Atomic Force Microscopy Study of Forces between a Silica Sphere

Jan 12, 2013 - The University of Queensland, School of Chemical Engineering, Brisbane, Qld 4072 Australia ... Copyright © 2013 American Chemical Soci...
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Atomic Force Microscopy Study of Forces between a Silica Sphere and an Oxidized Silicon Wafer in Aqueous Solutions of NaCl, KCl, and CsCl at Concentrations up to Saturation Yuhua Wang,† Liguang Wang,* Marc A. Hampton, and Anh V. Nguyen* The University of Queensland, School of Chemical Engineering, Brisbane, Qld 4072 Australia ABSTRACT: This paper describes the measured forces between a spherical silica particle and a planar oxidized silicon wafer in NaCl, KCl, and CsCl aqueous solutions using an atomic force microscope (AFM). The magnitudes of measured forces are sensitive to electrolyte type and concentration over a broad range of 0.01−4 M under study. Increasing NaCl and KCl concentrations finds the suppression of repulsion at low concentrations, the appearance of attraction at an intermediate concentration, and the suppression of the attraction at high concentrations. In contrast, no attractions were detected for CsCl solutions except at 0.5 M, and increasing the concentration would lead to suppression of repulsion at the low concentration range and enhancement of repulsion at the high concentration range. The deviation between the measured total force and the calculated double-layer repulsion can be represented in the form of a power law incorporating an effective Hamaker constant (Aeff) and an offset separation distance.

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10−2 M.10 Moreover, short-ranged forces, such as hydration force, which is not considered in the DLVO theory, may arise between silica surfaces in electrolyte solutions. A review on hydration force can be found elsewhere.11 The hydration force in electrolyte solutions was studied by Chapel12 using a surface force apparatus (SFA). This study showed that, at a concentration range from 1 × 10−4 to 0.1 M, the strength and range of the hydration force between silica surfaces decreased with a decreasing bare counterion size in the order of Cs+ > K+ > Na+ > Li+. A similar sequence of the surface force in the order of K+ > Na+ > Li+ between silica surfaces was found by Peschel et al.,13 who used a self-developed force measurement method to measure surface forces over a broad range of electrolyte concentrations (1 × 10−5 to 3 M). Despite the accumulating evidence for the existence of hydration force between silica surfaces in inorganic electrolyte solutions, the origin of the hydration force is not clear yet. Peschel et al.13 stated that the hydration force barrier is controlled by the structure of the bulk phase and solid surface. The pseudothermodynamic theory developed by Eriksson et al.14 also supports the water structuring mechanism of hydration force. Israelachvili15 stated that, at higher concentrations, specific to each electrolyte, hydrated cations bind to the negatively charged surfaces and give rise to a repulsive hydration force, which is attributed to the energy needed to dehydrate the bound cations. Yoon and Vivek16 suggested that the organization of the interfacial water should be responsible for the hydration force between silica surfaces. Valle-Delgado et

INTRODUCTION Understanding the surface forces between silica surfaces immersed in aqueous electrolyte solutions is of fundamental and practical importance. For example, the outstanding stability of silica suspensions in aqueous solutions at high salt concentrations has been the subject of several investigations,1−4 and the surface forces are fundamentally important for understanding this colloid stability phenomenon. In many industrial applications, such as the flotation separation of particles, salt crystals, and minerals, saline water with an ionic strength often exceeding 1 or 2 M has been used.5,6 In particular, the mineral flotation operation in many arid regions in Australia and South America has increasingly been forced to use saline groundwater and seawater to mitigate the shortage of fresh water. It is noted that silica and many other silicates represent the major unwanted component in the particle separation by flotation. However, direct measurements for the surface forces between silica surfaces at high electrolyte concentrations are limited. The colloidal probe technique by using an atomic force microscope (AFM) is widely used to measure surface forces between a colloidal particle and a solid substrate.7,8 Dishon et al.9 performed AFM force measurement for silica surfaces in NaCl, KCl, and CsCl solutions with concentrations of 1 × 10−3 to 1 M. They observed that the forces were electrolytedependent at concentrations above 0.01 M. Their data interpretation was made in the framework of the celebrated Derjaguin−Landau−Verwey−Overbeek (DLVO) theory with using surface potential as an adjustable parameter. Although the DLVO theory lays the foundation for explaining colloidal stability, it is known that the DLVO theory works only in a certain electrolyte concentration range, say 1 × 10−3 to 5 × © 2013 American Chemical Society

