Specific effects of divalent cation nitrates on the nanotribology of silica surfaces

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⁺, Sr ²⁺, Ca²⁺, and Mg²⁺ nitrate solutions. The measurements demonstrated strong lubrication effects for solutions of Ba ²⁺, 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.