NON-DLVO FORCES BETWEEN SURFACES -SOLVATION, HYDRATION AND CAPILLARY EFFECTS

Abstract

Direct measurements of surface forces not explicable within the framework of DLVO-theory are reviewed. Emphasis is placed on the interaction of molecularly smooth mica surfaces in both non-aqueous liquids and water. The results show that DLVO-theory does not hold at surface separations below 5 nm, with one apparent exception. This is mica in dilute electrolyte solutions in water, where the surface properties are determined by hydrogen ion adsorption. At high electrolyte concentrations in water and in all non-aqueous liquids oscillatory solvation forces, related to the finite size of molecules, replace the continuum Van der Waals force at short range. By contrast, the double-layer interaction appears to be well described by the non-linear Poisson-Boltzmann equation in all but a few special cases. In binary mixtures of incompletely miscible liquids the short-range forces may be dominated by capillary condensation. Between hydrophobic surfaces in water extremely long-range (~70 nm) deviations from DLVO-theory are found. Hydrophobic surfaces in close proximity give rise to cavitation - like capillary condensation an example of a surface-induced phase transition.