Specific Effects of Divalent Cation Nitrates on the Nanotribology of

May 3, 2006 - Recent investigations have shown that hydrated layers of monovalent cations adsorbed on silica (Donose, B. C.; Vakarelski, I. U.; Higash...
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Ind. Eng. Chem. Res. 2006, 45, 7035-7041

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Specific Effects of Divalent Cation Nitrates on the Nanotribology of Silica Surfaces Bogdan C. Donose, Ivan U. Vakarelski, Elena Taran, Hiroyuki Shinto, and Ko Higashitani* Department of Chemical Engineering, Kyoto UniVersity, Katsura,Nishikyo-ku, Kyoto 615-8510, Japan

Recent investigations have shown that hydrated layers of monovalent cations adsorbed on silica (Donose, B. C.; Vakarelski, I. U.; Higashitani, K. Silica surfaces lubrication by hydrated cations adsorption from electrolyte solutions. Langmuir 2005, 21, 1834) or mica (Raviv, U.; Klein, J. Fluidity of bound hydration layers. Science 2002, 297, 1540) surfaces can act as highly efficient boundary lubricants. Here, by using the lateral force microscopy (LFM) mode of atomic force microscopy (AFM), we extended these investigations to the case of divalent cations, measuring the frictional force between a completely hydrophilic silica particle and a silica wafer in Ba2+, Sr2+, Ca2+, and Mg2+ nitrate solutions. The measurements demonstrated strong lubrication effects for solutions of Ba2+, Sr2+, and Ca2+ nitrates starting even at a very low electrolyte concentration, such as 3 × 10-5 M. As for Mg2+ ions, however, it is surprising to know that the frictional force increases with increasing electrolyte concentration. Possible nanotribological mechanisms are discussed, and the value of friction was found to be more closely related to the average residence time of water molecules around cations than to the number of water molecules in their hydration shell. 1. Introduction Interactions between charged surfaces in electrolyte solutions are one of the major factors determining the stability of colloidal systems.1-3 These interactions have been extensively investigated on a molecular scale, using the surface force apparatus (SFA)3-6 and atomic force microscopy (AFM), and they have been correlated with the microstructure of the surfaces.6-9 Most of these investigations have been focused on the normal interactions between surfaces, and not many researchers have paid attention to the lateral interactions.1,2,10-13 However, lateral interactions between silica surfaces in solutions became of particular interest, because the frictional force is closely related with the planarization of Si/SiO2 surfaces, which is one of the key technologies in the many rapidly developing microelectronic processes involving the chemical mechanical polishing (CMP) and microelectromechanical systems (MEMS). In our recent study, we used the lateral force microscopy (LFM) mode of AFM to investigate the effects of monovalent cationic solutions of CsCl, NaCl, and LiCl on the nanotribology of silica surfaces.1 Hydrated cations adsorbed on the surfaces were found to act as highly efficient lubricants, providing a significant reduction of the friction coefficient compared with that of pure water. This lubrication effect was correlated with the hydration property of the cations; the strongly hydrated cations were found to be better lubricants than the poorly hydrated cations; that is, the effect follows the lyotropic series, Li+ > Na+ > Cs+. Another important finding was that, for all the systems studied, the friction decreased with increasing scan velocity at a fixed applied load and reached a saturation value at high scan rate. It was suggested that the layers of hydrated but freely movable water molecules around cations adsorbed on the surfaces play an important role for the reduction of the frictional force. The objective of this work is to extend the investigation on the nanotribology of silica surfaces in electrolyte solutions to the solutions of multivalent cations. Because of their abundance * To whom correspondence should be addressed. Tel.: +81-75-3832662. Fax: +81-75-383-2652. E-mail: [email protected].

