Measurement of forces between surfaces immersed in electrolyte solutions

Abstract

The forces between two mica surfaces immersed in various aqueous electrolyte solutions have been measured. From 200 nm down to contact separations four different types of force are operating. First, there are attractive van der Waals forces; these are non-retarded up to about 6.5 nm with a Hamaker constant of (2.2 ± 0.3)× 10^–20 J. The van der Waals forces appear to be slightly stronger than expected from the Lifshitz theory. Second, there are repulsive double-layer forces. In 10^–4 mol dm^–3 1 : 1, 2 : 1 and 2 : 2 electrolytes these are well accounted for by theory but at concentrations above 10^–3 mol dm^–3 there are discrepancies with theory, especially for 2 : 1 electrolytes. Third, there are repulsive “hydration” forces; these decay exponentially with distance with the characteristic decay length of about 1.0 nm. Their magnitude varies from mica to mica and is largely independent of ionic strength. They are negligible at separations above 7.5 nm. Fourth, there are adhesion forces; these are complex and depend on a host of factors such as pH, type of electrolyte cations, mica orientation, etc., and are not simply given by extrapolating the long-range forces down to separations of the order of interatomic spacings.