Received: September 17, 2012 Revised: January 7, 2013 Published: January 12, 2013 2113

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al.4 reviewed different theories and experimental data and concluded that the main contribution comes from the formation and rupture of hydrogen bonds between the surface silanol (Si-OH) groups and the neighboring water molecules. This view was supported by recent spectroscopic studies. Bailey and McGuire17 used ATR-FTIR to probe the water structure in colloidal silica and found a strongly hydrogen-bonded hydration layer at the surface of the colloidal particles and its impact of water structure on hydration force. Yang et al.18 used the sum frequency generation spectroscopy to probe the structure of water molecules at the interfaces of aqueous salt solutions and silica. They found the pronounced perturbation of the interfacial water structure on silica surfaces, and the magnitudes of perturbation are different for Li+, Na+, and K+, following the order of K+ > Li+ > Na+. It was summarized by Butt et al.11 that the origin of hydration force may be related to several effects working together. Most published work has focused on electrolyte concentration up to 0.5 M. No systematic force measurement has been undertaken to investigate a broader range of electrolyte concentrations up to the solubility limit, using a proven force measurement technique, such as AFM. No consensus has been reached on the effect of electrolyte type on hydration force. It is expected that higher electrolyte concentrations close to the solubility limits may change the water structure to an even larger extent, so the hydration force measurement at very high electrolyte concentrations should be of great interest to understanding the nature of hydration force. In the present work, we used AFM to measure the surface forces between a silica sphere and an oxidized silicon wafer in monovalent electrolyte solutions with a broad concentration range of 0.01− 4 M. The results have significant implications on colloid stability in saline or bore water.

hydrodynamic effects in their experimental system were already minimized. The scan rate was 0.05 Hz. The cantilever with silica particles was also immersed in ethanol for 20 min, followed by exposure to UV light for 1−2 h immediately before each measurement. Every force measurement was done after an equilibration time of 20−25 min. The radius of the silica colloidal probe was calculated using a “reverse imaging” technique.24 In this technique, the colloidal probe is scanned over a calibration grating consisting of an array of very sharp peaks (TGT1, NT-MDT, Moscow). A resulting AFM contact mode air image consists of a number of spherical domains. Each spherical domain represents a “reverse image” of the interaction area of the colloidal probe. From a section of the spherical domains, the radius of the colloidal probe can be calculated. The reliability of the experimental curve in the present work can be seen from Figure 1, in which the force curves obtained

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Figure 1. Typical normalized force, F/R, as a function of separation for a silica sphere (radius R = 9.5 μm) interacting with an oxidized silicon wafer in deionized water. F/R is equal to the energy per unit area multiplied by 2π, according to the Derjaguin approximation.25,26 The dashed line represents the experimental data obtained when retracting the silica sphere. The solid line represents those obtained when approaching the silica sphere to the flat sample surface.

