaviour
is obtained for the studied ionic strengths. The short range interactions are different
for both ionic strength studied. At 10−4 M ionic strength the short range attraction
decreases in the order Li+ >Na+ >K+ >Cs+ following the inverse Hofmeister series.
The model of Pashley [12] and Higashitani [11] can explain the interactions between the
silica particles at short range. It is possible that a gel layer is present at the silica surface
which also influences the interactions between the silica spheres. A decrease in pH in the
presence of electrolytes produces a synergetic effect between the protons and the cations,
giving rise to hydration repulsion. An increase of the ionic strength to 10−3 M produces
a short range repulsion due to hydration forces. It seems that the stability of silica under
the studied conditions is defined by a balance of electrostatic forces at long range and a
combination of hydration, depletion and steric forces at short range.
63
Bibliography
[1]
D. F. Evans and H. Wennerström. The colloidal domain: Where Physics, Chemistry,
Biology and Technology meet. VCH publishers, 1994.
[2]
J.-P. Chapel. In: Langmuir 10 (1994), pp. 4237–4243.
[3]
M. Colic et al. In: Langmuir 13 (1997), pp. 3129–3135.
[4]
J. J. Valle-Delgado et al. In: J. Chem. Phys. 123 (2005), pp. 034708–1.
[5]
S. Rentsch et al. In: Phys. Chem. Chem. Phys. 8 (2006), pp. 2531–2538.
[6]
J. J. Adler, Y. I. Rabinovich, and B. M. Mougdil. In: J. Colloid Interface Sci. 237
(2001), pp. 249–258.
[7]
G. Vigil et al. In: J. Colloid Interface Sci. 165 (1994), pp. 367–385.
[8]
B. V. Velamakanni et al. In: Langmuir 6 (1990), p. 1323.
[9]
J. Israelachvili. Intermolecular and Surfaces Forces. 2nd. Edition. Academic Press,
1991.
[10]
G. M. Torrie, P. G. Kusalik, and G. N. Patey. In: J. Chem. Phys. 91 (1989), p. 6367.
[11]
I. U. Vakarelski, K. Ishimura, and K. Higashitani. In: J. Colloid Interface Sci. 227
(2000), pp. 111–118.
[12]
[13]
D. F. Parsons et al. In: Langmuir 26 (2010), pp. 3323–3328.
[14]
G. V. Franks. In: J. Colloid Interface Sci. 249 (2002), pp. 44–51.
[15]
M. Boström et al. In: Adv. Colloid Interface Sci. 123-126 (2006), pp. 5–15.
[16]
Israelachvili. J. N. and R. M. Pashley. In: Nature 306 (1983), p. 249.
[17]
[18]
[19]
P. West and A. Ross. An Introduction to Atomic Force Microscopy Modes. Santa
Clara, CA: Pacific Nanotechnology, Inc., 2006.
[20]
J. Ralston et al. In: Pure Appl. Chem. 77 (2005), pp. 2149–2170.
[21]
W. A. Ducker, T. J. Senden, and R. M. Pashley. In: Langmuir 8 (1992), pp. 1831–
1836.
[22]
McKee. DLVO fitting. http://goo.gl/2Rh8c. June 2011.
64
Bibliography
[23]
D. Y. C. Chan, R. M. Pashley, and L. R. White. In: J. Colloid Interface Sci 77
(1980), p. 283.
[24]
W. Briscoe. Colloid Science: Principles, methods and applications. Ed. by T Cosgrove. John Wiley and Sons Ltd, 2010, p. 343.
[25]
P. G. Hartley, I. Larson, and P. J. Scales. In: Langmuir 13 (1997), pp. 2207–2214.
[26]
G. Toikka and R. A. Hayes. In: J. Colloid Interface Sci. 191 (1997), pp. 102–109.
[27]
C. Labbez et al. In: Langmuir 25 (2009), pp. 7209–7213.
[28]
I. U. Vakarelski and K. Higashitani. In: J. Colloid Interface Sci 242 (2001), pp. 110–
120.
[29]
M. Colic, M. L. Fisher, and G. V. Franks. In: Langmuir 14 (1998), pp. 6107–6112.
[30]
A-C. Johnsson. “On the electrolyte induced Aggregation of Concentrated Silica
Dispersions: An Experimental Investigation Using the Electrospray Technique”.
PhD thesis. 2011.
[31]
J. Lyklema and Th. F. Tadros. In: J. Electroanalytical Chem. 17 (1968), pp. 267–
275.
[32]
M. Skarba. “Interactions of colloidal particles with simple elctrolytes and Polyelectrolytes”. PhD thesis. 2008.
65
4. Interaction Forces between Silica
Surfaces in Cationic Surfactant
Solutions below the CMC
4.1. Introduction
The morphologies, functions, and applications of surfactants are diverse. There are
natural surfactants such as the phospholipid protein, a pulmonary surfactant [1] used
to reduce the surface tension in the lung. In addition synthetic surfactants are used
in industry, including textile, cleaning products, cosmetics, food, and others. Depending on the concentration the surfactant can act as a destabilizing or stabilizing agent.
Therefore, it is important to understand the interaction forces between two surfaces in
presence of the surfactant. Related to that, the morphology of the surfactant aggregates
on the surfaces has a dominant effect. The morphology of the surfactant aggregates in
the bulk depends on the critical packing parameter [2, 3]. Therefore, different aggregate
morphologies like spherical, cylindrical, globular, oblate micelles, single and multiwalled
vesicles, microtubules, bilayers, lamellar phases, and inverted structures can be found
depending on the size ratio between head group and hydrophobic tail of the surfactant
[3]. It is known that hexadecyltrimethylammonium bromide (CTAB) forms spherical
micelles in the bulk at the critical micelle concentration (CMC) [4–6]. Additional parameters, like surface charge, head groups and interaction between the hydrophobic tail,
define the structure of the adsorbate on surfaces [4, 7]. Several studies of the adsorption
of surfactant to a hydrophilic surface, like mica and silica, have been performed [6–9].
Tyrode et al. [8] mention the two limit cases for adsorbing charged ionic surfactant at an
oppositely charged hydrophilic surface below the CMC. For low surface charge systems,
where the electrostatic interaction between head groups and surface is weak, no monomer
adsorption at low concentration takes place. At a certain concentration, the critical surfactant aggregation concentration (csac), aggregates start to adsorbe to the surface. No
monolayer formation is present in this system, since a monolayer with aliphatic chains
facing towards the aqueous solution would be entropically unfavoured. Due to hydrophobic interactions additional surfactant molecules (with the hydrophilic groups facing out
to the water) will start to adsorbe before monolayer coverage is reached. In case of high
surface charge, monolayer formation is favoured due to strong electrostatic attraction
between the surfactant head groups and the oppositely charged surface. Subramanian et
al. [4] studied the effect of the counterion on the shape of the adsorbed aggregates on the
66
silica surface with AFM and demonstrated that it is possible to change the morphology
of the CTAB from spherical to cylindrical by changing the counterion. With Cl− ions
for example spherical micelles are obtained whereas with Br− ions cylinder aggregates
on the silica surface are reported. Velegol et al. [9] obtained similar results studying
the adsorbed layer at concentrations from 0.9 mM to 10 mM. Other authors [10, 11] reported the presence of micelles on the silica surface above the CMC studied with optical
reflectometry and surface force apparatus respectively. Stiernstedt et al. [2] measured
the surface force between silica particles across tetradecyltrimethylammonium bromide
(TTAB) with a bimorph surface force apparatus. The obtained adsorbed layer thickness
is 4 nm. Rutland et al. [11] could not conclude if a patchy bilayer or flattened micelles
were present on the silica surface close to the CMC. They observed a strong dependency
of the adsorption of CTAB molecules on the surface charge of the particles. Raman
scattering and sum frequency spectroscopy showed that the thickness of the CTAB layer
on silica particles is about 3 nm. Surface neutralization is obtained around 0.1 mM surfactant concentration. Neither of the two principal models mentioned in the literature
("The two step model" and "The four region model") can explain the adsorption to the
silica surface [8].
The surface forces in the presence of surfactants have been investigated by several authors.
Parker et al. [12] studied the interactions between glass particles with a surface force
apparatus across CTAB solutions of different concentrations. They observed attractive
interactions between two silica surfaces for distances at about 20 nm. The attractive
forces could not be explained by van der Waals forces and they were present after the
charge reversal and at higher concentrations. Several researchers try to explain the long
range attraction observed between hydrophobic surfaces. Craig et al. [13] reported long
range hydrophobic attraction of about 40 nm for cetylpyridimium chloride (CPC) in 100
mM NaCl adsorbed to silica surfaces. Carambassis et al. [14] associate the long range
attraction between hydrophobic surfaces with the presence of bubbles. In a further work
Craig et al. [15] obtained a slightly less attractive hydrophobic force for adsorbed CPC
layers on the silica surface in 100 mM NaCl when the surfactant solution was degassed.
Although the influence of dissolved gas on the hydrophobic interaction was suspected,
they could not prove that nanobubbles may be responsible for the observed attractions.
Kekicheff et al. [16] reported the correlation between the prefactor of the long range
hydrophobic interaction for silica particles in CTAB solutions and the ionic strength of the
solutions, supporting the hypothesis that the long range attraction between hydrophobic
surfaces may have electrostatic origin. In the work of Yaminski et al. [17] microcavitation
between the adsorbed patches of CTAB on the surface is considered to explain the large
range hydrophobic interactions. Pashley et al. [18] studied the phenomenon of cavitation
in CTAB monolayer. No evidence of cavitation was observed for this system since the
contact angle was less than 90 degrees.
So far, no systematic study of the interaction forces in the presence of cetyltrimethylammonium bromide (CTAB) has been carried out using AFM and still the mechanism for
the long range hydrophobic attraction present in those systems is debated. In this work
the interaction forces between a pair of silica particles (system I) and between a silica
67
particle and a silicon wafer (system II) in CTAB solutions were measured over a large
range, from well below the CMC to concentrations above the CMC. A prediction of the
aggregates structure on the silicon oxide surfaces in dependence on surfactant concentration is established based on the qualitative and quantitative analysis of the interaction
curves.
4.2.1. Materials
A suspension of silica particles of 4.63 µm in mean diameter (10% solid content) was
purchased from Bangs Laboratories. Solutions from cetyl trimethyl ammonium bromide
(CTAB, analytic grade, Aldrich) were prepared in a concentration range from 0.005 mM
to 1.2 mM in pure water (milli-Q). CTAB was water soluble up to 1.2 mM at room
temperature. Clear solutions were obtained at all concentrations, which shows that the
experiments were performed above the Krafft temperature.
Preparation
The attachment of silica particles to the glass slide and to the cantilevers were performed
as described in section 3.2.2. The silicon wafers (type-P Wacker Siltronic Burghausen)
were cut and cleaned in piranha solution H2 O2 /H2 SO4 50:50 for 30 minutes, thereafter
washed with milli-Q water, and then immediately used for the experiments. This method
allows the creation of an oxide layer at the silicon wafer surface and renders the surface
highly hydrophilic.
Methods
MFP-3D Asylum Research atomic force microscope mounted on an inverted optical microscope (Olympus IX71) (see section 3.2.2). The measurements performed with one pair
of spheres are reported. The force measurements between a silica particle and an oxidized
silicon wafer were performed with the same apparatus (Olympus IX71), instead of a glass
slide with attached silica particles, a clean silicon wafer is placed on the scanner. A new
clean silicon wafer was used for each concentration. At least 50 repetitions were done.
After each measurement an optical microscope was used to check, if the particle was still
attached to the cantilever. For the particle-particle system this check was performed
during the measurements using an inverse microscope placed in the AFM.
68
The zeta potentials were measured using a Malvern Zetasizer Nano ZS with a HeNe laser.
Different solutions of CTAB from 0.005 to 1.2 mM containing 0.1 % silica particles were
prepared. These suspensions were placed in a clear disposable zeta cell (DTS1060C) and
equilibrated for two minutes at 25o C in the equipment before starting measurements [20].
The average of 10 zeta potential measurements of the same sample was taken as the zeta
potential value.
4.2.3. Simulations
The simulations are based on the DLVO theory and were performed as described in
section 3.2.3.
4.3. Results
4.3.1. Interaction forces between two silica particles (system I)
Figure 4.1 shows the interaction forces between two silica particles at a surfactant concentration from 0 to 0.1 mM. The pH of the solutions was around 5.8 where silica is
negatively charged. The diameter of the particles was 4.63 µm. The fitting of the experimental curves to the DLVO theory provides the potential, ϕ and ionic strength, I .
Each experimental curve was fitted with both boundary conditions, constant charge and
constant potential. For reasons of clarity, in figure 4.1 only the fit for the experimental
curve at 0.1 mM ionic strength is shown. The constant charge boundary condition fits
very well at larger distances but for distances smaller than 20 nm, the experimental curve
lies between the two boundary conditions. Neither the charge nor the potential remains
constant. The respective potentials, ϕ and decay length, k−1 are shown in table 4.1.
The repulsion decreases with increasing surfactant concentration from 0 to 0.05 mM.
The interaction in water was repulsive for all distances. Below 5 nm a small attraction
was observed which can be interpreted as a van der Waals force. At 0.005 mM repulsion
was observed for the whole range of studied forces. The diffuse layer potential ϕ was
decreased from -30 mV (for water) to -14 mV and the Debye length k−1 of 42.87 nm was
similar to the Debye length of water and corresponds to an ionic strength I of 0.005 mM.
In force measurements, only the value of the potential can be determined. Based on zeta
potential measurements under the same conditions, the sign of the potential is inferred.