in natural systems and in various technological processes, the effects caused by the adsorption of multivalent ions are of fundamental importance and practical interest. We have been working on a series of measurements for solutions of divalent nitrates, Ba(NO3)2, Sr(NO3)2, Ca(NO3)2, and Mg(NO3)2, divalent chlorides, BaCl2, SrCl2, CaCl2, and MgCl2, and a trivalent chloride, LaCl3, and found their complex but interesting behavior; the correlation between the type and concentration of cations and the lubrication effects was not so straightforward as in the case of monovalent cations. In the present study, the results for solutions of divalent nitrates are discussed as the first step of this series on experiments of multivalent solutions. The results on the solutions of multivalent chlorides will be reported later. 2. Experimental Section The nitrate solutions of divalent cations with different hydration enthalpies were prepared using analytical grade Mg(NO3)2‚6H2O, Sr(NO3)2‚6H2O (Wako Pure Chem. Ind., Japan), Ca(NO3)2 (Chameleon Reagent, Japan), and Ba(NO3)2 (Cica Reagent, Japan). Their fundamental properties are shown in Table 1. Silicon wafers with a thermal oxide deposit of 10 µm thickness (100 crystal orientation, P-type, B-doped) were purchased from Silicon Quest International, U.S.A. The rootmean-square (rms) roughness of the silica surface measured by AFM was about 0.1 nm over 1 µm.2 We made the wafer surface clean and fully wetted, using the following procedure which is known as the RCA treatment for wafer cleaning.14 After washing with acetone, ethanol, and DI water, wafers were soaked in a mixture of 1:1:5 by weight of NH4OH, H2O2, and H2O at 70 °C for 15 min. Then the wafers were thoroughly washed with copious amount of DI water. The contact angle of the wafers treated by this procedure is known to be less than 5° so that the surfaces are regarded as completely hydrophilic. Pure water employed here, whose internal specific resistance is no less than 18.2 MΩ/cm, was prepared by purifying it further with a Milli-Q purification system (Millipore) after distillation and deionization by a distillation apparatus (WGA-28, Yamato Scientific). Nonporous colloidal silica particles of ca. 20 µm in diameter were kindly provided by Fuso Chemical Co., Japan. A silica

10.1021/ie060182r CCC: $33.50 © 2006 American Chemical Society Published on Web 05/03/2006

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Table 1. Fundamental Properties of Cations ions

bare ion radius (nm)

hydration no.

hydration enthalpy (kJ/mol)

τ1/τ0 (-)a

µe/µ0 at 3 × 10-3 M (-)b

binding energy to surface OH- (kJ/mol)c

Ba+2 Sr+2 Ca+2 Mg+2

0.148 0.132 0.099 0.065

6 6 6 6

-1289 -1456 -1577 -1908

1.03 1.79 1.62 3.94

0.29 0.55 0.40 1.27

-893.5 -961.5 -1010.4 -1153.0

Li+

0.068

4

-552

1.9

0.55d

-569.4

a

τ0 is the residence time of a water molecule in pure water; τ1 is the residence time of a water molecule contacting an ion (ref 28). b µ0 is the friction coefficient in pure water; µe is the friction coefficient in solutions. c The radius of OH- was assumed to be 0.176 nm (refs 2, 24). d Value measured in a 0.1 M solution whose ionic strength is equal to that of 3 × 10-3 M of divalent cation.

particle was attached to the cantilever end, using a small amount of thermoplastic epoxy adhesive (Shell Epikote 1004). Before use, the probe was washed by ethanol and DI water and then plasma-treated for 1 min in an argon-water moisture atmosphere, by using a basic plasma kit-BP1 (Samco Co., Kyoto) and a radio frequency plasma generator-ENI ACG 98 (13.6 MHz, 300 W). All measurements were carried out with a Digital Instruments Nanoscope III multimode AFM equipped with a fused silica liquid cell. Triangular cantilevers (Veeco NanoProbe NP) of spring constant KN ) 0.12 N/m were used to measure the normal interaction forces. Rectangular tipless cantilevers NSC12/tipless (MikroMasch) with the nominal spring constant of KN ) 14.0 N/m were used for the lateral force experiments. The exact value of KN for each cantilever was determined by the resonance frequency method, and the AFM sensitivity for the detection of lateral force by the method of Bogdanovic et al.15 The lateral spring constant, KL, was determined using the following equation derived from the geometrical characteristics of a rectangular cantilever:

KL )

2KNl2 3(1 + υ)