EXPERIMENTAL SECTION Pure deionized water was produced by a system consisting of a Reverse Osmosis Unit in combination with a Milli-Q Academic Unit (Millipore). Analytical grade NaCl, KCl, and CsCl were purchased from Sigma Aldrich (Australia). All the salts were roasted at 500 °C for at least 5 h, and the prepared stock solutions were filtered twice to eliminate particulate contaminations. The pH range of the salt solution is between 5.95 and 6.20 at different concentrations. Nonporous silica particles (Fuso Chemical Co., Japan) were cleaned by RCA SC-1 solution.19,20 The radii (R) of the silica particles were between 8.2 and 9.8 μm. Silicon wafers of (100) crystal orientation with a deposited thermal oxide layer of 100 nm in thickness were obtained from Silicon Valley Microelectronics. The particles were attached to the end of a goldcoated silicon nitride triangular cantilever (NP probe, Veeco) using a very small amount of thermoplastic epoxy resin Epikote 1004.21,22 A silicon wafer cleaned by the same procedure was completely wetted by water, which indicates that the cleaning method produces a hydrophilic surface. A MFP-3D (Asylum Research) atomic force microscope was used for all the force measurements. The AFM probes were calibrated by employing the thermal vibration method23 embedded in the Asylum Research AFM software. The cantilevers used in the present set of data were found to have a spring constant of 40−70 pN/nm. The approach velocity was kept low (50 nm/s) to minimize hydrodynamic effects, with the Reynolds number in our system being exceedingly low. Note that Dishon et al.9 used an approach speed of 100 nm/s, and they claimed that the

in the approach and retraction modes overlap with each other. The cleanliness of the experimental system was also checked by carrying out the force measurements between the same silica surfaces in deionized water immediately before and after the force measurement. Any pronounced changes in force curves indicate possible significant contamination, so the corresponding experimental data should be discarded. Work by Donose et al.20 shows that the cleaning procedure for similar surfaces used in this study did not affect the force curves.

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RESULTS AND DISCUSSION The measured force versus separation data are presented in Figures 2−4 for NaCl, KCl, and CsCl, respectively. In these Figures, only the curves in approach mode are shown. Figures 2 and 3, respectively, show the changes in the overall force from repulsion to attraction with increasing concentration from 0.01 to 1 M NaCl/KCl; further increasing the concentration decreased the strength of the attraction force. The crossover from the descending trend of the force curves to the ascending trend was seen at 1 M. In contrast, Figure 4 shows the measured forces for CsCl solutions at concentrations of 0.01−4 M. As shown, increasing the salt concentration would decrease the total repulsion, which passes through a minimum, after 2114

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Figure 2. Force curves between a silica sphere and an oxidized silicon wafer in aqueous solutions of NaCl at various concentrations. The dashed line represents the Lifshitz−van der Waals force with the Hamaker constant being 0.83 × 10−20 J.15

Figure 3. Force curves between a silica sphere and an oxidized silicon wafer in aqueous solutions of KCl at various concentrations. The dashed line represents the Lifshitz−van der Waals force (the same as in Figure 2).

Figure 4. Force curves between a silica sphere and an oxidized silicon wafer in aqueous solutions of CsCl at various concentrations.

regard to their range being longer than that of the double-layer repulsion. In general, as NaCl and KCl concentrations were increased, we observed the suppression of repulsion at low concentrations, the appearance of attraction at an intermediate concentration, and suppression of the attraction at high concentrations. In contrast, no attractions were detected for CsCl solutions except at 0.5 M, and increasing the concentration would lead to suppression of repulsion at the low concentration range and enhancement of repulsion at the high concentration range. Specifically, for a given electrolyte, increasing the bulk concentration from 0.01 to 0.5 M would decrease the overall surface force. This phenomenon was due to the double-layer compression, which reduces the dominant double-layer force at relatively low electrolyte concentrations. Further increasing the bulk concentration from 1 to 4 M for a given electrolyte made

which it increased. The measured forces at the concentration range were repulsive, with an exception at 0.5 M CsCl, where very weak attraction was detected below 4 nm. For NaCl and KCl (Figures 2 and 3), attractive forces were seen at high concentrations of electrolytes, so the force curves were compared to the Lifshitz−van der Waals force (vdW, represented by the dashed line) FvdW A = − H2 R 6D

(1)

where the Hamaker constant AH is equal to 0.83 × 10−20 J15 and D is the separation distance. It is clear that, at high electrolyte concentrations, the overall surface forces were less attractive than the Lifshitz−van der Waals forces, suggesting the existence of additional repulsive forces, which appears to be the secondary hydration force with 2115