A further increase of the CTAB concentration gave a weaker repulsion at separation >10
nm, and for smaller distances attraction was observed. At 0.03 mM surfactant concentration only weak repulsion was detected, and at 0.05 mM CTAB concentration no repulsion
was observed at all, only attractive interaction with a jump–in distance around 20 nm.
When the concentration was increased further to 0.1 mM repulsion occurred again. This
leads to the conclusion that the point of zero charge (pzc) is between 0.05 and 0.1 mM
CTAB concentration.
69
0.25
water, pH=5.8, ö= - 30 mV, ë=42.18 nm
0.005 mM
0.008 mM
0.01 mM
0.03 mM
0.05 mM
0.1 mM
DLVO_CC
DLVO_CP
0.20
0.15
F/2
]mINm[ Rð
0.10
0.05
0.00
0
5
10
15
20
25
30 35 40
Separation [nm]
45
50
55
60
65
70
Figure 4.1.: Forces between a pair of colloidal silica particles (system I) across CTAB
surfactant solution, from 0 to 0.1 mM surfactant concentration. Hamaker
(constant potential) fits are shown for 0.1 mM surfactant concentration
In figure 4.2 the respective interaction forces are shown for a concentration regime from
0.1 to 0.5 mM. For a better understanding only the best fits to the experimental curves are
shown. The constant charge boundary condition is fitting the experimental curve really
well at 0.2 and 0.3 mM for distances larger than 10 nm where repulsion was observed.
At smaller distances short range attraction dominates the interactions. At 0.4 mM, the
constant potential fits better almost until contact. Repulsive interactions are seen for
larger distances and at smaller distances only a small attraction is observed. At 0.5
mM the experimental curve lies between the constant charge and the constant potential
boundary conditions for distances smaller than 20 nm. No short range attraction is seen
anymore, the interactions are monotonic repulsive at this concentration. The diffuse
layer potential ϕ increased from 0.1 mM to 0.4 mM surfactant concentration (see table
4.1). The Debye length k−1 correlates well with the theoretical one and was decreasing
with increasing surfactant concentration, as expected.
In the concentration regime from 0.5 to 1.2 mM CTAB (see figure 4.3) repulsion domi-
70
CTAB
Concentr.
(mM)
0.00
0.005
0.008
0.01
0.03
0.05
0.10
0.20
0.30
0.40
0.50
0.80
1.00
1.20
Diffuse layer
potential
system I ϕ (mV)
-30.0±0.0
-14.0±1.6
-15.0±0.0
-15.0±0.0
0.0
0.0
+36.0±0.0
+48.0±0.0
+48.6±2.8
+72.2±6.2
+57.2 |+75.0∗∗ ± 7.5
+38.2 |+60.0∗∗ ± 0.6
+30.0 |+45.0∗∗ ± 0.0
+30.0 |+45.0∗∗ ± 0.0
Diffuse layer
potential
system II ϕ (mV)
-45.0±0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
+46.0±2.6
+75.0 |+50.0∗ ± 5.0
+158.0 |+80.0∗ ± 2.7
+44.0 |+85.0∗∗ ± 5.3
not measured
Experim.
decay len.
k−1 (nm)
42.2
42.9
42.9
42.9
n.d.
n.d.
30.0
21.4
17.5
15.1
13.6
10.7
8.8
8.78
Zeta
potential
ζ (mV)
-55.0
0.0
0.0
0.0
+21.3±4.2
+26.9±5.3
+20.5±2.0
+35.0±6.8
+62.7±3.6
+58.8±12.4
+95.8±1.3
+108.0±7.1
+127.0±2.7
+111.0±3.9
Table 4.1.: Results for simulations of direct force measurements by the Poisson-Boltzmann theory. ∗ From fitting with the plane of origin of charge taken at 4
nm from each surface. ∗∗ Assuming that the surfaces were not in contact
(micelles/bilayer adsorption).
nated the interaction over the whole range. The fits shown for 0.5 mM (figure 4.2), 0.8
mM and 1.0 mM (figure 4.3) were calculated assuming that the surfaces were in contact.
The constant charge and constant potential boundary conditions fit well at larger distances. For distances smaller than 20 nm, the experimental curve lies between the two
boundary conditions. No adhesion in the retraction curves (data not shown) was seen
for concentrations larger than 0.5 mM. An unexpected decrease in the repulsion with
increasing surfactant concentration was observed in this concentration regime. Therefore
the DLVO fits were also calculated assuming that the surfaces were not in contact taking
into account the adsorption of aggregates that cannot be removed from the surface by
the applied force. Under this assumption, the constant charge boundary condition fits
the experimental curves well until contact (data not shown). The fitted diffuse layer
potentials are reported in table 4.1).
4.3.2. Interaction forces between a silica particle and a silicon wafer
(system II)
In order to get information about the adsorption of surfactant at the silicon wafer, force
curves were recorded for interactions between a silica particle and a silicon wafer (system
II) for the same CTAB concentration range (0.005 to 1 mM). The interaction curves were
71
0.25
water, pH=5.8,
0.1 mM
0.2 mM
0.3 mM
0.4 mM
0.5 mM
DLVO_CC
DLVO_CP
0.20
F/2
]mINm[ Rð
0.15
0.10
0.05
0.00
0
5
10
15
20
25
30
35
40
Separation [nm]
45
50
55
60
65
70
Figure 4.2.: Forces between a pair of colloidal silica particles (system I) across CTAB
surfactant solutions from 0.1 to 0.5 mM surfactant concentration. Hamaker
(constant potential) fits are shown for 0.2 mM (DLVO_CC), 0.3 mM
(DLVO_CC - overlaps fit at 0.2 mM), 0.4 (DLVO_CP) and 0.5 mM
(DLVO_CC and DLVO_CP) surfactant concentration
fitted with the DLVO theory. The Derjaguin approximation is still valid for this system
and since it is difficult to simulate two different surfaces, a symmetric system (two planar
surfaces) was assumed. The obtained diffuse layer potential serves to show the changes
in the system. The fitted diffuse layer potentials are reported in table 4.1. The force
curves for two concentrations below 0.3 mM are shown in figure 4.4. Only attraction was
observed for these concentrations and the jump–in distance was around 20 nm for 0.005
mM and 50 nm for 0.05 mM. The interactions at 0.1 mM and 0.2 mM (data not shown)
remain attractive with a jump–in distance around 50 nm. The interactions between
two silica particles (system I) at 0.05 mM (jump–in distance around 15 nm) are also
represented in this figure. It can be seen that the attraction is larger in magnitude and
range for system II.
Figure 4.5 shows the force curves for concentrations ranging from 0.3 mM to 0.8 mM. At
0.3 mM CTAB concentration the observed attraction has a jump–in distance of about
43 nm and no repulsion occurs at long range. From 0.4 mM up to 0.8 mM repulsion was
72
0.25
water, pH=5.8
0.5 mM
0.8 mM
1 mM
1.2 mM
DLVO_CC
DLVO_CP
0.20
]mINm[ Rð
F/2
0.15
0.10
0.05
0.00
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
Separation [nm]
Figure 4.3.: Forces between a pair of colloidal silica particles (system I) across CTAB surfactant solutions from 0.5 to 1.2 mM surfactant concentration. Hamaker constant A= 8.5 × 10−21 J. DLVO_CC (constant charge) and DLVO_CP (constant potential) fits are shown for 0.8 and 1 mM surfactant concentration.
observed for distances larger than 20 nm. The better DLVO fits are shown for 0.4, 0.5 and
0.8 mM surfactant concentration. At 0.4 mM the constant charge as well as the constant
potential boundary condition fits the experimental curve very well at distances larger than
20 nm. The interactions for distances smaller than 20 nm were attractive. At 0.5 mM the
better fit is obtained by the constant charge boundary condition. A non-DLVO repulsion
was observed from 20 nm down to 10 nm followed by a plateau down to contact. At 0.8
mM also the constant charge boundary condition fits better and a non-DLVO repulsion
from 20 nm down to 13 nm was observed followed by a plateau down to contact. It
is important to note that the plateau did not occur between silica particles (see figure
4.3). Since a plateau around 10 nm was observed at 0.5 and 0.8 mM corresponding to
the adsorption of aggregates on the surface, the DLVO fits at these concentrations were
calculated with the plane of charge displaced by 4 nm from each surface (see table 4.1).
The obtained fitted diffuse layer potentials for the unshifted plane of charge are shown
in table 4.1. At 1 mM the interaction was monotonically repulsive (figure 4.6). The
experimental curve was shifted by 8 nm, considering the presence of aggregates on the
surface, which could not be removed by the applied force at this concentration. The
original and the shifted curves are both shown with the corresponding fits, the obtained
73
diffuse layer potentials are given in table 4.1.
0.10
0.005 mM (silica particle-silicon wafer)
0.05 mM (silica particle-silicon wafer)
0.05 mM (silica particle-silica particle)
0.08
0.06
]mINm[ Rð
0.04
F/2
0.02
0.00
-0.02
-0.04
-0.06
0
10
20
30
40
50
60
70
Separation, D [nm]
Figure 4.4.: Forces between a colloidal silica particle and a silicon wafer (system II) across
CTAB surfactant solutions at 0.005 and 0.05 mM surfactant concentration.
Forces between two colloidal silica particles (system I) at 0.05 mM surfactant
concentration. Hamaker constant A= 8.5 × 10−21 J.
4.3.3. Point of zero charge
From figure 4.1 one can conclude that the point of zero charge (pzc) for silica particles
(system I) occurred at about 0.05 mM surfactant concentration. Since a symmetric system is measured, the sign of the potential can also be inferred from the trend of the
force curves. The interactions are changing with concentration, from repulsion at low
surfactant concentration to attraction at 0.05 mM (pzc) to repulsion again at higher concentrations. Silica particles are negatively charged in water at normal pH [21]. Therefore,
a negative sign is assumed for surfactant concentrations below 0.05 mM and a positive
charge for concentrations above 0.05 mM. The dependence of the charge of the silica
on surfactant concentration and the point of zero charge was confirmed by zeta potential measurements. The zeta potentials and the fitted diffuse layer potentials are shown
in table 4.1. For surfactant concentrations below 0.03 mM the zeta potentials values
were low and no stable values were obtained. From 0.03 mM onwards, positive values
of zeta potentials were obtained, meaning that the pzc in zeta potential measurements
74
0.6
0.3 mM
0.4 mM
0.5 mM
0.8 mM
DLVO_CC
DLVO_CP
0.5
0.4
F/2
]mINm[ Rð
0.3
0.2
0.1
0.0
-0.1
0
10
20
30
40
Separation, D [nm]
50
60
70
Figure 4.5.: Forces between a colloidal silica particle and a silicon wafer (system II)
across CTAB surfactant solutions from 0.3 to 0.8 mM surfactant concentration. Hamaker constant A= 8.5 × 10−21 J. DLVO_CC (constant charge)
and DLVO_CP (constant potential) fits are shown for 0.4 mM surfactant
concentration. For the DLVO_CC fits shown at 0.5 mM and 0.8 mM the
plane of charge was set 4 nm away from each surface.
was around this concentration. The zeta potential increased with increasing surfactant
concentration indicating an increase in adsorbed amount of surfactant. For system I the
fitted diffuse layer and the zeta potentials show the same tendency at concentrations
below 0.4 mM. Both are increasing with surfactant concentration. At concentrations
above 0.4 mM the zeta potential continuously increased with concentration, as expected,
whereas the fitted diffuse layer potential shows an "apparent" decrease. From figure 4.5
the point of zero charge for system II can be deduced. It occurred at about 0.3 mM
surfactant concentration.
75
0.6
0.5
1 mM
1 mM (offset 8 nm)
DLVO_CC1
DLVO_CP1
DLVO_CC2
DLVO_CP2
0.4
F/2
]mINm[ Rð
0.3
0.2
0.1
0.0
-0.1
0
20
40
Separation, D [nm]
60
Figure 4.6.: Forces between a colloidal silica particle and a silicon wafer (system II) across
1 mM CTAB surfactant solution. Hamaker constant A= 8.5 × 10−21 J.
The experimental curve was offset 8 nm under the assumption that micelles/patchy bilayers are adsorbed at the surface. DLVO_CC (constant
charge) and DLVO_CP (constant potential) fits are shown for the experimental curve (DLVO_CC1 , DLVO_CP1 ) and the shifted curve (DLVO_CC2 ,
DLVO_CP2 ).
4.4. Discussion
4.4.1. Interaction between two silica particles (system I)
The fitted potentials are called "effective potentials" because although the DLVO theory
is applied for the analysis, there are other non DLVO forces which influence the interactions, like hydrophobic, hydration and steric forces. From zeta potential measurements
it is known that the silica particles have a negative surface potential in water. The addition of 0.005 mM CTAB to water causes a decrease in the diffuse layer potential from
ϕ=-30 mV to ϕ=-14 mV (see figure 4.1). The interaction curve is still repulsive. The
potential is decreased due to the adsorption of positively charged surfactant monomers
to the silica particles. The attraction observed at short range for the interaction in water
was overcome at this surfactant concentration and short range repulsion was observed.
A correlation between the measured interaction curves and the possible surfactant struc-
76
tures on the silica surface can be established. From the quantitative analysis of the
interaction curve (the fitting to the DLVO theory) it is known that adsorption occurred
at 0.005 mM because the diffuse layer potential is decreased with respect to the diffuse
layer potential for the interactions in water. Based on the qualitative analysis of the
force curve at this concentration a parallel arrangement is the most probable (see figure
4.7). If the surfactant molecules were arranged perpendicular to the surface, attraction
would be observed at short range due to hydrophobic interactions. Since short range
repulsion was observed, it is more probable that the surfactant monomers were arranged
in parallel to the surface and that the observed short range repulsion was due to the
dehydration of the ammonium head groups. At this concentration the hydration forces
overcome the hydrophobic forces and/or van der Waals forces (see figure 4.7). Other
authors [8] reported a perpendicular arrangement of the surfactant molecules with the
hydrophobic tails facing towards water at low surfactant concentration.