(1)

where υ is the Poisson ratio, which is 0.27 in the present case, and l is the cantilever length. All cantilevers used in the lateral force measurement experiments were taken from the same batch and were found to have lateral spring constants KL in the range of 59 ( 4 (nN m)/rad. The normal force interactions were measured following the method introduced by Ducker et al.7 The lateral force measurements were performed using the friction force mode of AFM. In this mode, the colloidal probe is pressed against the substrate at a constant applied load while the substrate is sliding horizontally underneath the cantilever. The lateral force was determined from the magnitude of the frictional loop voltage signal. Further details of the measuring procedure of lateral forces are given elsewhere.1,16 All the experiments were carried out at the room temperature of 25 ( 0.5 °C. The pH of solutions used in the experiments was 5.6 ( 0.5. 3. Results and Discussion 3.1. Normal Interaction Forces. First we examined the normal interaction forces between silica surfaces in pure water and various aqueous solutions of divalent nitrates. Figure 1 shows the approach and retraction force curves in pure water and in 3 × 10-5 and 3 × 10-3 M Ba(NO3)2 solutions. It is clear that the interaction is totally repulsive at all separations and there is no hysteresis between the approaching and the retracting force curves, as in our previous investigations on the solutions of monovalent cations.1 These features, which are typical in the case of highly hydrophilic silica surfaces in

Figure 1. Normal force curves for a 20 µm silica particle and silica wafer in pure water, 3 × 10-3 M Ba(NO3)2, and 3 × 10-5 M Ba(NO3)2 solutions: (a) approaching force curves; (b) retracting force curves.

aqueous solutions, have been considered to be due to the prevention of adhesive contact between surfaces by the shortrange hydration force.1,6,9 It must be emphasized also that the

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force curves for pure water in Figure 1 coincided completely with those for 10 times stronger loading force between the surfaces. This implies that the silica surfaces are coated by firmly adsorbed water layers.17 At the concentrations of 3 × 10-5 and 3 × 10-3 M, very similar normal force curves to those in Figure 1 were obtained for all the nitrate solutions investigated in the present study.17 When the concentration is 3 × 10-1 M as shown in Figure 2, the approaching and retracting force curves for Sr(NO3)2 are always repulsive and do not show any hysteresis as before, but in the cases of Ca(NO3)2 and Mg(NO3)2, the long-range repulsion disappeared completely in the approaching force curve, and the small and large adhesion forces appeared in the retraction force curves for Ca(NO3)2 and Mg(NO3)2 solutions, respectively. Here the force curves for Ba(NO3)2 are not shown because of the insolubility at this concentration. The abovementioned dependence of adhesion on the type of cations was observed before for the system of the silica particle and mica surface, where the dependence was correlated with the hydration enthalpies of the adsorbed monovalent cations.8 However, the mechanism of interactions for solutions of divalent cations may be more complicated especially in the case of concentrated solutions, because various interaction forces, such as the ioncorrelation, the divalent bridging, and the oscillatory force, may play a significant role in addition to the hydration forces.18-20 Further discussion is given later. 3.2. Lateral Forces for Pure Water. Now, we discuss the results of lateral force measurements. Our attention here was paid to the characteristics of the friction in the range of 10 times stronger loading force than that in the previous friction measurements in solutions of monovalent cations.1 As shown in Figure 3, the linearity of the loading force L versus the lateral force FL for pure water holds up to L ) 4800 nN, and the reduction of FL with the scan rate V is similar to that in the previous experiments. The coincidence among three runs of data taken at different locations on the silica wafer in the figure demonstrates the reliability of the present experiments and suggests the negligible wearing of the particle surface during the experiments. The average friction coefficient µ derived from the linear regression was found to be approximately 0.1, which is in the range of the previous values.14 The dependence of FL on V also showed the same trend as before, that is, a sharp initial decrease at the low scan rate and plateau region at the higher scan rate. This decrease is often called the boundary lubrication in the Stribeck chart, in which the large friction at the low scan rate is considered to be caused by the direct contact of solid surfaces.21 But we consider this mechanism may not be applicable to the present case as discussed later. 3.3. Lateral Forces for Solutions of Divalent Cations. Here we measured consecutively the friction forces in solutions of various concentrations of divalent cations. Values of FL measured at the fixed values of L and V are presented in Figures 4-7 for solutions of Ba(NO3)2, Sr(NO3)2, Ca(NO3)2, and Mg(NO3)2, respectively, in comparison with the data for pure water. For each cation studied, experiments were repeated using several different particles in order to confirm the reliability of the main trends observed. Figure 4 shows the dependence of FL for solutions of various Ba(NO3)2 concentrations on the values of L and V, respectively. It is clear that the friction increases linearly with the loading force, but the magnitude decreases with increasing electrolyte concentration, Ce. It is interesting to note that the reduction is significant even at an extremely low concentration, 3 × 10-5 M, and reaches the maximum at Ce ) 3 × 10-2 M. This clearly