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the force curves shift upward, indicating a lessened van der Waals force and/or an enhanced hydration repulsion. For all three electrolytes at 0.5 M, the overall surface forces (no matter repulsive or attractive) were rather weak. It is likely that the point of surface charge neutrality was reached near this concentration, similar to what is reported in Dishon et al.9 Dishon et al. found that the attraction at the point of neutrality was independent of ion type. Dishon et al.9 found a pronounced ion specific effect at concentrations above 0.01 M. At 1 × 10−3 M, they found that NaCl, KCl, and CsCl gave nearly identical force curves except at very short separation distances below 3 nm. On the other hand, Parsons and Ninham27 pointed out that sometimes, at low concentrations, the ion specific forces dominate electrostatics. To study the ion specific effects, we compared all the experimental data at given concentrations. Figures 5−7 show the replotted graphs for NaCl, KCl, and CsCl at each concentration.

Figure 6. Force curves replotted for aqueous solutions of NaCl, KCl, and CsCl at 0.5 M.

the overall surface force followed in the order of Cs+ > K+ > Na+. At such high ionic strengths, the double-layer force should have been effectively suppressed, so it is difficult to explain the measured force extending to 5−10 nm without considering additional non-DLVO forces. The ion specific effects presented in Figures 5−7 are summarized in Table 1. The results presented hitherto suggest lessened van der Waals forces at high electrolyte concentrations, in agreement with Adler et al.28 The difference in the DLVO forces and the measured force might be due to the additional hydration repulsion, which also seems long-ranged. Traditionally, the hydration force is considered as a new force, which is irrelevant to the van der Waals force. However, Parsons and Ninham27 recently showed that the long-ranged secondary hydration force is due to ion dispersion force. It appears difficult, therefore, to decouple the Lifshitz−van der Waals forces from the hydration force. Parsons and Ninham claimed that ionic dispersion, finite ion size, and surface hydration layers are the key factors for understanding hydration force. Moreover, the uncertainty for calculating the van der Waals force at high salt concentrations is still unknown. Therefore, we take a simple (rather primitive) approach by using Aeff to represent the contributions from both the Hamaker force and the hydration force. Similarly, Adler et al.28 also introduced the concept of effective Hamaker constant Aeff to represent the difference in force between the electrostatic repulsion and measured force. We used the following formula to fit the experimental data F (D) Aeff F(D) = el − R R 6(D + 2Δ)2

Figure 5. Force curves replotted for aqueous solutions of NaCl, KCl, and CsCl at the concentrations of (a) 0.01 M and (b) 0.1 M.

(2)

where Δ is the offset distance per surface, corresponding to the finite outward shifts in the Outer Helmholtz Plane (OHP) of 0.5 nm per surface.29 For two surfaces, the overall offset distance is 2Δ (=1 nm). Different locations of the van der Waals and double-layer planes are attributed to the charged silica hairs protruding from the surface. Protruding silanol and silicic acid groups can grow on silica surfaces in the presence of water.30 More information on the silica hairs (polysilicic acid) or gel is given elsewhere.28,30,31 In calculating the double-layer repulsion (Fel/R) as a function of separation (D), we used the full numerical solution to the Poison−Boltzmann equation with a constant charge boundary condition. According to ref 29, if two surfaces are brought together quickly, which is the case for AFM force measurement, the interaction may be at a constant charge condition and the

Figure 5 shows that at the electrolyte concentrations of 0.01 and 0.1 M, ion specific effects were observed in each set of experiments, and the magnitude of the overall surface force followed in the order of Cs+ > K+ > Na+, which is consistent with the order of the series reported by Chapel12 and Peschel et al.13 Figure 6 shows that, at a transition concentration of 0.5 M, all the measured forces were weak, and the ion specific effects were minute. Nevertheless, the magnitude of the overall surface force followed in the order of Na+ > Cs+ > K+. At this concentration, possible charge neutrality and reversal might have complicated the interpretation of the order of the series. Figure 7 shows that, at a given concentration in the electrolyte concentration range of 1−4 M, the magnitude of 2116

