Silica Surfaces Lubrication by Hydrated Cations Adsorption from

A significant lubrication effect was demonstrated for solutions of high ... can also play an important role for the adhesive interaction between surfa...
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Silica Surfaces Lubrication by Hydrated Cations Adsorption from Electrolyte Solutions Bogdan C. Donose, Ivan U. Vakarelski, and Ko Higashitani* Department of Chemical Engineering, Kyoto UniversitysKatsura, Nishikyo-ku, Kyoto 615-8510, Japan Received September 27, 2004. In Final Form: November 19, 2004 Adsorption of hydrated cations on hydrophilic surfaces has been related to a variety of phenomena associated with the short-range interaction forces and mechanisms of the adhesive contact between the surfaces. Here we have investigated the effect of the adsorption of cations on the lateral interaction. Using lateral force microscopy (LFM), we have measured the friction force between a silica particle and silica wafer in pure water and in electrolyte solutions of LiCl, NaCl, and CsCl salts. A significant lubrication effect was demonstrated for solutions of high electrolyte concentrations. It was found that the adsorbed layers of smaller and more hydrated cations have a higher lubrication capacity than the layers of larger and less hydrated cations. Additionally, we have demonstrated a characteristic dependence of the friction force on the sliding velocity of surfaces. A mechanism for the observed phenomena based on the microstructures of the adsorbed layers is proposed.

Introduction Understanding of tribological phenomena at a nanoscale level is of critical importance for the rapid development of existing and emerging technologies, such as chemical mechanical planarization (CMP) and microelectromechanical systems (MEMS).1,2 The colloidal and frictional behaviors of silica, which play an important role in these technologies, have been known to be far more complex than expected for materials with smooth and chemically inert surfaces.3-5 In the past decade, the lateral force microscopy (LFM) mode of the atomic force microscope (AFM) has been proved to be a powerful technique to investigate the frictional phenomenon down to an atomic scale.1,6 The influence of monovalent electrolyte solutions on the interaction forces between surfaces has been extensively studied by Israelachvili and Pashley,7-10 using the surface force apparatus (SFA) and mica surfaces, and the existence of short-range repulsive force caused by the hydrated cations adsorbed on the surface was postulated. The strength of this force was found to increase in the order of Cs+ < K+ < Na+ < Li+. Later, when silica surfaces were used instead of mica, a different dependence of the short-range interaction on the electrolyte was demonstrated.11-13 AFM studies by Vakarelski et al. have shown * Author to whom correspondence should be addressed. Tel: +81-(0)75-383-2662. Fax: +81-(0)75-383-2652. E-mail: k_higa@ cheme.kyoto-u.ac.jp. (1) Bhushan, B. Handbook of Micro/Nanotribology, 2nd ed.; CRC Press: Boca Raton, FL, 1995. (2) Basim, G. B.; Brown, S. C.; Vakarelski, I. U.; Moudgil, B. M. J. Dispersion Sci. Technol. 2003, 24, 499. (3) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979. (4) Cook, L. M. J. Non-Cryst. Solids 1990, 120, 152. (5) Vigil, G.; Xu, Z.; Steinberg, S.; Israelachvili, J. N. J. Colloid Interface Sci. 1994, 165, 367. (6) Carpik, R. W.; Salmeron, M. Chem. Rev. 1998, 97, 1163. (7) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975. (8) Pashley, R. M. J. Colloid Interface Sci. 1981, 80, 153. (9) Pashley, R. M. J. Colloid Interface Sci. 1981, 83, 531. (10) Pashley, R. M. Adv. Colloid Interface Sci. 1982, 16, 57. (11) Horn, R. G.; Smith, D. T.; Haller, W. Chem. Phys. Lett. 1989, 162, 404. (12) Grabe, A.; Horn, R. J. Colloid Interface Sci. 1993, 157, 157. (13) Chapel, J. P. Langmuir 1994, 10, 4243.

