Molecular theory of solvated ion dynamics

We present a simple molecular approach for understanding the dynamics of a single ion in a polar solvent. The approach is based on a heuristic development starting from a resolution of the force–force correlation function of a Brownian ion, into parts depending on the attractive ion–solvent interaction and a repulsive, hard core ion–solvent interaction. We show that in the limit of infinitely weak, long range attractive ion–solvent interactions the molecular theory reduces to the phenomenological dielectric friction picture for the drag on an ion. The molecular theory also gives sensible results in the case of strong, short range attractive interactions. In this case the theory essentially indicates that the drag on the ion reduces to the drag on a composite body, i.e., a solventberg consisting of the ion and the molecules in the first solvation shell.