Figure 2. Approach and retraction normal force curves in 3 × 10-1 M solutions of divalent cations: (a) Sr(NO3)2 solution, (b) Ca(NO3)2 solution, and (c) Mg(NO3)2 solution.

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Figure 3. Lateral force measurements in pure water: (a) dependence of the lateral force on the loading force at V ) 4 µm/s; (b) dependence of the lateral force on the scan rate at L ) 1900 nN.

Figure 4. Dependence of the lateral force on Ba(NO3)2 concentration: (a) lateral force vs loading force at V ) 4 µm/s; (b) lateral force vs scan rate at L ) 1900 nN.

indicates that the lowering of friction is related to the adsorption of the Ba2+ cations on the surface. As for the scan rate dependence of friction, similar trends as that in Figure 3 for pure water were also found in the case of Ba(NO3)2 solutions at all the concentrations. Very similar features were also observed for solutions of Sr(NO3)2 and Ca(NO3)2, as shown in Figures 5 and 6. All these characteristics on the friction reduction are very similar to those that we found previously for solutions of monovalent cations, especially for highly concentrated solutions of highly hydrated cations, that is, a 1 M LiCl solution. As for the Mg(NO3)2 solution, however, almost the reverse trends of the other solutions were observed; the friction increases linearly with the loading force as before, but the magnitude increases with the salt concentration. In addition, the dependence of the friction on the scan rate for the 3 × 10-3 and 3 × 10-1 M solutions shows a sharp initial increase, and then the maximum and a gradual decrease at the higher scan rate. It is surprising to find that there exist such complicated phenomena of friction in this simple series of experiments. 3.4. Possible Mechanism. Of course it is very difficult to estimate precisely the lubrication mechanism by divalent cations on the molecular scale, using the limited number of data obtained in the present experiments. Nevertheless, for future development, it is important to propose possible mechanisms by which the phenomena observed can be explained consistently and at least qualitatively. The important results obtained are summarized below.

(1) The friction increases linearly with the loading force, independent of the kind and concentration of cations. (2) The magnitude of friction decreases with increasing concentration of Ba(NO3)2, Sr(NO3)2, and Ca(NO3)2 solutions but increases in the case of Mg(NO3)2 solution, and the magnitude of friction is affected by an extremely small amount of divalent cations, such as 3 × 10-5 M. (3) The scan rate dependence of friction shows initially a sharp decrease in the region of low scan rate and reaches the plateau region at the higher scan rate in the cases of Ba(NO3)2, Sr(NO3)2, and Ca(NO3)2 solutions, while it shows a sharp initial increase, and then the maximum and a gradual decrease in the case of Mg(NO3)2 solutions of Ce g 3 × 10-3 M. According to our recent investigation of the dependence of the friction on the solution pH, the relation of FL versus L shows a nonlinearity when a gel layer is formed on the silica surfaces at high pH. Hence, the linear relation of FL versus L of the present experiments implies that there is no significant formation of gel layer on the silica surface at the normal pH.22 The characteristics of lubrication effects shown in Figures 4-6 are similar to those found in our previous experiments of monovalent cations, in which small but highly hydrated Li+ ions are superior as lubricants to large but poorly hydrated Cs+ ions. This similarity of the lubrication effect between monovalent and divalent cations implies that a similar tribological mechanism may be involved. Previously we related the lubrication effect

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Figure 6. Dependence of the lateral force on Ca(NO3)2 concentration: (a) lateral force vs loading force at V ) 4 µm/s; (b) lateral force vs scan rate at L ) 1900 nN.