that the adsorption of hydrated cations can also play an important role for the adhesive interaction between surfaces in electrolyte solutions.14,15 On the macroscopic scale, Franks et al.16-18 have demonstrated the strong effect of the adsorption of cations on the rheological behavior of concentrated silica and alumina slurries. Knowing all these effects for the short-range and adhesive interactions, it is expected that the electrolyte adsorption could significantly influence the frictional interaction between surfaces in the solutions. Using the SFA and AFM, the lateral interaction between silica surfaces has been approached from the point of view of the contact and adhesion mechanics.5,19-21 For example, Biggs and co-workers19 have investigated the relation between friction and load for silica surfaces of different hydrophobicity, using the colloidal probe AFM technique. Feiler et al.20 have investigated the dependence of the friction on the net interaction force between surfaces in electrolyte solutions of different pH. Recently using a modified SFA, Raviv and Klein21 have measured the shear force between mica surfaces, which are sliding past each other across aqueous salt solutions at the typical pressure and concentrations found in natural systems. They have concluded that hydration layers of adsorbed cations are keeping the compressed surfaces apart and act as a highly efficient boundary lubricant at the same time. In the present work, we are trying to extend the range of effects associated with the microstructures at the solid/ liquid interface to the lateral interaction between silica surfaces. Using the LFM technique, we have measured the frictional force between a silica particle of micrometer (14) Vakarelski, I. U.; Ishimura, K.; Higashitani, K. J. Colloid Interface Sci. 2000, 227, 111. (15) Vakarelski, I. U.; Higashitani, K. J. Colloid Interface Sci. 2001, 242, 110. (16) Franks, G. V.; Colic, M.; Fisher, M. L.; Lange, F. J. Colloid Interface Sci. 1997, 193, 96. (17) Colic, M.; Fisher; M. L.; Franks, G. V. Langmuir 1998, 14, 2207. (18) Franks, G. V.; Johnson, S. B.; Scales, P. J.; Boger, D. V.; Healy, T. W. Langmuir 1999, 15, 4411. (19) Biggs, S.; Cain, R. G.; Page, N. W. J. Colloid Interface Sci. 2000, 232, 133. (20) Feiler, A.; Larson, I.; Jenkins, P.; Attard, Ph. Langmuir 2000, 16, 10269-10277. (21) Raviv, U.; Klein, J. Science 2002, 297, 1540.

10.1021/la047609o CCC: $30.25 © 2005 American Chemical Society Published on Web 01/21/2005

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size and a silica wafer with a molecularly smooth surface in pure water and various electrolyte solutions of monovalent chlorides. Despite the significant difference of the normal interaction between silica and mica surfaces, we found that, as in the case of mica, the cations adsorbed on the surface exhibit a strong lubrication effect, which is correlated well with the hydration properties of the cations. A significant dependence of the frictional force on the scanning velocity, which varies with each solution condition, can be further related to the properties of the adsorbed layers. Experimental Section Silicon wafers coated by CVD silica of 1 µm thickness have been kindly provided by Shin-Etsu Chemical Co. Ltd., Japan. The root-mean-square (RMS) roughness of the silica surface was measured to be about 0.2 nm over 1 µm2 by AFM. Nonporous silica spheres of 6.8 µm in diameter were purchased from Bangs Laboratories Ltd. as a suspension of 10% volume in deionized water. Prior to usage, a small amount of suspension was slowly dried in a vacuum chamber (∼70 °C). Electrolyte solutions were prepared using LiCl, NaCl, and CsCl of analytical grade (Wako Chemical Co.). The salts were baked for several hours at 400 °C to remove the organic contaminants. The water used for all the experiments was produced by a Millipore filtration system, with an internal specific resistance no less than 17.6 MΩ/cm. The pH of solutions was 5.6 ( 0.5 in all the experiments. All measurements were carried out with a Digital Instruments Nanoscope III multimode atomic force microscope equipped with a fused silica liquid cell. Rectangular tipless cantilevers (MikroMasch) with a normal spring constant of KN ) 1.7 ( 0.3 N/m, which was determined by the frequency method, were used.22 The wafers were thoroughly washed with acetone, ethanol, and pure water. After the washing, they were plasma-treated for 3-5 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). Subsequent measurements of contact angles showed a gradual improvement of the surface decontamination, and the wafers became fully hydrophilic after the plasma treatment. A silica 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 plasmatreated in the same way as in the case of the wafers. Forces normal to the flat surface were measured, following the method introduced by Ducker et al.23 The frictional force measurements were performed using the “friction force” mode of the AFM. In this mode, the colloidal probe is pressed against the substrate at a constant applied load while the substrate slides horizontally underneath the cantilever. Further details of the measuring procedure are given elsewhere.20,24 The magnitude of the lateral frictional force, FL, was determined from half of the difference in the lateral force detector signal in one complete scan (cycle), VL, as expressed by eq 1:

FL ) 1/2VLSL

KL H

(1)

where SL is the lateral detector sensitivity, KL is the lateral spring constant, VL is the lateral force detector signal in one scanning loop, and H is the distance from the bottom of the sphere to the midpoint of the cantilever. The detector sensitivity in a lateral direction was determined using the method of Meurk et al.,25 and the lateral spring constant, KL, was determined from the rectangular lever dimensions using the following equation:24 (22) Cleveland, J. P.; Manne, S.; Bocker, D.; Hansma, P. K. Rev. Sci. Instrum. 1993, 64, 1. (23) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 1831. (24) Vakarelski, I. U.; Brown, S. C.; Rabinovich, Y. I.; Moudgil, B. Langmuir 2004, 20 (5), 1724. (25) Bogdanovic, G.; Meurk, A.; Rutland, M. W. Colloids Surf., B 2000, 19, 397.

KL )

2KnL2 3(1 + υ)

(2)

where υ is the Poisson ratio) 0.27,26 KN is the normal spring constant, and L is the cantilever length. For all the experiments, the room temperature was kept constant at 25 ( 0.5 °C.

Results and Discussions Normal Interaction. Typical approach-retraction force curves of the normal interaction between a silica particle and a silica wafer in pure water, 10-2 M LiCl, and 1 M LiCl solutions are given in Figure1a, 1b, and 1c, respectively. The upper and lower thin lines in Figure 1a and 1b are the fitting curves given by the DLVO theory of constant charge and constant potential, respectively, and the curve in Figure 1c represents the van der Waals attractive force. The actual ionic concentration in pure water is calculated to be about 4 × 10-5 M by the decay length of the long-range electrostatic interaction. This ionic strength most probably comes from the dissolved air CO2 and ions dissolved from the glass surface of the AFM cell. The pH of the solutions was not adjusted, and it was measured to be around pH 5.6. The most important feature of the normal interaction in pure water and in solutions of intermediate electrolyte concentration is that the interaction stays purely repulsive at all separations, as shown in Figure 1a and 1b. The particle rebounds from the surface without the adhesive contact, as predicted by the classical DLVO theory. Separate experiments, where the particle was pushed toward the surface up to 200 nN (F/R ) 50 mN/m) and stayed in contact from 0.1 to 100 s, showed no change of the force curves. As for the solution of 1 M LiCl, the longrange electrostatic contribution is screened and the interaction has a shallow secondary minimum. Nevertheless, the short-range repulsive force at the separation of the last few nanometers overcomes the van der Waals attraction and prevents the direct contact of surfaces. The results for the normal interactions shown here are typically observed for hydrophilic silica surfaces. For example, Chapel13 has demonstrated a similar interaction pattern for blown silica surfaces using the SFA. It should be noted that the interaction between silica surfaces depends strongly on the method of the surface preparation and the prehistory of contact.5,11-13,27-30 As explained in the experimental part, the surfaces employed here have been subject to the plasma treatment of mild water vapor and argon to ensure the removal of any kind of organic contamination. At the same time, this kind of treatment makes the silica surface highly hydrophilic, where the contact angle is less than 10°, and it is considered that at least one layer of water molecules is strongly bonded with the surface.31 The origin of the above-mentioned short-range repulsive force known as “hydration” or “structural” force has been of continuous interest. In the case of mica surfaces, the mechanism of cation exchange proposed by Pashley et al.8-10 has shown that the force arises from the energy (26) Hess, P. Appl. Surf. Sci. 1996, 106, 433. (27) Hartley, P. G.; Larson, I.; Scales, P. J. Langmuir 1977, 12, 22072214. (28) Atkins, D.; Ke´kicheff, P.; Spalla, O. J. Colloid Interface Sci. 1997, 188, 234-237. (29) Yaminsky, V. V.; Ninham, B. W.; Pashley, R. M. Langmuir 1998, 14, 3223-3235. (30) Bowen, W. R.; Hilal, N.; Lovitt, R. W.; Wright, C. J. Colloids Surf., A 1999, 157, 117-125. (31) DeRossa, R. L.; Schader, A. P.; Shelby, J. E. J. Non-Cryst. Solids 2003, 331, 32-40.