Increasing the concentration further from 0.008 mM to 0.05 mM (see figure 4.1) turned
the interactions from repulsive to attractive. More and more surfactant molecules interact
electrostatically with the silica surface. That is the most probable interaction because
the silica particles are hydrophilic and negatively charged and the CTAB head group is
positively charged. At 0.05 mM only attraction was observed with a jump–in distance at
around 20 nm. The range is larger than for pure van der Waals forces, which indicates
that additional interactions (like hydrophobic ones) come into play. The hydrophobic
interactions observed in the force curves suggest that the most probable arrangement
of surfactant aggregates is perpendicular to the surface (see figure 4.7). The point of
zero charge (pzc) was obtained at this concentration. Parker et al. [12] also observed
neutralization of the silica surface at this concentration. Although the range of the
interaction was similar, the magnitude was larger in Parker’s work. Yaminski et al. [17]
reported the point of zero charge for silica at the same concentration and argued that
below this concentration the formation of patches on the surface is possible, since the
energy gain is larger for patches than for isolated molecules. A further increase in the
surfactant concentration to 0.1 mM resulted in a repulsive long range interaction meaning
that a charge reversal from negative to positive occurred. The obtained potential was
+36 mV. The sign of the potential can be inferred from the trend of the force curves and
was also confirmed by zeta potential measurements. The interaction cannot be fitted by
the constant charge/potential model for distances smaller than 20 nm indicating that at
smaller distances there is a variation of both, the surface charge and the surface potential.
At distances <10 nm attraction was observed, which may be due to the expulsion of
aggregates present on both surfaces. Near to contact, for distances <2 nm, repulsion
was observed again which may be steric in origin. The charge reversal of the surface is
associated to the adsorption of surfactant molecules with the hydrophilic head facing out
to the water. The aggregates can easily be removed during the force measurements (see
figure 4.7). The repulsion continued increasing with increasing surfactant concentration
(figure 4.2). The effective potentials are shown in table 4.1. At 0.2 and 0.3 mM the
constant charge boundary condition matches the experimental curves really well at larger
distances. For distances smaller than 10 nm no fit is seen at all because the interactions
77
are not DLVO like and correspond to the expulsion of aggregates from the surface. At 0.3
mM the attraction is weak and starts to be replaced by a soft short range repulsion. At
0.4 mM surfactant concentration, the experimental curve is best fitted with the constant
potential boundary condition until smaller distances. A potential of 72 mV was obtained.
The change in short range interaction from attraction at 0.2 mM to soft repulsion at 0.3
mM finishing with a strong repulsion at 0.4 mM can be correlated with the stiffness of the
aggregates adsorbed to the surface. The aggregates are closer and stiffer with increasing
concentration (see figure 4.7).
A further increase in concentration to 0.5 mM led to a decrease in the diffuse layer potential to +57 mV (see table 4.1). The diffuse layer potential decreased until the critical
micelle concentration was reached where it stayed constant (see figure 4.3). This decrease
is unexpected. The experiments were repeated with another pair of silica particles showing the same tendency (data not shown). Interestingly, the interaction curves cannot
be fitted by the constant charge/potential model at distances smaller than 20 nm. The
fits were corrected for concentrations from 0.5 mM onwards, under the assumption that
the surfaces were not in contact due to the adsorption of micelles/patchy bilayers on the
surfaces. Still a decrease in diffuse layer potential is observed from 0.5 mM onwards (see
table 4.1). That is explained by condensation of Br− counterions to the highly charged
surfaces (surfaces with a denser outer layer of surfactant) during the force measurements.
Hence lower diffuse layer potentials are obtained. In contrast the zeta potential increases
with increasing surfactant concentration (see table 4.1), as expected. The interactions
are monotonic repulsive in this concentration regime (from 0.5 mM to 1 mM) indicating
that the aggregates are stiffer with increasing concentration.
4.4.2. Interaction between a silica particle and a silicon wafer (system II)
The interaction of a silica particle with a silicon wafer is represented in figures 4.4, 4.5
and 4.6. At concentrations as low as 0.005 mM a large attraction was observed. This
could be caused by aggregate patches of CTAB with the hydrophobic tails facing out to
the water present on the surface of the silicon wafer (see figure 4.7). At 0.05 mM the
interaction between a silica particle and an oxidized silicon wafer still remained attractive.
At this concentration the silica particle was neutralized, that means that the observed
attraction has to be produced by hydrophobic interactions between the two surfaces.
Therefore, it is probable that the surfactant was adsorbed at the silicon wafer surface
with the hydrophobic tails facing out to the water, as shown in figure 4.7. The long
range attraction occurred up to 0.3 mM (see figure 4.5). It is known from force and
zeta potential measurements that at this concentration the silica particle was positively
charged. Hence the observed attraction may be due to electrostatic interaction between
the already positive silica particle (charge reversal at 0.05 mM) and the bare silicon wafer
(the areas without surfactant adsorption). Therefore, it is proposed that the surfactant
adsorbed patchwise to the silicon wafer surface (see figure 4.7). The validity of this
statement is confirmed later in this paper (see figure 4.9).
78
Silica particle-silica particle interaction (system I)
0.005 mM
0.05 mM (pzc)
0.1 mM
0.4 mM
1 mM
0.4 mM
1 mM
Silica particle-silicon wafer interaction (system II)
0.005 mM
0.05 mM
0.3 mM (pzc)
Figure 4.7.: Possible surfactant morphologies depending on the concentration.
Figure 4.5 shows the interactions between a silica particle and a silicon wafer from 0.3
to 0.8 mM. The point of zero charge for this system was 0.3 mM. At 0.4 mM a long
range double layer repulsion was present. The short range attraction started at around
17 nm and is non-DLVO. The surfactant starts to be adsorbed with the hydrophilic tails
facing out to the water (see figure 4.7). At 0.5 mM the short range interaction occurred
at around 8 nm which corresponds to the expulsion of micelles/patchy bilayers from the
two opposing surfaces. At 1 mM (CMC) complete micelles/patchy bilayers were present
on the silica/silicon wafer surfaces, and they could not be removed under the applied
force. Two DLVO fits were done. The results are shown in table 4.1. The experimental
curve can be fitted well with a diffuse layer potential of 44 mV, assuming that the
surfaces (silica/silicon wafer) are in contact(D = 0) or with a diffuse layer potential of
85 mV assuming that the surfaces (silica/silicon wafer) are separated by 8 nm due to the
adsorption of micelles/patchy bilayers. The non-DLVO repulsion observed in the shifted
curve can be explained by steric forces between the micelles/patchy bilayers (see figure
4.6).
79
4.4.3. Comparison between the system silica particle–silica particle (I) and
the system silica particle–silicon wafer (II)
A comparison of the interactions in the two systems gives the following results. At low
CTAB concentration (e.g. 0.05 mM) the attraction is larger for system II. Since the
attractions were due to hydrophobic interactions, the silicon wafer was more hydrophobic than the silica particles. Consequently the hydrophobic patches were larger at the
silicon wafer (see figure 4.4). At 0.4 mM surfactant concentration (figure 4.8) the Debye
0.4
0.3
0.4 mM (silica particle-silicon wafer) , ö=+50mV,
ë=15.2nm
0.4mM (silica particle-silica particle), ö=+75mV, ë=15.2nm
]mINm[ Rð
F/2
0.2
0.1
0.0
0
10
20
30
40
50
60
70
Separation, D [nm]
Figure 4.8.: Interaction forces between two silica particles (system I) and between a particle and a silicon wafer (system II) at 0.4 mM surfactant concentration. The
Debye length is 15.2 nm for both cases.
length correlates well with the theoretical one for both systems but the interaction looks
different. A greater repulsion was obtained for system I compared to system II. That
indicates that the outer layer of surfactant was more dense at the silica particle surface.
The qualitative analysis of both systems from 0.4 mM onwards led to the conclusion that
it was not possible to remove the aggregates from the silica particles under the applied
force in system I whereas aggregate expulsion was seen for system II up to 0.8 mM. The
aggregates at the silica particles must be stiffer and more closely packed than on the
silicon wafer surface from 0.4 mM onwards. In general, the aggregate morphology at the
80
silica particles was different than at the silicon wafer, which was verified with the different force curves obtained for the same concentration. The silica particles and the silicon
wafer have the same surface chemistry, but the surface treatment has a great influence
on the surface charge and hence the type of aggregates on the surface. Other authors
confirm that different preparation methods cause differences in surface charge [22]. The
point of zero charge (pzc) for system I was obtained at 0.05 mM surfactant concentration
and for system II at 0.3 mM. That indicates that the silicon wafer carries a larger surface
charge.
4.4.4. Non DLVO forces
Non-DLVO forces were observed in system I and II at 0.05 mM. The jump–in distance for
system II was about 43 nm which is larger than that for system I. The bridging mechanisms through nanobubbles is an explanation for the hydrophobic forces observed at this
concentration. In previous experiments performed in our group nanobubbles were found
on the silicon oxide surface at 0.05 mM surfactant concentration (see chapter 5). When
two surfaces with nanobubbles are approaching, a thin free–standing lamella is formed
between the bubbles. When it breaks the bubbles will bridge and a jump-in contact will
be observed in the force curves. At 0.3 mM a large attraction was observed for system
II. It was proposed that the surfactant is adsorbed patchwise and that electrostatic interactions between the already positive silica particle and the still negative bare areas
at the silicon wafer also play a role in the observed attraction. An AFM image of the
silicon wafer at this concentration (see figure 4.9) shows that two different morphologies were present on the surface, so-called "micropancake" (thin layers of air) with some
nanobubbles on the top and some areas without surfactant adsorption. The observed
morphology is similar to that in reference [24]. It indicates that the adsorption at the
silicon wafer occurs patchwise at low surfactant concentration. Patchwise adsorption is
also reported at mica surfaces in the presence of low concentration of C16 TAB [25] and
at silica surfaces in the presence of low concentration of C18 TACl [26]. Micropancakes as
well as nanobubbles may be covered with surfactant. Attractive electrostatic interaction
between the silica particle (positive) and the silicon wafer (negative areas without adsorption) as well as the bridging through nanobubbles can explain the attractive interaction
observed at this concentration. Non DLVO forces due to expulsion of aggregates or steric
forces between micelles/patchy bilayers were also observed at higher concentrations.
81
micropancake
300
250
4
200
2
nm
nm
3
150
100
1
50
0
0
0
50
100
150
200
250
Height
300
nanobubble
nm
2
1
0
-1
-2nm
0
50
100
150
nm
200
250
300
Figure 4.9.: AFM tapping mode of a silicon wafer at 0.3 mM surfactant concentration.
4.5. Conclusions
The interaction forces between two silica particles (system I) and between a silica particle and a silicon wafer (system II) in the presence of aqueous CTAB solutions with
concentrations between 0.005 and 1 mM were measured using AFM. The force curves
were correlated to the surfactant morphologies (figure 4.7). Both systems show a charge
reversal from negative to positive caused by the adsorption of cationic surfactant (CTAB)
at the former negatively charged silicon oxide surface. The interactions of the two systems were different for the same studied surfactant concentration. The point of zero
charge was obtained at 0.05 mM for the silica particle–silica particle system (system I)
as in Parker’s work [12] and at 0.3 mM for the silica particle–silicon wafer system (system
II). This leads to different aggregate morphologies at the silica particle surface and at the
surface of the silicon wafer. An explanation for the difference might be the surface treatment: the silica particles were plasma cleaned, whereas the silicon wafers were treated
with a piranha solution. In another study the same surface treatment was applied to the
silica particles and the silicon wafers and no differences in the interaction were observed
[27]. At a low CTAB concentration (for example 0.05 mM) long-range attraction was
observed. The attraction was larger in range and magnitude for the silica particle–silicon
wafer system and starts at distances larger than 40 nm. They cannot be caused by van
der Waals attraction, but they are explained by the presence of nanobubbles probing hy-
82
drophobic patches on the surfaces. The attraction occurs when the nanobubbles bridge.
Obviously, on a silicon wafer surface larger hydrophobic patches are present than on
the surface of the silica particles. Another explanation for long-range attraction is the
electrostatic attraction between oppositely charged patches. At higher surfactant concentration (0.4 mM onwards), monotonic repulsion between the two silica particles was
observed. In contrast to this, aggregate expulsion could be observed in the interaction
curves for the silica particle–silicon wafer system up to a CTAB concentration of 0.8
mM. The difference is explained by the different stiffness of the surfactant aggregates at
the two surfaces (silica particle and silicon wafer). For the same studied concentrations,
the outer layer of surfactant was denser for the silica particles and the aggregates on
the silica particles were stiffer and more closely packed. The stiffer aggregates are more
difficult to remove.
83
Bibliography
[1]
J. A. Zasadzinski et al. In: Biophys. J 89 (2005), pp. 1621–1629.
[2]
J. Stiernstedt et al. In: Langmuir 21 (2005), pp. 1875–1883.
[3]
D. F. Evans and B. W. Ninhman. In: J. Phys. Chem. 90 (1986), pp. 226–234.
[4]
V. Subramanian and W. A. Ducker. In: Langmuir 16 (2000), pp. 4447–4454.
[5]
S. Berr, R. R. M. Jones, and J. S. Johnson Jr. In: J. Phys. Chem. 96 (1992),
pp. 5611–5614.
