Solvation forces in charged fluids

Abstract

A formalism based on linear response theory is used to obtain an expression for the free energy of a non-uniform charged fluid in terms of the local ion number density and the bulk direct correlation functions. When the fluid is a restricted primitive model electrolyte the free energy splits into two independent parts, the minimization of which leads to expressions for the equilibrium charge and density distributions. From the free energy an expression for the force between two thick plates immersed in an electrolyte is obtained. In the limit of point ions, the expressions we obtain reduce to those of the Debye-Hückel theory of electrolytes. The equations are solved numerically and at low bulk electrolyte concentrations the monotonically decaying repulsive force of the classic Verwey and Overbeek results is found. But at higher concentrations and larger inverse Debye screening lengths the force displays pronounced oscillations. Correspondingly, the electric potential displays oscillations which have consequences for the zeta potential