(38f) Specific Effects of the Multivalent Cations on Friction between Silica Surfaces in Electrolyte Solutions

Lateral force measurements for interactions between a silica sphere attached to a rectangular cantilever and a flat silica wafer were performed using an atomic force microscope (AFM). The lateral force was measured as a function of applied normal loads and scan rate in solutions of divalent and trivalent electrolytes, chlorides and nitrates, with concentrations varying between 10^-5 and 10^-1 M. It is found that the friction coefficient is decreasing with the salt concentration increase for barium and strontium salts. This effect is reversed for magnesium salts. In the group of alkaline-earth metal salts, friction in calcium chloride has a transitional behavior: friction coefficient is decreasing up to 3x10^-3M, and at 3x10^-1 it is sharply increasing. This type of behavior is not encountered in the case of calcium nitrate, suggesting that the anion co-adsorption plays an important role in determining the tribological performance of the symmetrical silica-silica system. It is found that the friction force increases with the concentration of lanthanum chloride and nitrate increase. The results indicate that the friction coefficients measured in chloride solutions have higher values compared with nitrate solution. Additionally, we have demonstrated specific dependence of the friction force on the sliding velocity. These results imply that the tribological properties of silica in electrolyte solutions are the outcome of a combined effect of both, anions and cations, and their hydration properties. The friction trends behavior is discussed and a model is proposed.