Figure 5. Dependence of the lateral force on Sr(NO3)2 concentration: (a) lateral force vs loading force at V ) 4 µm/s; (b) lateral force vs scan rate at L ) 1900 nN.

with the lateral mobility of the water molecules in the hydration shell around cations adsorbed on the surfaces. This hypothesis was first proposed by Raviv and Klein,2 by which the lowering of friction between mica surfaces in concentrated LiCl and NaCl solutions was explained. The recent molecular dynamic simulation given by Leng and Cummings23 justified this hypothesis by simulating the shear thinning behavior of confined hydration layers composed of monovalent cations and water molecules. A significant lowering of the friction coefficient was observed even at an extremely low concentration (3 × 10-5 M) of Ba2+, while the corresponding lowering occurs at about 10-1 M in the case of the monovalent cation, Li+. The estimation of the density of adsorbed cations indicates that only an extremely small number of adsorption sites on the surface is covered by Ba2+,18,20 while the surface will be predominantly covered by Li+.4,6 This reminds us to consider that the strong lubrication effect of divalent cations is essentially attributable to their high electric charge. A rough estimation of the binding energy between a divalent cation and a function OH- on the surface shows that divalent cations will be bound firmly on the surface, as shown in Table 1.24 Besides, according to the theoretical investigations by Ruckenstein and co-workers, multivalent ions

adsorbed on the surface induce a field to the neighboring water molecules which propagates further toward the bulk phase of the solution.25,26 This is consistent with our recent results claiming that a fragile structure composed of water molecules, ions, and hydrated ions exists outside of the primary layer of water molecules and ions adsorbed firmly on surfaces, especially in the case of multivalent ions.27 These results imply that the overall surface property is greatly influenced by a small number of adsorbed divalent cations; even if the surfaces are not fully covered, the adsorbed cations will be strong enough to isolate effectively the surfaces and accompany a large number of freely movable water molecules into the gap of surfaces. This provides the formation of the thick and soft adsorbed layers composed of freely movable water molecules acting as lubricants, as illustrated in Figure 8b.1 As shown in Table 1, the characteristics of Ba2+, Sr2+, and Ca2+ are more or less the same in terms of the hydration number and relative residence time τ1/τ0 of water molecules around cations, where τ0 is the residence time of water molecules in pure water.28 We consider that the hydration number is a “static” measure of the number of water molecules accompanying with the adsorption of cations, and the residence time τ1 is a “dynamic” measure of the degree of freedom of water molecules, which may be related with freely movable water molecules as lubricants. However, the detailed comparison between the residence time and the relative friction coefficient

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Figure 8. Schematic images of ions and water molecules between sliding surfaces: (a) firmly structured layers of pure water on highly hydrophilic surfaces; (b) adsorbed Ba2+ with movable water molecules of small residence time; (c) adsorbed Mg2+ with firmly hydrated water molecules of large residence time.

Figure 7. Dependence of the lateral force on Mg(NO3)2 concentration: (a) lateral force vs loading force at V ) 4 µm/s; (b) lateral force vs scan rate at L ) 1900 nN.

µe/µ0 at Ce ) 3 × 10-3 M indicates that the order of the magnitude of µe/µ0 coincides well with that of τ1/τ0, although it does not with the order of the hydration, enthalpy, and ionic size, where µ0 is the friction in pure water. This indicates that the magnitude of friction should be correlated with the dynamic parameters, instead of the static ones. It is also interesting to note that the magnitudes of τ1/τ0 and µe/µ0 of lubricating divalent cations are nearly the same as those of Li+ of the same ionic strength, that is, 0.1 M. On the other hand, the values of τ1/τ0 and µe/µ0 for the nonlubricating Mg2+ ions are much larger than those of the other ions. This implies that the movement of water molecules around Mg2+ will be strongly restricted, just like water molecules sticking firmly on the surface, as illustrated in Figure 8a. Besides, the binding energy toward the wall is the strongest, as shown in Table 1, which is consistent with the strong adhesion at Ce ) 3 × 10-1 M shown in Figure 2. We believe that this strong restriction for the movement of water molecules in the gap and the molecular-scale roughness of the surface by the adsorption of Mg2+ ions are the origins for the fact that the friction increases with increasing concentration of Mg(NO3)2, although the hydration number is the same as the others. Now we consider the dependence of friction on the scan rate shown in Figures 4-7. According to the lubrication text book,21 the rapid decrease at the small scan rate for Ba(NO3)2, Sr(NO3)2, and Ca(NO3)2 solutions may be regarded as the boundary

