Molecular layering of water in thin films between mica surfaces and its relation to hydration forces

The force as a function of distance between mica surfaces in aqueous KC1 solutions has been measured with particular attention given to the forces at separations below 2 nm. As previously reported, the forces in dilute electrolyte solutions are well described by the DLVO theory (i.e., repulsive double-layer forces and attractive van der Waals forces), but above a certain electrolyte concentration an additional short-range repulsive hydration force arises as hydrated cations adsorb to the mica surfaces. As more cations adsorb, the hydration force increases in both magnitude and range (attaining 4–5 nm). We now find that the repulsive hydration force is not purely monotonic, but has an oscillatory component superimposed on it which is particularly pronounced at separations below about 1 nm. The periodicity of the oscillations is 0.25 ± 0.03 nm, corresponding to the diameter of water molecules. The results are compared with those obtained using other, nonaqueous, liquids and with the crystalline swelling properties of clays. The finding that hydration forces are oscillatory at short-range carries implications for the theoretical understanding of hydration effects in general, and its significance for other systems is discussed.