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Figure 1. Typical force-separation curve for the normal interaction between a 6.8-µm silica particle and a silica wafer in (a) pure water, (b) 10-2 M LiCl, and (c) 1.0 M LiCl. Solid symbols (b) are for approach, and empty symbols (O) are for the retraction interaction. Upper and lower curves indicate the fitting curves for DLVO theory of constant-charge and constantpotential, respectively. Hamaker constant was taken as 1.2 × 10-20 J; the surface potential was -73 mV for pure water and -40 mV for 10-2 M LiCl solution.

necessary to dehydrate the cations adsorbed on the mica surfaces at high electrolyte concentration. As for silica surfaces, the hydration repulsion is observed even at low electrolyte concentrations, and the origin of the extra force is more controversial. One of the primary explanations is that water molecules are firmly structured around the surface hydroxyl groups.13 Several quantitative theories for this extra repulsive force have been proposed, but the effects are considered to be related to the water structuring or ionic adsorption around the surface in all the theories.33,34 It is also possible that this lack of adhesion is due to the steric stabilization by a gel layer of protruding silanol and silicic acid.5,29 Some studies have shown that both phenomena, that is, the surface hydration and the surface gelation, might play a certain role, depending on the kind of silica used. The silica treatment employed here may render the surfaces highly hydrophilic, and a more or less thick layer of organized water and hydrated cations (32) Torrie, G. M.; Kusalik, P. G.; Patey, G. N. J. Chem. Phys. 1989, 91, 6367. (33) Paunov, V. N.; Binks, B. P. Langmuir 1999, 15, 2015. (34) Ruckenstein, E.; Manciu, M. Langmuir 2002, 18, 7584.