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Figure 7. Force curves replotted for aqueous solutions of NaCl, KCl, and CsCl at the concentrations of (a) 1, (b) 2, (c) 3, and (d) 4 M.

the classical DLVO fit will always show the force maximum and the predominance of vdW at short separation distances. Figure 8 illustrates that the fitted values of ψ and Aeff decreased with increasing NaCl concentration from 0.01 to 0.5 M. The change in Aeff suggests an increase in hydration repulsion as the electrolyte concentration is increased. This is in agreement with the prediction made by the polarization model.3 In contrast, Rabinovich et al.33 and Peschel et al.13 found that the decay length of the hydration force decreases with increasing electrolyte concentration. At higher NaCl concentrations (1−4 M), Figure 9 shows that the absolute values of the fitted ψ were less than 2 mV. These surface potentials could carry a positive sign, with charge reversal being assumed to have taken place. Again, one can see a decrease in Aeff with increasing NaCl concentration. Notably, there is an unusual increase in Aeff from 0.5 to 1 M NaCl, which may be related to the complex changes in electric field because of charge neutrality or reversal. At a given high electrolyte concentration, 4 M, Figure 10 shows the fitted force curves with the experimental ones for three electrolytes. CsCl gave negative Aeff values, whereas NaCl and KCl gave positive ones. The negative values of Aeff indicate very strong repulsive forces, as shown in Figure 10c. Note that, at such high ionic strengths, the double-layer force should have been effectively suppressed at above several nanometers. Figures 8−10 show that eq 2 may be used to fit the present experimental data satisfactorily. This approach uses the effective Hamaker constant (Aeff) to incorporate the possible effects of ionic dispersion force, ion sizes, and surface layer of hydration. Moreover, the necessity of using the offset distance (Δ) for force curve fitting with eq 2 supports the view that charged silica hairs are present at the solid−liquid interfaces, resulting in different locations of the double-layer repulsion and van der Waals planes.

Table 1. Ion Specific Effects on the Magnitude of Forces Measured at Various Electrolyte Concentration Ranges electrolyte concentration (M)

measured force decreasing in the order of

0.01, 0.1, 1, 2, 3, 4 0.5

Cs+ > K+ > Na+ Na+ > Cs+ > K+

boundary condition of constant surface charge density is appropriately chosen. Specifically, the nonlinear Poison− Boltzmann equation for the potential between two planar surfaces was numerically solved within Matlab employing the finite difference method. The Neumann boundary condition of the constant surface charge was implemented using the shooting technique.32 The potential data were then used to numerically determine the pressure and interaction energy between planar surfaces. Finally, the interaction force between a sphere and a planar surface as measured by AFM was also numerically determined using the Derjaguin approximation. In the numerical calculation of energy from the pressure, the upper limit of the integration was offset to a finite separation distance (e.g., 15 D lengths) and the residual energy was accurately calculated using the analytical solution of the superposition approximation. The Debye length was determined by using the following formula15 0.304 [nm] κ −1 = (3) C where C is the bulk concentration of the electrolyte in mol/L. Equation 2 was used to fit the experimental data by adjusting the electrical surface potential (ψ) and the effective Hamaker constant Aeff. With 2Δ = 1 nm, the experimental data were fitted satisfactorily (see Figures 8−10). We also found that the selection of the vdW offset distance 2Δ significantly affects the calculated forces. Note that, at low electrolyte concentrations, 2117

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Figure 10. Force curves between a silica sphere and an oxidized silicon wafer in aqueous solutions of NaCl, KCl, and CsCl at 4 M. The solid lines represent eq 2, where the Debye length is determined by eq 3, and the surface electrical potential (ψ) and the effective Hamaker constant (Aeff) are the fitted parameters.

