Oscillatory surface forces: A test of the superposition approximation

We report on the density functional theory calculations of electrolyte solutions confined between charged plates. Experiments with the surface forces apparatus have shown that for charged cylinders in simple solvents the surface force is oscillatory at short range. It has been suggested that the total force be considered a superposition of the electrostatic repulsion, the solvation force of a pure solvent, and the van der Waals attraction. In this paper we use the Rosenfeld-Kierlik-Rosinberg (RKR) functional to study a three-component electrolyte solution and explicitly test the superposition approximation, We find that the superposition on approximation is a good approximation when the surface charge is low and the salt concentration is high. We also find that a three-component electrolyte can display short-range equilibrium separations in the absence of van der Waals interactions. (C) 1996 American Institute of Physics.