lubrication, as mentioned before. However, it is clear that the value of friction at V ) 0 µm/s depends on the kind and concentration of electrolytes, and it is known that the hydrophilic surfaces are firmly covered by water molecules, as speculated before from the data in Figure 1. Hence, we consider that this reduction of friction is not due to the direct contact between solid surfaces but to the breakage of the structured layers formed by the adsorbed cations and water molecules. The plateau region is regarded as the hydrodynamic lubrication region. In the case of Mg(NO3)2 solutions, the friction increases with the scan rate in the region of small scan rate. It is known that the friction increases with the surface roughness. Because Mg2+ ions adsorbed on the surface with firmly bound water molecules, as illustrated in Figure 8c, make the surface rougher on the molecular scale, this surface roughness results in the initial increase of the friction. After the maximum of friction, the gradual decrease will be due to the breakage of the structured layer of ions and water molecules adsorbed on the surface. Of course, these mechanisms are derived from simple speculation, so further complementary experiments and molecular dynamic simulations are definitely needed to justify the mechanisms proposed here, especially to clarify the nature of the reverse lubrication observed for Mg2+ ions. As an additional issue, the role of anions, which could as well participate in forming hydrated layers of multivalent cations on surfaces, must be clarified in the future. 4. Conclusions Lateral forces between silica surfaces in electrolyte solutions of divalent nitrates were investigated by using the LFM mode of AFM. It was found that even very small concentrations of divalent cations could significantly change the frictional force between the surfaces. The Ba2+, Sr2+, and Ca2+ cations were found to act effectively as lubricants, as in the case of monovalent cations previously examined.1 In the case of monovalent cations, the degree of lubrication followed the order of the hydration enthalpy of cations. In the case of divalent cations, however, the mechanism of friction was not so simple; the precise correlation of the friction with the hydration enthalpy

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was not obtained, but it was successfully correlated with the relative residence time, which is a dynamic measure of the degree of freedom of water molecules between surfaces. But apparently these mechanisms postulated here must be clarified by further complementary experiments and molecular dynamic simulations. Acknowledgment The authors are grateful to the Japan Society for the Promotion of Science for the financial support for this project (Project No. 15206085) and also for the financial support by Keihanna Company. Literature Cited (1) Donose, B. C.; Vakarelski, I. U.; Higashitani, K. Silica surfaces lubrication by hydrated cations adsorption from electrolyte solutions. Langmuir 2005, 21, 1834. (2) Raviv, U.; Klein, J. Fluidity of bound hydration layers. Science 2002, 297, 1540. (3) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: San Diego, CA, 1992. (4) Pashley, R. M. DLVO and hydration forces between mica surfaces in lithium, sodium, potassium, and cesium ions electrolyte solutions: a correlation of double-layer and hydration forces with surface cation exchange properties. J. Colloid Interface Sci. 1981, 83, 531. (5) Pashley, R. M.; Israelachvili, J. N. DLVO and hydration forces between mica surfaces in Mg2+, Ca2+, Sr2+ and Ba2+ chloride solutions. J. Colloid Interface Sci. 1984, 97, 446. (6) Chapel, J. P. Electrolyte species dependent hydration forces between silica surfaces. Langmuir 1994, 10, 4243. (7) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Measurement of forces in liquids using a force microscope. Langmuir 1992, 8, 1831. (8) Vakarelski, I. U.; Ishimura, K.; Higashitani, K. Adhesion between silica particle and mica surfaces in water and electrolyte solutions. J. Colloid Interface Sci. 2000, 227, 111. (9) Kohonen, M. M.; Karaman, M. E.; Pashley, R. M. Debye length in multivalent electrolyte solutions. Langmuir 2000, 16, 5749. (10) Vigil, G.; Xu, Z.; Steinberg, S.; Israelachvili, J. N. Interactions of silica surfaces. J. Colloid Interface Sci. 1994, 165, 367. (11) Feiler, A.; Larson, I.; Jenkins, P.; Attard, Ph. A quantitative study of interaction forces and friction in aqueous colloidal systems. Langmuir 2000, 16, 10269. (12) Biggs, S.; Cain, R. G.; Page, N. W. Lateral force microscopy study of the friction between silica surfaces. J. Colloid Interface Sci. 2000, 232, 133.

