Ion solvation dynamics in binary mixtures

Abstract

The dynamics of selective ion solvation in binary Stockmayer solvents is investigated using molecular dynamics simulations. The dependence of the usual solvation response function, S(t), on solvent composition and on the relative polarity of the solvent species is examined and discussed. We also introduce particle solvation response functions which describe the compositional relaxation of the first solvation shell. It is shown that the selective solvation process can be well described by a simple phenomenological model based on the ideas of elementary chemical kinetics. This model is useful and helps in the identification of two distinct time scales present in the selective solvation process. These are associated with a rapid electrostriction step during which the total number of particles in the first shell increases to its equilibrium value, and a slower spatial redistribution process during which the composition of the first shell achieves equilibrium. The redistribution phase depends on the rate of mutual ion-solvent diffusion and also on the rate of particle exchange between the first and second shells. A detailed analysis of the exchange process indicates that exchanges occur on virtually a one-to-one basis with the insertion of a stronger dipole into the first shell being mirrored by an almost immediate ejection of a weaker one.