[6]
H. N. Patrick et al. In: Langmuir 15 (1999), pp. 1685–1692.
[7]
S. C. Biswas and D. K. Chattoraj. In: J. Colloid Interface Sci. 205 (1998), pp. 12–
20.
[8]
E. Tyrode, M. W. Rutland, and C. D. Bain. In: J. Am. Chem. Soc. 130 (2008),
pp. 17434–17445.
[9]
[10]
R. Atkin, V. S. J. Craig, and S. Bigg. In: Langmuir 16 (2000), pp. 9374–9380.
[11]
M. W. Rutland and J. L. Parker. In: Langmuir 10 (1994), pp. 1110–1121.
[12]
J. L. Parker, V. V. Yaminski, and P. M. Claeson. In: J. Phys. Chem. 97 (), pp. 7706–
7710.
[13]
3332.
[14]
[15]
1569.
[16]
P. Kekicheff and O. Spalla. In: Phys. Rev. Lett. 75 (1995), pp. 1851–1854.
[17]
V. V. Yaminski et al. In: Langmuir 12 (1996), pp. 1936–1943.
[18]
R. M. Pashley et al. In: Science 229 (1985), pp. 1088–1089.
[19]
[20] Zetasizer Nano Series User Manual. url: http://www.biophysics.bioc.cam.
ac.uk/files/Zetasizer_Nano_user_manual_Man0317-1.1.pdf.
[21]
Y. Zeng. “Structuring of colloidal dispersion in slit-pore confinement”. PhD thesis.
2011.
84
Bibliography
[22]
J. Nawrocki. In: J. of Chromatography A 779 (1997), pp. 29 –71.
[23]
L. A. C. Lüderitz and R. von Klitzing. In: Langmuir 28 (2012), pp. 3360–3368.
[24]
X. H. Zhang et al. In: Langmuir 23 (2007), pp. 1778–1783.
[25]
B. G. Sharma, S. Basu, and M. M. Sharma. In: Langmuir 12 (1996), pp. 6506–6512.
[26]
J. Zhang et al. In: Langmuir 21 (2005), pp. 5831–5841.
[27]
85
5. Scanning of Silicon Wafers in Contact
with Aqueous CTAB Solutions below
the CMC
5.1. Introduction
Surfactants can be used as stabilizers/emulsifiers (above the CMC) in the cosmetic industry. Below the CMC, surfactants find applications in flotation processes since they
can be adsorbed on a hydrophilic surface rendering the surface hydrophobic [1]. The
surfactant morphologies found at the surface are a function of the surface charge, the
type of head groups, and the hydrophobic part of the surfactant [2, 3].
Many studies have been performed to clarify the structure of surfactants at the surface
of mica or silica surfaces above the CMC. At the silica surface, micelles or flattened bilayers of CTAB close to the CMC have been reported [4]. Velegol et al. [5] described the
CTAB adsorbed layer at the silica surface in the presence of 0.9×CMC and 10×CMC
solutions. At 0.9×CMC surfactant concentration a coexistence of spheres and short rods
was observed at the silica surface whereas wormlike micelles were observed at 10×CMC
surfactant concentration. In some cases, a transition from the wormlike micelles to a
laterally homogeneous structure (interpreted as a bilayer) similar to that observed on
mica occurred. Ducker et al. [6] studied the adsorption of CTAB on mica at a concentration of 2×CMC. They obtained a flat sheet CTAB morphology in the absence of salt
at the mica surface. Sharma et al. [7] reported that the adsorption of CTAB on mica at
low concentration 10−5 M occurs patchwise. The distances between the patches was not
constant, and a coexistence between patches of different heights was also observed, which
was interpreted as surfactant molecules or aggregates. An increase of the concentration
produced more closely packed surfactant patches. At 10−3 M surfactant concentration, a
continuous wormlike admicellar structure with reduced separation compared to previous
concentrations was observed at the mica surface. A further increase of the concentration
to 10−2 M produced a continuous bilayer structure at the mica surface. They demonstrated that the variation of pH with the consequent variation in surface charge density
influences the structure of the adsorbed micelles. The surface charge can also be varied
by surface treatment [8]. Different substrates may also have different surface charges.
The p.z.c for the adsorption of CTAB depends on the surface used: at the silica surface,
5 × 10−5 M was obtained, whereas the neutralization of the mica surface occurred at a
lower concentration of 3.5 × 10−6 M [9]. Yaminski et al. [10] studied the adsorption of
86
5. Scanning of Silicon Wafers in Contact with Aqueous CTAB Solutions below the CMC
CTAB to a silica surface in the presence of sodium acetate using a surface force apparatus.
They reported a pronounced attraction between surfaces when the CTAB concentration
is increased to 5 × 10−5 M. The attraction is explained by so-called hydrophobic interactions. Hence, one mechanism to explain the hydrophobic interactions is through bridging
of nanobubbles, which are present at the opposing hydrophobic surfaces [11–15]. The
presence of nanobubbles corresponds to a reduced density of water, which was detected
at hydrophobic surfaces by neutron reflectometry [16].
It is known from the literature that the nanobubbles are stable for several days at hydrophobic interfaces and that they are present in solutions saturated with gas [17]. So
far, nanobubbles have been studied on surfaces hydrophobized by chemical pretreatment
(HOPG [18], OTS [18, 19]). The liquid phase was either water or surfactant solutions like
SDS or CTAB [18]. Recently, nanobubbles were imaged on ultraflat gold covered with
binary self-assembled monolayers (SAMs) with variable hydrophilic/hydrophobic balance
[20]. Still, nanobubbles were found at the surface of SAMs with a macroscopic contact
angle of 15◦ , but they were very tiny. Ducker [17] explains the stability of nanobubbles at a hydrophobic surface by surface active contaminants. Under this assumption,
the adsorption of surface active material to the nanobubble avoids the diffusion of gas
out of the nanobubbles. The surface active material will stabilize the bubbles through
creation of a diffusional barrier. The surface active contaminant may be adsorbed at
the solid–liquid interface with the corresponding decrease of the solid–liquid interfacial
tension. In that case, the liquid–vapor interfacial tension has to become extremely small
to fit the observed low nanobubble contact angle (θ ≈ 16◦ ), which leads to a flattening
of the nanobubbles [17]. Zhang et al. [18] studied the nanobubbles in the presence of
two different surfactants, hexadecyl trimethyl ammonium bromide (CTAB) and (SDS)
sodium dodecylsulfate at 0.5×CMC. Little or no variation in contact angle was observed
for the nanobubbles present at HOPG or OTS surfaces when surfactant was added to the
solution. This was later explained by the fact that the nanobubbles were already covered
with some kind of surface active material (contaminants) so that the effect of surfactant
on the nanobubbles was not seen [17]. A further proof for stabilization of nanobubbles by
contaminants was their decreased stability in surfactant solutions well above the CMC,
where the contamination is solubilized within the micelles. The bubbles on a graphite
surface disappeared after 15 minutes exposure to 5×CMC SDS solution.
A systematic investigation of the influence of the surfactant concentration on the stability
of nanobubbles at a solid–liquid interface is still missing. Another open question is the
hydrophilic/hydrophobic balance of silicon oxide surfaces at low CTAB concentration
(below 0.5 mM). Therefore, in the present paper, hydrophilic silicon oxide surfaces are
studied against aqueous solutions of a broad CTAB concentration regime (0.05–0.8 mM).
In this regime, the silicon oxide surface is partially hydrophobic and nanobubbles may
be present at the surface. Hence the surfactant fulfills two tasks: (1) modification of
the silicon oxide surface via physisorption and (2) the stabilization of the nanobubbles.
The nanobubbles are studied via Scanning Force Microscopy (SFM) and the results are
correlated with the interactions between a silica microsphere and a planar silicon oxide
surface against aqueous CTAB solutions in a Colloidal Probe AFM.
87
5.2.1. Materials
Solutions from cetyl trimethyl ammonium bromide (CTAB, analytic grade, Aldrich, purity > 99%) were prepared in a range of concentrations, 0.05 mM to 0.8 mM in pure
water (milli-Q). CTAB was used as received. Surface tension measurements were made
in the range of the studied concentration at 298 K with a tensiometer Krüss K11 using
the ring method. The CMC of CTAB obtained was 1 mM, no minima was detected
around the CMC, which indicates that no surface active contaminants were present in
the CTAB used. The value of surface tension at 1 mM CMC of 36.2 mN/m correlates
well with the literature value [21]. Nonporous silica particles, 4.63 µm in diameter, were
used for the force measurements.
Preparation
The silicon wafers (type-P Wacker Siltronic Burghausen) were prepared as described in
section 4.2.2.
Methods
The scanning was performed using a MFP-3D Asylum Research atomic force microscope
(AFM). The images of the spherical features in liquid were obtained in iDrive tapping
mode using iDrive compatible cantilevers from Asylum Research. These were gold coated
with a nominal spring constant of 0.09 N/m. iDrive is a patented technique that uses
Lorentz force to magnetically actuate a cantilever with an oscillating current that flows
through the legs [22]. This technique is recommended for imaging of extremely soft
matter in liquid. The setpoint was adjusted to minimize the force on the sample while
still tracking the surface. The tip curvature radius measured by SEM is Rtip = 15 nm ± 5
nm. A new experiment was performed for each concentration. Further, the images were
flattened with a first or second order polynomial fit. The cross section of the observed
spherical features was fitted to an arc of a circle, which allows the determination of
the bubble parameters (see references [19, 23, 24]). At least 10 spherical features were
analyzed (when possible) to obtain the parameters. The roughness (RMS) was obtained
from a 300×300 nm image.
The force measurements between a silica particle and an oxidized silicon wafer were
also performed with a MFP-3D Asylum Research AFM mounted on an inverted optical
microscope (Olympus IX71) (see section 4.2.2). The contact angles were measured with a
Goniometer (OCA5) using the Sessile drop method. The clean silicon wafers were placed
88
on the sample stage. A syringe was used to place a drop of the surfactant solution on
the silicon wafer surface. The software SCA 20 determined the contact angle using the
Young–Laplace fitting method [25].
5.2.3. Simulations
The DLVO theory is used to simulate the experimental curves (see section 3.2.3 for
further details).
5.3. Results
Scanning of a Silicon Wafer from 0.05 to 1 mM CTAB
Figure 5.1 represents the surface of a silicon oxide surface in milli-Q water. A homogeneous surface is observed in water with a surface roughness of 0.43 nm. The contact
angle of the cleaned silicon wafer is close to 0◦ .
300
250
2.0
200
150
1.0
nm
nm
1.5
100
0.5
50
0.0
0
0
100
200
300
nm
Figure 5.1.: AFM tapping mode of a silicon oxide surface in water. AFM images taken
with a magnetic actuated cantilever; nominal spring constant: 0.09 N/m,
amplitude setpoint: 0.265 V, setpoint ratio: 0.26, scan rate: 0.5 Hz, drive
frequency: 6.91 KHz
89
When the concentration of surfactant is increased to 0.05 mM spherical features (figure
5.2), which resemble nanobubbles, are observed at the surface. At this concentration
the features diameter is 50–80 nm. In the phase image, the spherical features are also
recognized since a phase drop is observed along the features indicating the different nature
of features and substrate. At 0.3 mM the features are still spherical, and the diameter is
around 57 nm (figure 5.3). The spherical features dominate the surface topology. From
this concentration onwards a slight phase shift between substrate and feature is observed
in the phase image. At 0.4 mM surfactant concentration, a few spherical features could
still be found at the surface (see figure 5.4).
Since the observed features resemble a spherical cap, their parameters can be obtained
by fitting a cross section to an arc of a circle (see table 5.1).
Figures 5.5 and 5.6 represent the adsorbed surfactant layer at concentrations 0.5 and
0.8 mM. For 0.5 mM other types of features were found at the surface. They are flatter
(height about 2 nm) than the features at lower CTAB concentration. At 0.8 mM, the
roughness decreases to 0.27 ± 0.03 nm, which is lower than the original silicon wafer
roughness (0.43 nm), and no features could be identified.