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adsorbed on surfaces might be expected. At the same time, the effects due to the gel layer may be predominant over those due to a water layer bonded strongly with silica surfaces. As for such surfaces, it is expected that the interaction patterns change with the time of contact with water and show the hysteresis between the approach and separation interactions.5,29 However, in the case of our surfaces, no change of the interaction force curves was observed, even if the surfaces were immersed into water for several hours; no matter how strongly or how long two surfaces were pushed together, no measurable hysteresis or adhesion, which has been associated with the energy dissipation of the gel layer, was detected. To our knowledge, there are no reports on the investigation of the silica gel layers using AFM imaging. However, it should be noted that, according to the AFM images of the wafers and the particles used, their surfaces are extremely smooth (RMS of 0.2 nm over 1 µm2). In some preliminary experiments, we examined the existence of the gel layer by indenting the silica surface with a bare cantilever tip, but no measurable compressibility was found at the contact with the silica surface. Although we cannot exclude the possibility of the existence of a gel layer, the abovementioned observations make us believe that, in the case of our silica surfaces and experimental conditions, the results obtained in the present study are predominantly associated with surface hydration and cation adsorption. In the following sections, we examine how the phenomena observed above are associated with the lateral interaction between two silica surfaces in pure water and electrolyte solutions of various concentrations and types of cations. Friction in Pure Water. Initially we investigated the tribological behavior of silica surfaces in pure water. Results for the friction force versus the applied load at fixed scanning velocity and those for the friction force versus the scanning velocity at fixed applied load are shown in Figure 2a and Figure 2b, respectively. The reproducibility of data was confirmed for both types of measurements by repeating the same experiments as shown in the figure. Measurements were repeated at a fixed place on the silica wafer by gradually increasing the applied load or the scanning rate. After each change of the load or the scan rate, several friction force cycles were allowed to transpire in order to allow the lateral force magnitude to stabilize prior to taking a reading. The reproducibility of the data shown in Figure 2 suggests not only that there was no significant wearing of the surfaces in the time scale of the present experiment, but also that these data were taken under a quasi-equilibrium state and independent of the prehistory of surface contact. It should be noted that the range of the applied load in friction experiments (up to 500 nN or F/R of about 150 mN/m) far exceeds the magnitude of the normal force between the surfaces (F/R up to 2 mN/m) given in Figure 1. Data in Figure 2a show a linear dependence of the friction force on the applied load, in good agreement with the classical Amontons’ law. The previous AFM study by Biggs et al. has shown that the friction dependence on load can obey a linear or the Hertz law, depending on the surface properties.19 The friction coefficient value, µ ) 0.39, given by the data fitting in Figure 2a, is in good agreement with previous results.24 More intriguing here is the friction dependence on the scanning velocity presented in Figure 2b. With the initial increase of scan rate, there is a sharp decrease of the frictional force,

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Figure 3. Lateral force as a function of the applied load for a 6.8-µm silica sphere interacting with a silica wafer in pure water, 10-2 M LiCl, 10-1 M LiCl, and 1.0 M LiCl solutions.

Figure 2. Lateral force for a 6.8-µm silica particle interacting with a silica wafer in pure water: (a) Dependence of friction on the applied load at a fixed scan rate of 2 µm/s, and (b) dependence of friction on the scan rate at a fixed applied load of 175 nN. Data were taken at different places of the silica wafer.

followed by an almost saturated value above the range of 20 µm/s. Recently, Sherge et al.35 and Optiz et al.36 have done a series of nanotribology experiments using silicon oxide surfaces in ultrahigh vacuum and in the presence of controlled water vapors. They demonstrated that the friction is lowered with increasing adsorption of water on the surfaces, suggesting that two surfaces contact directly at the beginning but, as the adsorbed water residing between the sliding surfaces increases, the water acts as a boundary lubricant. These effects were strongly pronounced when hydrophilic surfaces were used.36 In a similar way, we can suggest here that the hydrated water layer shielding the surfaces from direct contact acts like a lubricant. The rate-dependent curve in Figure 2b resembles the transition between boundary and hydrodynamic lubrication in the classical Stribeck curve for the lubrication between solid bodies.37 At the same time, the velocity at which the hydrodynamic lubrication should take place gives about 4 orders of magnitude higher values (about 10 m/s) if the viscosity of the bulk water is employed in the estimation.37 However, this assumption may not be applicable, because of the anomalously high viscosity of the water layer adjacent to surfaces suggested by some studies.38,39 An alternative mechanism for the rate dependence of friction was also suggested; the friction is (35) Scherge, M.; Li, X.; Schaefer, J. A. Tribology Lett. 1999, 6, 215. (36) Opitz, A.; Ahmed, S. I.-U.; Schaefer, J. A.; Scherge, M. Wear 2003, 254, 924. (37) Persson, B. N. J. Sliding Friction: Physical Principles and Applications (Nanoscience & Technology Series); Springer-Verlag: Berlin and Heidelberg GmbH & Co. K, 2000. (38) Kim, H. I.; Kushmerick, J. G.; Houston, J. E.; Bunker, B. C. Langmuir 2003, 19, 9271.

