Effects of solvent exclusion on the force between charged surfaces in electrolyte solution

Effects of hard-core exclusions of solvent molecules on the force between charged surfaces immersed in electrolyte solution are investigated theoretically. A three-component model of electrolyte solution is used, in which the solvent is modeled as a fluid of neutral hard spheres. A free energy density functional theory, which has been applied previously to study the interaction between primitive electrical double layers, is extended to include ion–solvent and solvent–solvent interactions. Hard-core correlations are incorporated in a nonlocal free energy density functional, while electrostatic correlations are evaluated using the mean spherical approximation. The force between charged surfaces as a function of surface separation is found highly oscillatory, with a period approximately equal to the particle diameter. Because of the dominant role of the hard-core exclusions of the solvent fluid, the force does not depend strongly on either electrolyte concentration or surface charge density.