(13) Zhu, Y.; Granick, S. Viscosity of interfacial water. Phys. ReV. Lett. 2001, 87, 096104. (14) Donose, B. C.; Taran, E.; Vakarelski, I. U.; Shinto, H.; Higashitani, K. Effects of cleaning procedures of silica wafers on their friction characteristics. J. Colloid Interface Sci., in press. (15) Bogdanovic, G.; Meurk, A.; Rutland, M. W. Tip friction-torsional spring constant determination. Colloids Surf., B 2000, 19, 397. (16) Vakarelski, I. U.; Brown, S. C.; Rabinovich, Y. I.; Moudgil, B. Lateral force microscopy investigation of surfactant mediated lubrication from aqueous solution. Langmuir 2004, 20, 1724. (17) Donose, B. C. Silica-Silica Nanotribology in Electrolyte Solutions Studied by Atomic Force Microscopy. Ph.D. Thesis, Kyoto University, 2005. (18) Kjellander, R.; Marcelja, S.; Pashley, R. M.; Quirk, J. P. A theoretical and experimental study of forces between charged mica surfaces in aqueous calcium chloride solutions. J. Chem. Phys. 1990, 92, 4399. (19) Kekicheff, P.; Marcelja, S.; Senden, T. J.; Shubin, V. E. Charge reversal seen in electrical double layer interaction of surfaces immersed in 2-1 calcium electrolyte. J. Chem. Phys. 1993, 99, 6098. (20) Fielden, M. L.; Hayes, R. A.; Ralston, J. Oscillatory and ioncorrelation forces observed in direct force measurements between silica surfaces in concentrated CaCl2 solutions. Phys. Chem. Chem. Phys. 2000, 2, 2623. (21) Persson, B. N. J. Sliding Friction, 2nd ed.; Springer: Berlin, 2000. (22) Taran, E.; Donose, B. C.; Vakarelski, I. U.; Higashitani, K. pH dependence of friction forces between silica surfaces in solutions. J. Colloid Interface Sci. 2006, 297, 199. (23) Leng, Y.; Cummings, P. T. Fluidity of hydration layers nanoconfined between mica surfaces. Phys. ReV. Lett. 2005, 94, 26101. (24) Shinto, H.; Morisada, S.; Miyahara, M.; Higashitani, K. A reexamination of mean force potentials for the methane pair and the constituent ion pairs of NaCl in water. J. Chem. Eng. Jpn. 2003, 36, 57. (25) Ruckenstein, E.; Manciu, M. The coupling between the hydration and double layer interactions. Langmuir 2002, 18, 7584. (26) Ruckenstein, E.; Huang, H. Colloid restabilization at high electrolyte concentrations: effect of ion valency. Langmuir 2003, 19, 3049. (27) Li, Y.; Kanda, Y.; Shinto, H.; Vakarelski, I. U.; Higashitani, K. Fragile structured layers on surfaces in highly concentrated solutions of electrolytes of various valencies. Colloids Surf., A 2005, 260, 39. (28) Uedaira, H. Mizu no Bunshi Kogaku; Kodansha Press: Tokyo, 1998.

ReceiVed for reView February 14, 2006 ReVised manuscript receiVed March 27, 2006 Accepted April 4, 2006 IE060182R