Note on standard free energy of transfer and partitioning of ionic species between two fluid phases

We investigate the partition of ionic species in a system of two phases that are immiscible even in the limit of vanishing ionic concentrations. An analytic expression for the solvation free energy and for the standard free energy of transfer is obtained for dipolar hard-sphere solvents in the mean spherical approximation. We find that both dipolar hard-sphere solvents and continuum solvent models yield an electric potential difference (equilibrium junction potential) between two pure solvents in contact that is identically zero. Simple analytic expressions for partition coefficients and junction potentials are obtained in the ideal-solution limit for these solvent models. We note that for the junction potential the ideal solution limits differ in general from the pure-solvent values. For nonideal solutions, approximations based on the mean spherical approximation can be applied conveniently to calculate the activities of ionic species and to obtain the partition coefficients.