high at the lower scan rate because of the sintering (chemical bonding) between the two surfaces,3,5,28 but the friction is lowered at the higher scan rate because two surfaces do not have enough time to sinter and bond to each other. However, we excluded this mechanism because the sintering processes are usually manifested at longer time scales and there was no adhesion between the surfaces in the range of the applied loads and contact times studied. Further investigations was carried out, as shown in the succeeding section, to confirm that the effects of scan rate are related to the property of the layer of water molecules, by repeating the experiments in various electrolyte solutions. Friction in Electrolyte Solutions. Dependence of the frictional force on the applied load is compared between pure water and LiCl solutions of various concentrations in Figure 3. When the medium had to be changed, the electrolyte solutions of higher concentration were consecutively injected into the AFM cell. After finishing the measurements of the highest electrolyte concentration, the cell was flushed thoroughly with pure water to check whether the newly measured values in pure water agree with those of the initial measurements. It is found that the difference is within the error range. As shown in Figure 3, the value of frictional force is reduced slightly from pure water to 10-2 M LiCl solution, but the significant reduction was observed when the concentration is changed from 10-1 M to 1 M. This indicates that the lower friction is related to the adsorption of the Li+ cations on the surface. Following a charge regulation model, the ion density of lithium adsorbed on the surface can be estimated to exceed the density of hydrogen ions adsorbed on the surface, when the bulk concentration is above 10-2 M to 10-1 M LiCl.13 Comparison of this charge regulation model and the data in Figure 3 implies that the lateral force is affected significantly by adsorbed Li+, once the adsorption sites on the silica surface are predominantly occupied by Li+ cations. This friction lowering by the adsorbed layer of cations is in good agreement with the recent experimental results of Raviv and Klein.21 They have shown, using a sensitive surface force balance and mica surfaces, that the hydrated layer of adsorbed cations acts as a highly efficient lubricant between mica surfaces in concentrated electrolyte solutions, and were suggesting that the lubrication effect is due to the high mobility of water molecules inside the (39) Raviv, U.; Perkin, S.; Laurat, P.; Klein, J. Langmuir 2004, 20, 5322.

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Figure 4. Lateral force of a 6.8-µm silica particle interacting with a silica wafer in pure water and CsCl, NaCl, or LiCl solutions of 1 M: (a) dependence of friction on the applied load at a fixed scan rate of 2 µm/s, and (b) dependence of friction on the scan rate at a fixed applied load of 175 nN.

hydration layer residing between the sliding surfaces. Compared with the friction mechanism for mica surfaces, the frictional mechanism for the silica surfaces is less straightforward. In the case of mica surfaces, there is no hydration interaction in pure water, but the adsorbed cations in solutions of high electrolyte concentrations prevent the direct contact of surfaces and act as a lubricant at the same time. In the case of silica surfaces, the shortrange interaction prevents the direct contact of surfaces, even in pure water, as shown in the previous section. However, the lateral force measurements indicate that the hydrated layers supported by the adsorbed Li+ cations lubricate more effectively than the hydrated layers in pure water. To gain further understanding of the lubrication mechanism of cations, we have measured the lateral forces in solutions containing cations of different hydration enthalpy. In Figure 4a and 4b are shown the results of the friction force versus applied load and the friction force versus scan rate for pure water and LiCl, NaCl, and CsCl solutions of 1 M. The data in Figure 4 clearly show the following: (1) all the cations show better lubrication than pure water, and (2) the degree of lubrication follows the order of hydration of cations, that is, Li+ > Na+ > Cs+. The rate dependence of the lateral force demonstrated in Figure 4b is also dependent on the type of cation. This suggests that these rate effects are related to the structure of cations within the confined space between surfaces, but not to the gel layers on the silica surface. We conducted previously the measurements of the normal interaction and adhesive forces between silica particle and mica surfaces in the same solutions employed here, although the substrate surfaces were cleaned by a different procedure.14 Even though the system was not