Figure 8. Force curves between a silica sphere and an oxidized silicon wafer in aqueous solutions of NaCl at relatively low concentrations. The solid lines represent eq 2, where the Debye length is determined by eq 3, and the surface electrical potential (ψ) and the effective Hamaker constant (Aeff) are the fitted parameters.

Figure 9. Force curves between a silica sphere and an oxidized silicon wafer in aqueous solutions of NaCl at relatively high concentrations. The solid lines represent eq 2, where the Debye length is determined by eq 3, and the surface electrical potential (ψ) and the effective Hamaker constant (Aeff) are the fitted parameters. 2118

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Table 2. Fitted Values of Electrical Surface Potential and Effective Hamaker Constant NaCl C (M) 0.01 0.1 0.5 1 2 3 4

κ−1 (nm) 3.04 0.964 0.431 0.304 0.216 0.176 0.152

|ψ| (mV) 8.0 5.0 1.0 1.0 2.0 1.8 0.3

KCl Aeff (J) 2.3 8 2 6 4 2.5 9

× × × × × × ×

|ψ| (mV)

−21

10 10−22 10−22 10−22 10−22 10−22 10−23

17 13 1.0 0.7 1.5 1.7 2.4

Table 2 summarizes the fitted values of ψ and Aeff for all the force curves obtained in the present work. In general, as the electrolyte concentration was increased, we observed the decrease in the magnitude of Aeff, suggesting the decrease in van der Waals attraction or increase in hydration repulsion, or both. This trend is consistent with the polarization model, which predicts an increase in hydration force at higher ionic strength.3 Higher electrolyte concentrations close to the solubility limits may change the water structure to an even larger extent, so the hydration force would become stronger. For each type of electrolyte, as the electrolyte concentration was increased, |ψ| become smaller. At very high electrolyte concentrations, |ψ| is close to zero, with the sign of surface potential being either negative or positive. Similarly, other hydration force models find a fitted surface potential of zero at electrolyte concentrations above 1 M.4 At a given electrolyte concentration, say 0.1 M, |ψ| is 13 mV for silica in KCl solution and 15 mV in CsCl solution. The small difference in |ψ| obtained in the present work is consistent with the experimentally observed equal surface charge densities of silica in KCl and CsCl solutions at pH 6.0.34 Note that |ψ| is 5 mV for silica in 0.1 M NaCl solution, much lower than those in KCl and CsCl solutions. A similar order of surface charge density was seen by Tadros and Lyklema34 at 0.1 M at pH ≥ 7. The pH values in the present work were around 6.0. This difference in pH or surface potential may be largely due to the different silica surfaces used in their experiments and the present work and errors within cantilever spring constant determination. It is reported that silica surfaces that were produced by flame treatment or cleaned by plasma would eventually gel.35,36 Each of our experimental runs lasted for a few hours, and it is likely that the silica surfaces in the present work were also gelled and covered by charged silica hairs. Dishon et al.9 treated their silica surfaces with plasma, so there was a high probability of forming silica gel on their silica surface. Dishon et al. found an opposite trend of surface potential at 0.01 M and pH 5.5. However, these authors obtained the fitted surface potential values with using the DLVO theory without taking into consideration the hydration force, a non-DLVO force. Note that the ion-dependent force series at high electrolyte concentrations of 1−4 M remained the same for NaCl, KCl, and CsCl. In contrast, Colic et al.35 measured the rheological properties of silica suspensions at 4 M of various chlorides and found that, at pH 6.0, the inferred hydration force should decrease in the order of Cs+ > K+ > Na+, whereas at 1 M of various chlorides, the hydration force appeared to be indifferent to electrolyte type. On the whole, the force measured in the present work follows the order of Cs+ > K+ > Na+. Colic et al.35 also found that the short-range repulsive force with the strongest extent is

