The interionic potential of mean force in a molecular polar solvent from an extended RISM equation

The generalization of a recently proposed extension of the RISM integral equation to infinitely dilute isolated ions and ion pairs in polar, interaction site model, molecular solvents is outlined. An essential element of the development is the explicit separation of the contributions which yield a continuum dielectric solvent model from the remainder. Thus, it is only for this correction that one relies on the integral equation. Application is made to a system consisting of a diatomic polar solvent and atomic ions of varied charge and radius. The results of these calculations show that the present approach produces sensible qualitative features of ionic solvation including an appropriately varying degree of solvent orientational saturation with ionic charge and radius. Correspondingly, the calculated interionic potentials of mean force reproduce the same basic features manifest in already available studies of dipolar hard sphere solvents, including the positions of oscillatory features in the structure and the relatively short spatial range of the corrections to the continuum dielectric theory. A quantitative evaluation of the accuracy of the present predictions is not possible with presently available data.