90
300
250
15
150
10
100
5
nm
nm
200
0
50
-5
0
0
100
200
300
nm
Height
10nm
5
0
-5
0
50
100
150
nm
200
250
300
0
50
100
150
nm
200
250
300
15°
Phase
10
5
0
-5
Figure 5.2.: AFM tapping mode of a silicon oxide surface at 0.05 mM CTAB concentration. AFM images taken with a magnetic actuated cantilever; nominal spring
constant: 0.09 N/m, amplitude setpoint: 0.430 V, setpoint ratio: 0.43, scan
rate: 0.5 Hz, drive frequency: 6.34 KHz
91
300
250
8
200
4
100
nm
nm
6
150
2
0
50
-2
0
0
100
200
300
nm
Height
6nm
4
2
0
-2
0
50
100
150
nm
200
250
300
0
50
100
150
nm
200
250
300
Phase
2°
0
-2
92
300
200
4
150
3
2
100
nm
nm
250
1
50
0
0
0
100
200
300
Phase
Height
nm
6nm
4
2
0
-2
0
50
100
150
nm
200
250
300
0
50
100
150
nm
200
250
300
2
1
0
-1
-2
-3°
93
300
250
400
200
0
100
-200
50
-400
pm
nm
200
150
0
0
100
200
300
nm
Height
800pm
600
400
200
0
-200
0
50
100
150
nm
200
250
300
94
300
250
600
400
200
150
0
100
pm
nm
200
-200
-400
50
-600
0
0
100
200
300
nm
Height
200
0
-200pm
0
50
100
150
nm
200
250
300
5.4. Discussion
5.4.1. Nanobubbles
No features are observed at the surface of a cleaned hydrophilic silicon wafer. That correlates with the findings of reference [26], where no spontaneous formation of bubbles
was observed on flat hydrophilic silicon wafers. The surface is smooth and hydrophilic
with a roughness of 0.43 nm determined in water by iDrive tapping mode. The roughness correlates well with the values obtained in the literature [26]. In the presence of
0.05-0.4 mM CTAB, some features appear at the silicon wafer surface, which resemble
nanobubbles. They are spherical, at 0.5 mM flatter (micropancakes), and they vanish
close to CMC (0.8×CMC). Similar morphologies have been observed by other authors
[19, 20, 27–31]. From the phase image one can conclude that the features are deformable
at least at a concentration of 0.05 mM. The features are not present when a hydrophilic
silicon wafer is imaged in water. The features with heights between 8 and 15 nm correlate well with that reported by other authors [32]. At low concentration of 0.05 mM,
the feature height is around 15 nm. The observed features cannot be micelles because
95
micelles would have a diameter of 3.4 nm [5]. The properties of the spherical features
suggest that they are nanobubbles on the surface of a partially hydrophobic silicon wafer
(see figure 5.7). The presence of nanobubbles at the surface in figure 5.2 is an evidence
that a partial hydrophobization of the silicon wafer surface occurs due to the adsorption
of surfactant. The nucleation of the nanobubbles may be produced by air dissolved in
water [17]. The nanobubbles have a regular shape and are stabilized by the surfactant
in the solution. It cannot be excluded that surface active contaminants may also be
present at the bubble interface [17, 33]. The contaminants in the reference [17] are due
to the surface preparation (silanization process). Since in the present experiments the
partial hydrophobization occurs in situ on a clean hydrophilic silicon wafer, only a tiny
amount of contaminants should be present, if any. Another source of contaminant could
be the tip itself [34]. Borkent et al. [34] proposed that contaminants coming from the
gel package and deposited on the tip can precipitate on the organic surface once the tip
is immersed in water for scanning. The polysiloxanes (from the gel package) could be
adsorbed at the air–liquid interface (it is energetically unfavourable that polysiloxanes
deposit on hydrophilic surfaces immersed in liquid [20]) and might be the reason for
the discrepancy observed between the nanoscopic and macroscopic contact angles [34].
Song et al. [20] did not find experimental evidence that oligomeric siloxanes from the
gel package influence the contact angle. Therefore, this source of contaminants can also
be excluded from our experiments. The nanobubbles seem to be flattened at the surface
(see table 5.1). We propose that the nanobubbles are seated at the hydrophobic tail of
the CTAB (see figure 5.7) and that they label the hydrophobic domains. The size and
distribution of the aggregates in the presence of nanobubbles may differ from the size
and distribution of the aggregates in degassed solution since the bubbles could modify
the distribution of surfactant.
Water
Nanobubble
Cationic surfactant
molecule
Silicon Wafer
Figure 5.7.: Schematic picture of nanobubbles (not to scale) seated on the hydrophobic
tails of the surfactant molecules
The bubble parameters were obtained by fitting a cross section of the nanobubble to
an arc of a circle [19, 23, 24]. At 0.05 mM surfactant concentration, the height of
the bubbles is 13 ± 2.2 nm and the radius is 37.7 ± 4.6 nm. With increasing surfactant
concentration, the bubbles become smaller. At a surfactant concentration of 0.3 mM, the
bubble height is 7.6 ± 1 nm and the radius 28 ± 4.3 nm (see figure 5.3). At 0.4 mM, the
bubbles are smaller, which introduces more error in the analysis. At this concentration,
nanobubbles with heights of 5–8 nm were found with a radius between 20 and 33 nm.
96
The nanoscopic contact angle remains constant between 140◦ and 150◦ irrespective of
the CTAB concentration (see table 5.1) and is in good agreement with the literature [18,
34, 35].
Surfactant
Avg.
Avg. bubble
Avg. radius of
Contact
Macroscopic
Avg. nanoscopic
concentration
height
radius
curvature
angle
contact angle
contact angle
I (mM)
h (nm)
r (nm)
Rc (nm)
θair (degrees)
θwater (degrees)
θwater (degrees)
0.05
0.3
0.4
13.1
7.5
5.2
37.7
28.0
20.4
58.6
58.0
46.5
38.4
30.4
29.0
48.4
41.0
42.4
141.6
149.6
151.0
Table 5.1.: Parameters of the nanobubbles obtained by fitting the cross section to an arc
of a circle. The small size of the nanobubbles at 0.4 mM CTAB concentration
introduces more errors in the fitting and in the obtained parameters, but still
the parameters of the nanobubbles are shown for comparison.
Surfactant
Avg.
Avg. bubble
Avg. radius of
Contact
Avg. nanoscopic
Nanoscopic
concentration
height
radius
curvature
angle
contact angle
∆θwater (d)
I (mM)
h (nm)
rd (nm)
Rcd (nm)
θair (d) (degrees)
θwater (d) (degrees)
(degrees)
0.05
0.3
0.4
13.1
7.5
5.2
31.0
24.3
17.0
43.6
43.0
31.5
45.4
34.3
32.8
134.6
145.6
147.0
3.0
3.3
5.1
Table 5.2.: Parameters of the nanobubbles obtained after tip deconvolution, Rtip =15
nm±5 nm
Zhang et al. [19] also obtained no variation in contact angle in spite of a high polydispersity. This is in contrast to the work of Yang et al. [36] where a variation of the nanoscopic
contact angle of the nanobubbles was observed after adding 2-butanol surfactant. They
reported a slight decrease in the height of the nanobubbles and a more pronounced decrease in width after adding surfactant, which led to a decrease of the nanoscopic contact
angle. The diameters of the nanobubbles in this work are smaller than those reported
by other authors (see [18, 19]), because they are associated to the hydrophobic domains
at the surface. They are stable at least for 30 minutes observation time. Yang et al.
[36] reported small bubbles on a hydrophobic surface with a diameter of 100 nm whereas
Kameda et al. [37] reported nanobubbles with a diameter between 10 and 100 nm on a
Au(111) surface.
Since the size of the bubbles is in the same order of magnitude as the size of the tip,
it is necessary to perform tip deconvolution. The end of the tip is spherical with a
curvature radius of Rtip = 15 nm ± 5 nm obtained by scanning electron microscopy. The
deconvoluted radius of the curvature of the nanobubbles can be calculated as follows
[20]:
97
Rcd = Rc − Rtip
(5.1)
(Rcd : deconvoluted curvature radius of the nanobubble, Rc : convoluted curvature radius
of the nanobubble, Rtip : tip curvature radius). The error propagation was stated (see
table 5.2) taking into account the error in fitting the nanobubble cross section to an arc
of a circle and the error due to the uncertainty in the determination of the tip radius.
Note that the nanobubble parameters vary slightly after tip deconvolution and that the
nanoscopic contact angle still remains constant within the experimental errors (see table
5.2).
The Laplace equation predicts, that small bubbles will have a large internal pressure. The
pressure inside the bubbles is about 30 times the atmospheric pressure for the studied
concentrations. Song et al [20] obtained similar Laplace pressures for the nanobubbles.
In comparison, the internal pressure of the nanobubbles in water with a diameter of
about 1 µm is in the range of 1 to 1.7 atm [19].
According to the Laplace equation
∆P =
2σvl
Rc
(5.2)
(∆P :Laplace pressure, σvl :interfacial tension at liquid-vapor interface, Rc :curvature radius), the increase in surfactant concentration CTAB and the related decrease in surface
tension leads to a decrease in bubble radius at constant ∆P (see figures 5.2, 5.3, 5.4). The
nanoscopic contact angle througth water is much larger than the macroscopic one (see
table 5.1). Song et al. [20] reported a correlation within the experimental errors of the
macroscopic and nanoscopic contact angle of nanobubbles found on a hydrophilic sample
(binary self-assembly monolayers; SAMs, θmacro = 37◦ ). In our experiments, only the
local hydrophobicity of the (hydrophobic) domains/surfactant aggregates is taken into
account for the determination of the nanoscopic contact angle. In contrast, different areas including hydrophobic domains and hydrophilic areas contribute to the macroscopic
contact angle. A drop in phase angle is detected at the position of the nanobubbles at
low surfactant concentration (see figure 5.2). At higher concentrations, the phase shift
was small or not detected (see figures 5.3, 5.4, 5.5). Simonsen et al. [38] argued that
the phase change is due to the deformation of the bubbles when the tip approaches the
surface. The lack in phase shift for higher surfactant concentrations is due to the fact
that the bubbles become stiffer and that their viscoelasticity approaches the one of the
substrate.
5.4.2. Correlation with Force Curves
In order to correlate the appearance of nanobubbles with interactions between hydrophilic
surfaces in the presence of CTAB, force curves between a silica particle and a silicon wafer
98
were recorded. As shown in figure 5.8, they cannot be fitted with the DLVO theory at
distances smaller than 20 nm.
0.6
0.4mMC16TAB, ö=+50 mV, ë=15.2 nm
0.5mMC16TAB, ö=+70 mV, ë=13.6 nm
DLVO_CC
DLVO_CP
DLVO_CC (plane of charge taken 8 nm from the surface)
DLVO_CP (plane of charge taken 8 nm from the surface)
0.5
0.4
F/2
]mINm[ Rð
0.3
0.2
0.1
0.0
0
10
20
30
40
Separation, D [nm]
50
60
70
Figure 5.8.: Force curves between a silica particle and a silicon oxide surface in the presence of 0.4 and 0.5 mM CTAB concentration
The obtained Debye length coincides well with the theoretical one, which indicates that
almost all surfactant molecules in solution are dissociated. From force measurements we
obtained, that for concentrations below 0.4 mM only attraction is observed during the
whole range (data not shown). At 0.4 mM surfactant concentration, a slight repulsion
is observed at long range, which indicates a weak charge reversal (see figure 5.8). At
this concentration, the nanobubbles have a height varying from 5 to 8 nm. The jumpin contact takes place at a distance of about 18 nm. Under the assumption that both
opposing surfaces are decorated with nanobubbles, locally a foam lamella may be formed
between 2 opposing nanobubbles (5 nm height), which ruptures at a distance of about
8 nm between the outer surface. This might lead to the observed attraction. Similar
arguments are published in reference [39] for the rupture of aqueous wetting films on a
hydrophobic surface [40].
When increasing the concentration to 0.5 mM, a strong repulsion is detected up to a
distance of 10 nm. Then a jump-in contact is observed. The force curve can be fitted via
Gouy–Chapman theory for distances larger than 20 nm. The strong repulsion between 10
and 20 nm cannot be fitted by DLVO theory, which might be an indication for steric forces
99
0.6
1mMC16TAB, ë= 9.6 nm
0.5
DLVO_CC
DLVO_CP
0.4
F/2
]mINm[ Rð
0.3
0.2
0.1
0.0
0
20
40
Separation, D [nm]
60
Figure 5.9.: Force curves between a silica particle and a silicon oxide surface in the presence of 1mM CTAB concentration
due to the formation of micelles or (patchy) bilayers at the silica/silicon oxide surface,
respectively. Velegol et al. [5] found a jump-in contact at 5 nm for the interaction
between a tip and a silica surface in the presence of CTAB solution at 0.9×CMC. In the
present study, the jump-in contact occurs at 10 nm distance since the interactions are
between a silica particle and a silicon oxide surface. The size of micelles in solution is
about 5 nm and, at the surface, 3.4 nm [5]. In both studies, the jump-in contact is due
to the expulsion of micelles or bilayer patches between the surfaces. In the height image,
only micropancakes were detected at this concentration. Micropancakes (thin gas layers)
and micelles or patchy bilayers may coexist at this concentration. The obtained Debye
lengths at 0.4 mM and 0.5 mM are 15.2 nm and 13.6 nm respectively, which correlates
well with the theoretical values.
At 1 mM surfactant concentration, a monotonous repulsive interaction between a silica
particle and a silicon wafer is obtained (see figure 5.9). According to the present results,
the repulsion is assumed to be caused by the approach of two opposing micelles or bilayers
adsorbed on the silicon oxide/silica surfaces. The micelles/patchy bilayers are then so
closely packed and stiff, that they cannot be pressed out from the surfaces. No jump-in
contact is observed, and the 0 distance refers to the point of contact between opposing
micelles/bilayers. No bubbles were detected by SFM at this concentration since the
100
silicon wafer surface is already hydrophilic.
A decrease in height and radius of the bubble in the presence of surfactant with decreasing amplitude setpoint was reported in reference [18]. Interestingly, this effect was not
seen in the absence of surfactant. Therefore, in the present study, a similar amplitude
setpoint was used to compare the variation of height of the nanobubbles with surfactant
concentration. We concluded that the flattening of the nanobubbles at 0.5 mM is due to
the decrease in surface tension.
5.5. Conclusions
We reported the presence of small nanobubbles at the surface of a silicon oxide surface
exposed to aqueous CTAB solutions. The effect of the surfactant is twofold, it can
partially hydrophobize the silicon wafer surface and stabilize the nanobubbles. The
hydrophobic surfactant patches present at the silicon oxide surface at low concentration
(below 0.5 mM) are labeled by nanobubbles, which are imaged. The diameter of the
nanobubbles varies from 30 to 60 nm (after tip deconvolution). The nanoscopic contact
angle through water remains constant between 140◦ and 150◦ and is independent of the
CTAB concentration. This angle verifies the hydrophobicity of the domains formed by the
surfactant aggregates. It is much higher than the macroscopic contact angle of a CTAB
solution droplet (about 40◦ ), which presents an average of hydrophilic and hydrophobic
areas on a silicon wafer partially covered with CTAB. The Laplace pressure within the
nanobubbles is about 30 atm.