CsCl Aeff (J) 2.3 8 3.5 3 2.7 2 1

× × × × × × ×

−21

10 10−22 10−22 10−22 10−22 10−22 10−22

|ψ| (mV) 17.3 15 1.0 0.7 0.2 0.15 0.1

Aeff (J) 2.3 8 3 −8 −1.2 −1.7 −1.3

× × × × × × ×

10−21 10−22 10−23 10−21 10−20 10−20 10−20

observed with the largest Cs+ ion in the unhydrated state. Dishon et al.9 argued that the difference in measured forces for different ions is due to the ion specific adsorption of cations on the solid surfaces or overcharging of silica surfaces by monovalent cation condensation, which becomes increasingly more pronounced at higher electrolyte concentrations. They attributed the reappearance of repulsion at high concentrations to reappearance of the double-layer force. Indeed, at low concentrations, it cannot be excluded that possible changes in surface charge density could play an important role in the double-layer repulsion. At very high electrolyte concentrations, however, the observed range and magnitude of the measured repulsive forces were much larger than those of the doublelayer force. This discrepancy may be attributed to the presence of hydration force.

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CONCLUSIONS

Direct force measurements between a spherical silica particle and a planar oxidized silicon wafer in NaCl, KCl, and CsCl aqueous solutions were performed using an atomic force microscope (AFM). The magnitudes of measured forces are sensitive to electrolyte type and concentration over a broad range of 0.01−4 M under study. For a given electrolyte at low concentrations, 0.01−0.5 M, increasing the bulk concentration decreases the strength of the overall surface force, which may be attributed to the suppression of double-layer repulsion as a result of increased ionic strength and/or reduced surface charge density associated with cation adsorption. Increasing the bulk concentration from 1 to 4 M would increase the strength of the overall repulsive surface force, probably because the non-DLVO hydration force was increased while the double-layer force remained negligibly weak. At a given low electrolyte concentration, the magnitude of the overall force decreases in the order of Cs+ > K+ > Na+. At a given high concentration, the magnitude of the overall surface force also follows in the order of Cs+ > K+ > Na+. A numerical solution with a constant charge boundary condition was applied to calculate the double-layer repulsion, whereas the other forces were evaluated using a single term that incorporates the effective Hamaker constant (Aeff) and an offset separation distance. The experimental data were fitted successfully, supporting the assumption of charged silica hairs, which may lead to different locations of the van der Waals and double-layer planes. The present work suggests that a nonDLVO repulsive force exists between silica surfaces in concentrated NaCl, KCl, and CsCl solutions, and it becomes stronger at higher electrolyte concentrations up to saturation. 2119

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(30) Vigil, G.; Xu, Z.; Steinberg, S.; Israelachvili, J. N. J. Colloid Interface Sci. 1994, 165, 367. (31) General discussion: Faraday Discuss. Chem. Soc. 1978, 65, 44. (32) Press, W. H.; Teukolsky, S. A.; Vetterling, W. T.; Flannery, B. P. Numerical Recipes in C: The Art of Scientific Computing; Cambridge University Press: New York, 1992. (33) Rabinovich, Ya. I.; Derjaguin, B. V.; Churaev, N. V. Adv. Colloid Interface Sci. 1982, 16, 63. (34) Tadros, Th. F.; Lyklema, J. J. Electroanal. Chem. 1968, 17, 267. (35) Colic, M.; Fisher, M. L.; Franks, G. V. Langmuir 1998, 14, 6107. (36) Yaminsky, V. V.; Ninham, B. W.; Pashley, R. M. Langmuir 1998, 14, 3223.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.W.), [email protected]. edu.au (A.V.N.). Tel: 61 7 3365 7942 (L.W.), 61 7 3365 3665 (A.V.N.). Fax: 61 7 3365 4199 (L.W.), 61 7 3365 4199 (A.V.N.). Present Address †

School of Mineral Processing and Bioengineering, Central South University, Changsha 410083 People’s Republic of China. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the China Scholarship Council (CSC) for the visiting Research Fellowship, which made the stay of Dr. Yuhua Wang at The University of Queensland possible. This research is also supported under the Australian Research Council’s Discovery Projects funding scheme (Grant DP0985079).

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REFERENCES

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