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Figure 5. Schematic representation of the hypothetical frictional mechanisms.

completely the same as the present one, the results obtained there may be used to speculate the friction mechanism of the present study, because both results are somehow related to the surface microstructures. The important findings in the previous study are as follows. (1) The amount of disruption of the adsorbed layer increases and the distance between the surfaces decreases as the time of contact at a given normal force increases. (2) The thickness of adsorbed layer increases with increasing electrolyte concentration. (3) Highly hydrated cations, such as Li+, form a thick but soft adsorbed layer, but poorly hydrated cations, such as Cs+, form a firm adsorbed layer on the surface. These adhesive force results might be related to the friction results, as follows. According to the above result (1), as the scan rate increases, the contact time of two surfaces is shortened, so that the degree of disruption of the adsorbed layer decreases, that is, the gap between sliding surfaces increases. This may explain why the friction force decreases with increasing scan rate. The above result (2) indicates that the gap between sliding surfaces increases with increasing electrolyte concentration. This may explain why the friction force is reduced with increasing electrolyte concentration, as shown in Figure 3. However, these analogies cannot be made so easily explaining the dependence on the cation type, because contrary to the friction trend the less hydrated cations were found to be more effective in lowering the adhesion. This comparison demonstrates that whether the thickness of the confined layer is of significant importance, for the case of lateral interaction the mobility of the water molecules trapped inside can also be an important factor. On the basis of our experimental findings and results from previous studies, the possible mechanisms for confined liquid layers between the sliding silica surfaces at different solution conditions are schematically illustrated in Figure 5. In the case of pure water, there is

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at least one layer of water molecules strongly adsorbed on the silica surfaces, as shown in Figure 5a. However, this layer is relevantly thin and firmly adsorbed to the silica surfaces so that it behaves like a rigid structure. In the case of high electrolyte concentrations, there is a thick hydration layer separating the surfaces. According to the Raviv and Klein model for mica surfaces, the water molecules in the layer with hydrated cations are difficult to remove from the underlying ions, but they remain very fluidlike in a lateral direction and promote the lubrication.21 The smaller Li+ cations have a thicker and more effective lubricating shell composed of hydrated water molecules, but the bigger Cs+ cations have a thin and less effective lubricating shell, as schematically shown in Figure 5b and 5c. Because of the complexity and uncertainty of the silica surface, the proposed model is rather preliminary and may be oversimplified. Future detailed experiments and precise molecular dynamic simulations will be necessary to confirm the validity of the proposed mechanism. Our work demonstrates that the microstructure of liquid molecules confined between sliding surfaces can have a dramatic effect on the nanotribological behavior. Because of the wide abundance of electrolyte solutions in many technological processes and in the living systems, our findings are expected to have some important practical implications.

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Conclusions Measuring the lateral interaction between silica surfaces in pure water and electrolyte solutions of monovalent cations with different hydration enthalpy, several strong effects were demonstrated. In pure water, the friction force was found to increase linearly with the applied load at a fixed scan rate. At fixed applied load, the friction decreases with the increase of the scan velocity and reaches a saturation value at higher scan rates. This scan-rate dependence cannot be explained using viscous properties of bulk water, and it is suggested that the layer of hydrated cations and water molecules preventing the direct contact of surfaces plays an important role in the nanotribological behavior of the system. When an electrolyte was added to the solution and surfaces are predominantly covered by hydrated cations molecules, a significant lowering of the frictional coefficient occurs. An important finding was that the lubrication property can be related to the hydration capacity of the cations adsorbed on surfaces; the smaller and more hydrated the cations are, the stronger the lubrication is. The most probable mechanism is that the lubrication is attributable to the lateral mobility of the water molecule in the hydration shell of adsorbed cations. LA047609O