With increasing CTAB concentration, the nanobubbles become smaller and less prominent. This indicates that the silicon wafer surface becomes more hydrophilic. At low
CTAB concentration (0.05–0.4 mM) the surface is partially covered with hydrophobic
domains where the nanobubbles can be placed. Since nanobubbles were only observed at
low surfactant concentration (below 0.5 mM), they may play a role in the hydrophobic
interactions. A strong attraction (jump-in) is observed at short distances (≤ 20 nm)
which is explained by the rupture of the lamella between opposing nanobubbles seated
on the hydrophobic domains. With increasing concentration, more and more hydrophilic
domains, i.e. micelles or patchy bilayers, are formed until there are no bubbles observed
close to the CMC (1 mM). At 0.5 mM long range electrostatic forces as well as steric repulsion followed by micelle/bilayer expulsion are observed in the interaction curve between
the two opposing silica/silicon oxide surfaces. In the height image, only micropancakes
were detected. At 1 mM, the exerted force was not high enough to expulse the adsorbed
aggregates from the surface.
101
Bibliography
[1]
D. W. Fuerstenau and R. Herrera-Urbina. Cationic Surfactant: Physical Chemistry.
Ed. by D. N. Rubingh and P. M. Holland. Vol. 37. Marcel Dekker: New York, 1991,
p. 408.
[2]
V. Subramanian and W. A. Ducker. In: Langmuir 16 (2000), pp. 4447–4454.
[3]
S. C. Biswas and D. K. Chattoraj. In: J. Colloid Interface Sci. 205 (1998), pp. 12–
20.
[4]
M. W. Rutland and J. L. Parker. In: Langmuir 10 (1994), pp. 1110–1121.
[5]
[6]
W. A. Ducker and E. J. Wanless. In: Langmuir 15 (1999), pp. 160–168.
[7]
B. G. Sharma, S. Basu, and M. M. Sharma. In: Langmuir 12 (1996), pp. 6506–6512.
[8]
R. Atkin et al. In: Adv. Colloid Interface Sci. 103 (2003), pp. 219–304.
[9]
7710.
[10]
V. Yaminski et al. In: Langmuir 12 (1996), pp. 3531–3535.
[11]
176104–4.
[12]
L. A. Palmer, D. Cookson, and R. N. Lamb. In: Langmuir 27 (2011), pp. 144–147.
[13]
[14]
Y. Takata et al. In: Langmuir 22 (2006), pp. 1715–1721.
[15]
J. W. G. Tyrrell and P. Attard. In: Langmuir 18 (2002), pp. 160–167.
[16]
D. A. Doshi et al. In: Proc. Natl. Acad. Sci. U.S.A. 102 (2005), pp. 9458–9462.
[17]
[18]
X. H. Zhang, N. Maeda, and V. S. J. Craig. In: Langmuir 22 (2006), pp. 5025–5035.
[19]
X. H. Zhang, A. Quinn, and W. A. Ducker. In: Langmuir 24 (2008), pp. 4756–4764.
[20]
B. Song, V. Walczyk, and H. Schönherr. In: Langmuir 27 (2011), pp. 8223–8232.
[21]
J. Mata, D. Varade, and P. Bahadur. In: Thermochim. Acta 428 (2005), pp. 147–
155.
[22]
S. Hohlbauch, H. Cavazos, and R. Proksch. iDrive: Theory, Applications, Installation and Operation. Asylum Research. Santa Barbara, CA, 2008.
102
Bibliography
[23]
Y. Wang, B. Bhushan, and X. Zhao. In: Nanotechnology 20 (2009), p. 045301.
[24]
L. Zhang et al. In: Soft Matter 6 (2010), pp. 4515–4519.
[25] SCA 20, Version 2.04. DataPhysics Instruments GmBH. 2002.
[26]
A. Agrawal and G. H. McKinley. In: Mater. Res. Soc. Symp. Proc. 899E (2006),
NO7–37.1.
[27]
B. M. Borkent et al. In: Phys. Rev. Lett. 98 (2007), p. 204502.
[28]
B. M. Borkent et al. In: Phys. Rev. E 80 (2009), p. 036315.
[29]
M. Holmberg et al. In: Langmuir 19 (2003), pp. 10510–10513.
[30]
X. H. Zhang, A. Khan, and W. A. Ducker. In: Phys. Rev. Lett. 98 (2007), p. 136101.
[31]
J. R. T. Seddon and D. Lohse. In: J. Phys. Condens. Matter. 23 (2011), p. 133001.
[32]
R. Steitz et al. In: Langmuir 19 (2003), pp. 2409–2418.
[33]
D. R. Evans, V. S. J. Craig, and T. J. Senden. In: Phys. A 339 (2004), pp. 101–105.
[34]
B. M. Borkent et al. In: Langmuir 26 (2010), pp. 260–268.
[35]
H. Elnaiem et al. In: Comput. Mat. Continua 14 (2009), pp. 23–34.
[36]
S. Yang et al. In: Langmuir 23 (2007), pp. 7072–7077.
[37]
N. Kameda and S. Nakabayashi. In: Chem. Phys. Lett. 461 (2008), pp. 122–126.
[38]
A. C. Simonsen, P. L. Hansen, and B. Klösgen. In: J. Colloid Interface Sci. 273
(2004), pp. 291–299.
[39]
K. W. Stöckkelhuber et al. In: Langmuir 20 (2004), pp. 164–168.
[40]
R. v. Klitzing. In: Adv. Colloid Interface Sci. 114-115 (2005), pp. 253–266.
103
6. Nanobubbles at the Surface of a
Divinyl Disilazan modified Silicon
Wafer
6.1. Introduction
Hydrophobic interactions are attractive strong interactions between hydrophobic objects
or nonpolar molecules. These interactions play an important role in biology, since they
determine the conformation of proteins and the structure of the biological membrane [1].
Several theories have been proposed to explain hydrophobic interactions. One theory
explains that hydrophobic surfaces induce the formation of hydrogen bonds in the water
molecules closer to the surface, this effect propagates at larger distances and the disruption of the order produces a long range attractive force between the surfaces [2]. Another
theory takes the electrostatic fluctuations at the hydrophobic surface as an explanation
for the long range attraction observed between neutral bodies [3, 4]. Yaminski et al. [5]
argued that cavitation can explain the long range attractive forces between hydrophobic
surfaces. Considine et al. [6] measured the interaction forces between two latex particles
in an electrolyte solution. A big attraction was observed which varies from 20-400 nm,
depending on the pair of spheres used. The proposed model to explain the attraction
was based on the presence of gas bubbles attached to the particle surface. The bridging
mechanism between bubbles of opposing surfaces results in an attractive force. They
obtained a variation of the attractive force when the water was degassed. Carambassis
et al. [7] also concluded that nanobubbles are responsible for the long range attractions
observed between a silica and an oxidized silicon wafer hydrophobically modified with
Tridecafluoro 1,1,2,2-tetrahydrooctyl-methyldichlorosilane. The range of the attraction
from 50-100 nm was a measure of the size of the bubble. Craig et al. [8] studied the
hydrophobic interactions between silica surfaces immersed in solutions of CTAB and
CPC in the presence of 0.1 M NaCl and observed a decrease in the attractive force after
removing the dissolved gas. Nonetheless the attraction was still present. They also obtained a larger attractive force for surfaces immersed in CPC solutions than for surfaces
immersed in CTAB solutions. The magnitude and the range of the hydrophobic force
depends on the material conforming the hydrophobic surface and the method of preparation [9]. Kokkoli et al. [9] cited that the range of the force between surfaces prepared
from Langmuir-Blodgett deposition and silanization processes is about 250 nm, whereas
hydrophobic surfaces formed from equilibrium adsorption have short range hydrophobic
104
interactions of about 10 nm. The hydrophobic interactions between polymer surfaces are
measurable at distances of about 30 nm [9].
Divinyldisilazan is a silane with application in the rubber industry. This silane renders the
surface hydrophobic with a contact angle close to 90◦ . It is proven that nanobubbles are
present at the interface of water and hydrophobic materials [10–12]. The contact angle of
the nanobubbles varies depending on the composition of the solid surface [13]. This study
was outlined to characterize the nanobubbles present at the interface of a divinyldisilazan
modified silicon wafer immersed in water and surfactant solutions, and to study the
influence of the surfactant on the nanoscopic contact angle of the observed nanobubbles.
The interaction forces between two hydrophobically (divinyldisilazan) modified particles
in the presence of surfactant solutions are also analyzed.
6.2.1. Materials
A powder of silica particles 4.74 µm mean diameter was purchased from Bangs Laboratories and 1,1,3,3-tetramethyl-1,3-divinyldisilazan analytic grade was purchased from
Aldrich. Solutions from cetyl-trimethyl-ammonium bromide (CTAB, analytic grade,
Aldrich) were prepared in a range of concentration from 0.005 mM to 1.2 mM in pure water (milli-Q). The surface tension measurements were performed as described in section
5.2.1.
Preparation
The colloidal silica particles were exposed to a vapor of 1,1,3,3-tetramethyl-1,3-divinyldisilazan under vacuum conditions. A small beaker containing 0.5 g of silica particles was
placed in the reaction flask. The system was heated to 120◦ C under vacuum (10−2 T orr)
for two hours to remove the adsorbed water. Thereafter the silane was added dropwise
to the reaction flask. The system was left at room temperature and the particles were
exposed to the silane vapor for 24 hours. The excess silane was removed at 120◦ C under
vacuum (10−2 T orr). Finally hydrophobically modified silica particles were obtained
[14]. The hydrophobic particle was glued to the end of a tipless AFM cantilever (CSC12,
µ-mach, Lithonia) and another one to a glass slide (Menzel-Gläser, Germany) using
an optical microscope and a micromanipulator. Colloidal probes and glass slides with
attached particles were cleaned with ethanol and water and placed in an air plasma
cleaner for 20 minutes (Diener electronic. Femto timer). The silicon wafers (type-P)
Wacker Siltronic Burghausen were cut and cleaned in piranha solution for 30 minutes,
105
thereafter washed with milli-Q water and then modified by the same procedure as the
silica particles.
Methods
MFP-3D Asylum Research atomic force microscope mounted in an inverted optical microscope (Olympus IX71) as described in section 3.2.2. The scanning was performed
using the same apparatus (for further details see section 5.2.2). The contact angle of the
modified silicon wafer was measured with a Goniometer (OCA5) using the Sessile drop
method as described in section 5.2.2.
6.2.3. Simulations
The simulations are based on the DLVO theory. The description is found in section
3.2.3.
6.3. Results
Effect of Surfactant
The silicon wafers were modified using the same method as for silica particle modification.
The macroscopic contact angle of water on a modified silicon wafer is 89◦ . The modified
silicon wafer was exposed to the different surfactant solutions. Figure 6.1 shows the
image of the silicon wafer in water. The surface is covered with nanobubbles of different
sizes with an average height of 27 nm. The nanobubbles are stable and have an irregular
form. The obtained roughness is about 9.492 nm. The fit of the cross section of the
bubble to an arc of a circle [16–18] allows the determination of the bubble parameters.
The radius of curvature Rc was 96 nm and the nanoscopic contact angle was about 136◦
on average.
The morphology and distribution of the nanobubbles in the presence of 0.1 mM surfactant
concentration are represented in figure 6.2. Different bubble sizes are present at the
surface from 100 to 300 nm in diameter. It is not possible to obtain the nanobubble
parameters with these experimental conditions, because the bubbles are flattened at the
modified silicon wafer. The big nanobubbles are still recognized in the phase image.
At 0.3 surfactant concentration, nanobubbles were still observed at the modified silicon
wafer (see figure 6.3). The average height is between 4 and 8 nm and they are flattened
at the surface. An increase of the concentration to 0.4 mM (see figure 6.4) does cause the
nanobubbles at the surface to disappear. There are also other features which resemble
micropancakes [19]. Nanobubbles will be present at the surface of the modified silicon
106
1.0
0.8
5
0.4
0
nm
µm
0.6
-5
0.2
0.0
0.0
0.2
0.4
0.6
0.8
1.0
µm
Height
20nm
10
0
-10
0.0
0.2
0.4
0.6
0.8
1.0
0.6
0.8
1.0
Phase
µm
20°
10
0
-10
-20
0.0
0.2
0.4
µm
Figure 6.1.: AFM tapping mode of a hydrophobically modified silicon wafer in water.
AFM images taken with a magnetic actuated cantilever, nominal spring constant 0.09 N/m, amplitude setpoint: 0.292 V, setpoint ratio: 0.29, scan rate:
0.5 Hz, drive frequency: 6.27 KHz
wafer until 1.2 mM (see figures 6.5, 6.6). It seems that the nanobubbles are deformed at
1.2 mM surfactant concentration.
