Solvation forces in charged fluids

Abstract

From an exact expression for the free energy of a non-uniform fluid mixture a closure approximation for the inhomogeneous direct correlation functions is used to develop a theory of solvation forces in charged fluids based upon non-linear equations for the equilibrium ion number densities. In the limit of point ions, the expressions obtained reduce to those of the Poisson-Boltzmann theory of electrolytes. The numerical results obtained for a restricted primitive model electrolyte are compared with those of earlier work based on linear response theory and Poisson-Boltzmann theory with a simple Stern layer modification. At low electrolyte concentrations the agreement between all three theories is good. But at high electrolyte concentrations the Poisson-Boltzmann theory with a simple Stern layer correction fails to display the oscillations in the solvation force which characterize both the linear and non-linear theories.