Interaction forces between two hydrophobically modified silica particles were also performed in order to get information about the range and magnitude of the hydrophobic
forces. Figure 6.8 shows the interaction between two hydrophobic particles in water
and at different surfactant concentrations. The diameter of the modified particles was
obtained by SEM and is around 4.7 µm. The pH of the solutions is around 5.8. The
experimental curves were fitted to the DLVO theory. It may be that nanobubbles are
also present at the silica surfaces. Hence the fitted diffuse layer potential are not representative of the surface potential. The Debye length is a bulk parameter and can be used
to obtain the experimental ionic strength. The interactions in the presence of water are
attractive during the whole range with a jump into contact around 40 nm. An increase
of the concentration to 0.03 mM gives rise to a slight long range repulsion with an attractive interaction at short range, which cannot be explained by van der Waals forces. The
Debye length is 43 nm, which correlates well with the Debye length in water. A further
increase in the concentration to 0.3 mM does not increase the repulsion, and attractive
interactions are still observed at short range, but they are smaller than at lower surfactant concentration. The jump into contact is around 13 nm. The obtained Debye length
at this concentration is 17.6 nm. At 0.4 mM still a small attraction is detectable at short
range, but at long range strong repulsive forces dominate the interaction. For higher
107
1.0
0.8
5
0.4
0
nm
µm
0.6
-5
0.2
0.0
0.0
0.2
0.4
0.6
0.8
1.0
Height
µm
20nm
15
10
5
0
-5
0.0
0.2
0.4
0.6
0.8
1.0
0.6
0.8
1.0
Phase
µm
100°
96
92
0.0
0.2
0.4
µm
Figure 6.2.: AFM tapping mode of a hydrophobically modified silicon wafer in 0.1 mM
ratio: 0.24, scan rate: 0.5 Hz, drive frequency: 5.1 KHz
300
250
200
0
nm
nm
5
150
100
-5
50
0
0
100
200
300
Phase
Height
nm
6nm
4
2
0
-2
0
50
100
150
nm
200
250
300
0
50
100
150
nm
200
250
300
8°
4
0
-4
108
300
250
2
1
150
nm
nm
200
0
100
-1
50
-2
0
0
100
200
300
nm
Phase
Height
8nm
4
0
0
50
100
150
nm
200
250
300
0
50
100
150
nm
200
250
300
4
0
-4
-8°
300
250
2
1
150
0
nm
nm
200
100
-1
50
-2
0
0
100
200
300
Phase
Height
nm
6nm
4
2
0
-2
-4
0
50
100
150
nm
200
250
300
0
50
100
150
nm
200
250
300
10°
5
0
-5
-10
109
300
250
1.5
1.0
0.5
150
0.0
100
nm
nm
200
-0.5
-1.0
50
-1.5
0
0
100
200
300
nm
Height
4nm
2
0
Phase
-2
0
50
100
150
nm
200
250
300
0
50
100
150
nm
200
250
300
5
0
-5
-10°
300
250
800
400
150
0
pm
nm
200
100
-400
50
-800
0
0
100
200
300
Phase
Height
nm
3nm
2
1
0
-1
0
50
100
150
nm
200
250
300
0
50
100
150
nm
200
250
300
2°
0
-2
110
0.4
water
0.03 mM
0.3
F/2
0.2
0.1
0.0
-0.1
0
20
40
60
Separation, D [nm]
80
100
Figure 6.8.: Forces between a pair of hydrophobically modified silica particles in water
and from 0.03 to 1.2 mM surfactant concentration at pH=5.8, Hamaker constant A= 8.5 × 10−21 J. The continuous lines correspond to constant charge.
DLVO_CC (constant charge) fits are shown for 0.03 mM, 0.3 mM, 0.4 mM
and 1 mM surfactant concentration
concentrations repulsion is observed during the whole range and no adhesion is seen in
the retraction curve. The experimental Debye lengths agree well with the theoretical
ones.
6.4. Discussion
Effect of the Surfactant Concentration on the Nanobubble Properties
The image of the hydrophobically modified silicon wafer in water is shown in figure 6.1.
The surface is covered with nanobubbles of different sizes. The silanation was performed
under vacuum (see section 6.2.2). Therefore, it is assumed that air was not trapped at
111
the surface before the AFM experiments and that nanobubbles formed spontaneously
once the surface was exposed to water or aqueous solutions. It has been proven by Lou
et al. [20] that nanobubbles can be induced on a hydrophobic surface after immersing the
surface in water. Ducker et al. [21] argued that the stability of nanobubbles is due to the
presence of contaminants at the vapor-liquid interface. That may be possible because
an organic compound (divinyl disilazan) was used in the present study to modify the
silica and the silicon wafer. However, an effect of the surfactant concentration on the
bubble morphology is observed; the bubbles become flatter with increasing surfactant
concentration, which means that surfactant is adsorbed at the vapor-liquid interface.
The morphology of the bubbles at a divinyl disilazan interface exposed to water resembles
a spherical cap. The parameters of the bubbles are obtained by fitting the cross section
to an arc of a circle [16–18]. The average curvature radius Rc and the nanoscopic contact
angle θwater are 96 nm and 136◦ respectively without tip deconvolution. Since the bubbles
and the tip radius are in nanometer scale, tip deconvolution is needed. The end of the
tip is spherical with a curvatures radius of Rtip = 15 nm ±5 nm obtained by scanning
electron microscopy. The deconvoluted radius of curvature of the nanobubbles can be
calculated as follows [22]:
Rcd = Rc − Rtip
(6.1)
(Rcd : deconvoluted curvature radius of the nanobubble, Rc : convoluted curvature radius
of the nanobubble, Rtip : tip curvature radius). The deconvoluted curvatures radius is 81
nm and from there the deconvoluted nanoscopic contact angle can be calculated (132◦ ±
13◦ ). The average nanoscopic contact angle of 132◦ is larger than the macroscopic one
of 89◦ . According to the Laplace equation
∆P =
2σvl
Rc
(6.2)
(∆P : Laplace pressure, σvl : interfacial tension at liquid-vapor interface, Rc : curvature
radius), taken σvl = 72 mN/m for water, the internal pressure of the nanobubbles in water
is about 18 atm. Zhang et al. [16] reported an internal pressure of the nanobubbles in
water with a diameter of about 1 µm in the range of 1 to 1.7 atm. At 0.01 mM surfactant
concentration the surface is still covered with the same amount of nanobubbles and these
nanobubbles still have a spherical cap morphology (data not shown). No interaction
between tip and sample is seen at long range, but at short range the interactions are
attractive (see figure 6.9) with a jump into contact around 10 nm. The origin of these
attractive forces may be due to van der Waals forces or due to the penetration of the tip
into the nanobubbles. The jump into contact correlates well with the observed height of
the nanobubbles.
The increase in the concentration to 0.3 mM does not reduce the amount of bubbles at
the surface (see figure 6.3), but a decrease in height is detected in comparison with the
112
3nm
2
Defl
1
0
-1
-2
-3
0
10
20
30
40nm
Sep
Figure 6.9.: Interaction force between a tip and a bubble in 0.01 mM CTAB concentration
bubbles on the modified silicon wafer without any surfactant. It seems that the surfactant
is adsorbed at the gas–liquid interface decreasing the Laplace pressure, which leads to
a flattening of the nanobubbles. The bubbles are not so well defined (more flattened),
therefore it is difficult to determine the nanoscopic contact angle. Still nanobubbles
are present at 0.4 mM surfactant concentration, but other kinds of features are also
visualized. The features resemble flat air layers (micropancakes). A similar morphology
has also been observed by other authors [19, 23]. At 0.8 mM nanobubbles were still found
at the surface. For higher concentrations flattened circular domains are present at the
surface with a typical hight of 3-4 nm. They may also be interpreted as aggregates at the
modified silicon wafer surface (see figure 6.7). The 300 × 300 nm scale image shows flat
domains at the surface with a typical height of 4 nm. Interestingly, at 1.2 mM surfactant
concentration nanobubbles were still found at the surface, but they are flatter and more
uniform than the bubbles present at the surface of a modified silicon wafer without any
surfactant (see figure 6.7).
Nanobubbles may also be present at the surface of the modified silica particles. The interaction of the hydrophobic silica particles in water are attractive over the whole range.
It is proposed that the rupture of a water film between two opposing nanobubbles may
cause this long range attraction (see figure 6.8). In that way the jump into contact will
be related to the rupture thickness of the water film around 40 nm between two opposing
bubbles. This value correlates well with the rupture of a foam film in water [24]. A similar behaviour was obtained by Ishida [25] for the interaction between two hydrophobic
surfaces in water. At 0.03 mM a slight repulsion at large separation, due to the recharging of the surface, is seen (see figure 6.8). The jump into contact is at around 17 nm.
Long range repulsive interactions are observed at 0.3 mM surfactant concentration. For
distances smaller than 10 nm a non-DLVO repulsion is observed until 8 nm, which may
be attributed to the deformation of the bubble or to the steric repulsion of some aggre-
113
gates. The repulsion is followed by an attractive jump into contact. The obtained Debye
length of 17.6 nm correlates well with the theoretical one. The "effective" diffuse layer
potential was 42 mV. The interaction forces at 0.8 mM and 1 mM surfactant concentration are monotonic repulsive with a Debye length of 10.7 nm and 9.61 nm respectively.
The corresponding effective diffuse layer potentials are +49 mV and +30 mV. Additional
force curves, performed with another pair of modified silica particles at 0.8 mM and 1
mM (data not shown), yield an effective diffuse layer potential of +40 mV and +38 mV
respectively. The obtained Debye lengths of 10.7 nm and 9.61 nm correlate well with the
theoretical ones. The addition of surfactant increases the diffuse layer potential to more
positive values. The apparent decrease in potential is due to the condensation of the
Br− ions on the adsorbed layer during the force measurements, which leads to a screening of the surface charge. Also the uncertainty of the plane of charge due to aggregates
at the surfaces or nanobubbles contribute to the low fitted potential obtained at higher
surfactant concentrations. The lack of attraction at short range at higher surfactant
concentration may be related to a closer packing of the aggregates at the surface or may
be due to an increase in bubble elasticity with surfactant concentration (the nanobubbles
will be covered with surfactant). The stiffness of a nanobubble will be greater than that
of a microbubble 0.065N/m−1 [26]. That means that at large surfactant concentration,
a weak cantilever, with a typical spring constant of 0.03N/m−1 , is not able to penetrate
the nanobubbles and come into contact with the hard surface because the nanobubble is
more compliant than the cantilever itself. The same result was obtained by Zhang et al.
[27]. The nanobubbles described in chapter 5 have similar nanoscopic contact angles as
the nanobubbles at the modified silicon wafer. For the same studied concentration (e.g.
0.3 mM), the nanobubbles at the CTAB hydrophobic patches (see chapter 5) resemble
a spherical cap, whereas the nanobubbles at the divinyldisilazan interface are more flattened at the surface. They are also bigger in size and their nanoscopic contact angle
is closer to the macroscopical one. Different surfaces may induce different nanobubble
morphologies.
6.5. Conclusions
Nanobubbles are formed at the interface of a hydrophobically modified silicon wafer
(Divinyl-disilazan) exposed to water and surfactant solutions. In water the nanobubbles
resemble a spherical cap with a height of around 27 nm. The nanoscopic contact angle
through water of the modified silicon wafer after tip deconvolution is about 132◦ , which
is larger than the macroscopic contact angle of 89◦ . The Laplace pressure inside the
nanobubbles is 18 atm. A decrease in nanobubble height is observed with increasing
surfactant concentration. At 0.4 mM micropancake like morphologies were imaged at the
modified silicon wafer surface. At 1.2 mM surfactant concentration circular aggregates
were visualized. Condensation of the Br− ions on the adsorbed surfactant layer, together
with the presence of aggregates at the surface, may explain the low diffuse layer potential
obtained at larger surfactant concentrations.
114
Bibliography
[1]
J. Israelachvili and R. Pashley. In: Nature 300 (1982), pp. 341–342.
[2]
J. Ch. Erikkson, S. Ljunggren, and P.M. Claesson. In: J. Chem. Soc. Faraday Trans.
2 85 (1989), pp. 163–176.
[3]
R. Podgornik. In: Chem. Phys. Lett. 156 (1989), pp. 71.–75.
[4]
P. Attard. In: J. Phys. Chem. 93 (1989), pp. 6441–6444.
[5]
V. V. Yaminski and B. Ninham. In: Langmuir 9 (1993), pp. 3618–3624.
[6]
R. F. Considine, R. A. Hayes, and R. G. Horn. In: Langmuir 15 (1999), pp. 1657–
1659.
[7]
[8]
1569.
[9]
E. Kokkoli and Ch. F Zukoski. In: Langmuir 14 (1998), pp. 1189–1195.
[10]
176104–4.
[11]
B. M. Borkent et al. In: Langmuir 26 (2010), pp. 260–268.
[12]
[13]
H. Elnaiem et al. In: Comput. Mat. Continua 14 (2009), pp. 23–34.
[14]
D. Akcakayiran. “Funktionalisierung der Porenwände und Einlagerung von MetallKoordinations-Polyelektrolyten in SBA-15 Silika”. MA thesis. 2004.
[15]
[16]
X. H. Zhang, A. Quinn, and W. A. Ducker. In: Langmuir 24 (2008), pp. 4756–4764.
[17]
Y. Wang, B. Bhushan, and X. Zhao. In: Nanotechnology 20 (2009), p. 045301.
[18]
L. Zhang et al. In: Soft Matter 6 (2010), pp. 4515–4519.
[19]
J. R. T. Seddon and D. Lohse. In: J. Phys. Condens. Matter. 23 (2011), p. 133001.
[20]
Shi-Tao. Lou et al. In: J. Vac. Sci. Technol. B 18 (2000), pp. 2573–2575.
[21]
[22]
B. Song, V. Walczyk, and H. Schönherr. In: Langmuir 27 (2011), pp. 8223–8232.
[23]
X. H. Zhang et al. In: Langmuir 23 (2007), pp. 1778–1783.
115
Bibliography
[24]
D. Exerowa et al. In: Colloid Journal 63 (2001), pp. 50–56.
[25]
[26]
W. A. Ducker, Z. Xu, and J. N. Israeachvili. In: Langmuir 10 (1994), pp. 3279–
3289.
[27]
X. H. Zhang, N. Maeda, and V. S. J. Craig. In: Langmuir 22 (2006), pp. 5025–5035.
116
7.1. General Conclusions
The discussed experiments show that ion specific effects are still present even at low ionic
strengths. The long range interactions are similar for all the studied salts, but at 10−3
M ionic strength a slight tendency for Cs+ to be adsorbed at the surface is observed.
The surface potentials are slightly lower for 10−3 M ionic strength indicating additional
adsorption. The Debye length is different for 10−4 M and 10−3 M ionic strength and
coincides with the ideal values. The same qualitative long range interaction behaviour
is obtained for the studied ionic strengths. The short range interactions are different
for both ionic strength studied. At 10−4 M ionic strength, the short range attraction
decreases in the order Li+ >Na+ >K+ >Cs+ following the inverse Hofmeister series.
The model of Pashley [1] and Higashitani [2] can explain the interaction between the
silica particles at short range. It is possible that a gel layer is present at the silica surface
which also influences the interactions between the silica spheres. A decrease in pH in the
presence of electrolytes produces a synergetic effect between the protons and the cations,
giving rise to hydration repulsion. An increase of the ionic strength to 10−3 M produces
a short range repulsion due to hydration forces. It seems that the stability of silica under
the studied conditions is defined by a balance of electrostatic forces at long range and
a combination of hydration, depletion, and steric forces at short range. The interaction
forces between two silica particles (system I) and between a silica particle and a silicon
wafer (system II) in the presence of aqueous CTAB solutions with concentrations between 0.005 and 1 mM were measured using AFM. The force curves were correlated to
the surfactant morphologies (figure 4.7). Both systems show a charge reversal from negative to positive caused by the adsorption of cationic surfactant (CTAB) at the former
negatively charged silicon oxide surface. The interactions of the two systems were different for the same studied surfactant concentration. The point of zero charge was obtained
at 0.05 mM for the silica particle–silica particle system (system I) as in Parker’s work
[3] and at 0.3 mM for the silica particle–silicon wafer system (system II). This leads to
different aggregate morphologies at the silica particle surface and at the surface of the
silicon wafer. An explanation for the difference might be the surface treatment: the silica particles were plasma cleaned, whereas the silicon wafers were treated with a piranha
solution. In another study the same surface treatment was applied to the silica particles
and the silicon wafers and no differences in the interaction were observed [4]. At a low
CTAB concentration (for example 0.05 mM) long-range attraction was observed. The
attraction was larger in range and magnitude for the silica particle–silicon wafer system
117
and starts at distances larger than 40 nm. They cannot be caused by van der Waals
attraction, but they are explained by the presence of nanobubbles probing hydrophobic
patches on the surfaces. The attraction occurs when the nanobubbles bridge. Obviously,
on a silicon wafer surface larger hydrophobic patches are present than on the surface
of the silica particles. Another explanation for long-range attraction is the electrostatic
attraction between oppositely charged patches. At higher surfactant concentration (0.4
mM onwards), monotonic repulsion between the two silica particles was observed. In
contrast to this, aggregate expulsion could be observed in the interaction curves for the
silica particle–silicon wafer system up to a CTAB concentration of 0.8 mM. The difference is explained by the different stiffness of the surfactant aggregates at the two surfaces
(silica particle and silicon wafer). For the same studied concentrations, the outer layer
of surfactant was denser for the silica particles and the aggregates on the silica particles
were stiffer and more closely packed. The stiffer aggregates are more difficult to remove.
A systematic study of the CTAB adsorption to a silicon wafer was performed by AFM.
Small nanobubbles were present at the silicon oxide surface exposed to aqueous CTAB
solutions. The effect of the surfactant is twofold, it can partially hydrophobize the silicon wafer surface and stabilize the nanobubbles. The hydrophobic surfactant patches
present at the silicon oxide surface at low concentration (below 0.5 mM) are labeled
by nanobubbles which are imaged. The diameter of the nanobubbles varies from 30 to
60 nm (after tip deconvolution). The nanoscopic contact angle through water remains
constant between 140◦ and 150◦ and is independent of the CTAB concentration. This
angle verifies the hydrophobicity of the domains formed by the surfactant aggregates. It
is much higher than the macroscopic contact angle of a CTAB solution droplet (about
40◦ ), which presents an average of hydrophilic and hydrophobic areas on a silicon wafer
partially covered with CTAB. The Laplace pressure within the nanobubbles is about 30
atm. With increasing CTAB concentration the nanobubbles become smaller and less
prominent. This indicates that the silicon wafer surface becomes more hydrophilic. At
low CTAB concentration (0.05-0.4 mM) the surface is partially covered with hydrophobic
domains where the nanobubbles can be placed. Since nanobubbles were only observed
at low surfactant concentration (below 0.5 mM) they may play a role in the hydrophobic
interactions. A strong attraction (jump-in) is observed at short distances (≤ 20 nm),
which is explained by the rupture of the lamella between opposing nanobubbles seated
on the hydrophobic domains. With increasing concentration more and more hydrophilic
domains, i.e. micelles or patchy bilayers, are formed, until there are no bubbles observed close to the CMC (1 mM). At 0.5 mM long range electrostatic forces as well
as steric repulsion followed by micelle/bilayer expulsion are observed in the interaction
curve between the two opposing silica/silicon oxide surfaces. In the height image only
micropancakes were detected. At 1 mM the exerted force was not high enough to expulse
the adsorbed aggregates from the surface. Nanobubbles are also formed at the interface
of a hydrophobically modified silicon wafer (Divinyl-disilazan) exposed to water and surfactant solutions. In water the nanobubbles resemble a spherical cap with a height of
around 27 nm. The nanoscopic contact angle through water of the modified silicon wafer
after tip deconvolution is about 132◦ , which is larger than the macroscopic contact angle
of 89◦ . The Laplace pressure inside the nanobubbles is 18 atm. A decrease in nanobubble
118
height is observed with increasing surfactant concentration. At 0.4 mM micropancake
like morphologies were imaged at the modified silicon wafer surface. At 1.2 mM surfactant concentration circular aggregates were visualized. Condensation of Br− ions on the
adsorbed surfactant layer together with the presence of aggregates at the surface may
explain the low diffuse layer potential obtained at larger surfactant concentrations.
7.2. Future Work
Modification of AFM Tips with Nanoparticles
Although the colloidal probe atomic force microscope allows the measurements of the
interactions between the so called "colloidal particles", the particles are actually not real
colloids. Their dimensions are in the µm range. Spalla et al. [5] postulated, that for
the study of the hydrophobic interactions the use of macroscopic surfaces is a problem,
since a huge area has to be hydrophobized and it is difficult to obtain a homogeneous
hydrophobic layer. Rentsch et al. [6] studied the interaction forces between particles
of different sizes and proved that the Derjaguin approximation holds true even at small
distances. Will the Derjaguin approximation remain valid for the interaction between
a nanoparticle of 500 nm diameter and a flat surface? The challenge is to modify an
AFM tip with a nanoparticle of 500 nm or less in diameter in order to get nanoparticle
terminated tips. Thereafter, interactions between the nanoparticle modified tip and
other particles or planar surfaces have to be measured. Some previous experiments were
performed using the following method.
AFM cantilevers (PL2-CONT-10, NaNoAndMore) composed of silicon were cleaned with
piranha solution. The cantilevers were then modified with APTS (aminopropyltriethoxysilane) in the following way. First 0.1g of APTS was dispersed in toluene anhydrous.
The cantilevers were placed in this solution. After 3 hours the cantilevers were removed
and washed 3 times with toluene. The cantilevers were stored in a glass desiccator until
further use. Modified silica particles around 500 nm in diameter with -COOH terminated
groups were obtained from Bangs Laboratories. The idea is to perform an amidation
reaction between the -NH2 terminated tips and the -COOH terminated silica particles.
Surface functionalization with carboxylic acid functionalized silica was obtained by An et
al. [7]. A suspension of the -COOH terminated silica particles was placed in DMF. The
suspension was treated with a sonicator for 10 minutes. A small amount of DCC (N,N’dicyclohexylcarbodimide) was dissolved in 10 ml of DMF (Dimethylformamide). The
DCC solution was added to the particle suspension. Then the cantilevers were cleaned
in a plasma cleaner (Diener electronic. Femto timer) and placed in the suspension for
24 hours. Thereafter, the expected particle terminated cantilevers were taken from the
suspension and washed with DMF and water. Figures 7.1 and 7.2 show the resulting tip
after the amidation reaction. Some particles are observed at the cantilever, but since the
whole cantilever was modified, the particles can get attached to any part of the cantilever,
not only to the end of the tip. Further work has to be done to find the right method for
119
the modification of the tip with a single nanoparticle. The use of focus ion beam may
facilitate the attachment of a nanoparticle to the cantilever.
Figure 7.1.: Scanning electron microscopy of a modified cantilever
Figure 7.2.: Scanning electron microscopy of a modified cantilever
120
Bibliography
[1]
[2]
I. U. Vakarelski, K. Ishimura, and K. Higashitani. In: J. Colloid Interface Sci. 227
(2000), pp. 111–118.
[3]
7710.
[4]
[5]
O. Spalla. In: Current Opinion in Colloid and Interface Sci. 5 (2000), pp. 5–12.
[6]
S. Rentsch et al. In: Phys. Chem. Chem. Phys. 8 (2006), pp. 2531–2538.
[7]
Yanqing An et al. In: Journal of Colloid and Interface 311 (2007), pp. 507–513.
121
A. Appendix
Figure A.1.: Scanning electron microscopy of silica particles
Figure A.2.: Scanning electron microscopy of a magnetic actuated cantilever
122
A. Appendix
Figure A.3.: Scanning electron microscopy of a magnetic actuated cantilever
Nanoscopic
contact angle
WATER
Height H
AIR
θnano
Width W
Radius of curvature Rc
Figure A.4.: Schematic cross section of a nanobubble
123
A. Appendix
300
250
15
10
150
nm
nm
200
5
100
0
50
-5
0
0
100
200
300
nm
300
250
15
10
150
5
100
nm
nm
200
0
50
-5
0
0
100
200
300
nm
Amplitude
48nm
47
46
45
44
43
0
50
100
150
nm
200
250
300
Figure A.5.: Amplitude-distance data of nanobubbles on a silicon oxide surface at 0.05
mM CTAB concentration (see figure 5.2)
124
A. Appendix
300
250
8
6
150
4
100
2
nm
nm
200
0
50
-2
0
0
100
200
300
nm
300
250
8
6
150
4
100
2
nm
nm
200
0
50
-2
0
0
100
200
300
nm
Amplitude
22.0nm
21.5
21.0
20.5
20.0
0
50
100
150
nm
200
250
300
Figure A.6.: Amplitude-distance data of nanobubbles on a silicon oxide surface at 0.3 mM
CTAB concentration (see figure 5.3)
125
A. Appendix
300
200
4
150
3
2
100
nm
nm
250
1
50
0
0
0
100
200
300
nm
300
250
26
25
150
24
23
100
nm
nm
200
22
50
21
0
0
100
200
300
nm
Amplitude
24.0nm
23.6
23.2
22.8
0
50
100
150
nm
200
250
300
126
A. Appendix
300
250
400
200
150
pm
nm
200
0
100
-200
50
-400
0
0
100
200
300
nm
300
250
400
200
150
0
pm
nm
200
100
-200
50
-400
0
0
100
200
300
nm
Amplitude
26.0nm
25.5
25.0
24.5
0
50
100
150
nm
200
250
300
127
A. Appendix
1.0
0.8
nm
15
10
5
µm
0.6
0.4
0
-5
0.2
-10
-15
0.0
0.0
0.2
0.4
0.6
0.8
1.0
µm
1.0
0.8
20
15
µm
0.6
5
0.4
nm
10
0
0.2
-5
-10
0.0
0.0
0.2
0.4
0.6
0.8
1.0
Amplitude
µm
11nm
10
9
8
7
6
0.0
0.2
0.4
0.6
0.8
1.0
µm
Figure A.9.: Amplitude-distance data of nanobubbles on a modified silicon wafer immersed in water (see figure 6.1)
128
A. Appendix
1.0
0.8
10
0.6
0
0.4
nm
µm
5
-5
0.2
-10
0.0
0.0
0.2
0.4
0.6
0.8
1.0
µm
1.0
0.8
30
0.6
26
0.4
nm
µm
28
24
0.2
22
0.0
0.0
0.2
0.4
0.6
0.8
1.0
µm
Amplitude
28nm
26
24
22
20
0.0
0.2
0.4
0.6
0.8
1.0
µm
Figure A.10.: Amplitude-distance data of nanobubbles on a modified silicon wafer at 0.1
129
A. Appendix
300
250
3
2
1
150
0
100
nm
nm
200
-1
-2
50
-3
0
0
100
200
300
nm
300
200
4
150
2
0
100
nm
nm
250
-2
-4
50
0
0
100
200
300
Amplitude
nm
28nm
27
26
25
24
23
0
50
100
150
nm
200
250
300
130
A. Appendix
300
250
4
2
150
0
nm
nm
200
100
-2
50
-4
0
0
100
200
300
nm
300
250
40
200
30
25
100
nm
nm
35
150
20
15
50
0
0
100
200
300
nm
Amplitude
28nm
27
26
25
24
0
50
100
150
nm
200
250
300
131
A. Appendix
300
250
2
150
0
100
nm
nm
200
-2
50
0
0
100
200
300
nm
300
200
30
150
28
26
100
nm
nm
250
24
50
22
0
0
100
200
300
nm
Amplitude
30nm
28
26
24
22
0
50
100
150
nm
200
250
300
132
A. Appendix
300
200
2
150
1
0
100
nm
nm
250
-1
50
-2
0
0
100
200
300
nm
300
200
30
150
28
26
100
nm
nm
250
24
50
22
0
0
100
200
300
Amplitude
nm
28nm
26
24
22
0
50
100
150
nm
200
250
300
133

An AFM study of the interactions between colloidal

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