ION SOLVATION DYNAMICS IN BINARY-MIXTURES

The dynamics of selective ion solvation in binary Stockmayer solvents is investigated using molecular dynamics simulations. The dependence o f the usual solvation response function, S(t), on solvent composition and on the relative polarity of the solvent species is examined and di scussed. We also introduce particle solvation response functions which describe the compositional relaxation of the first solvation shell. I t is shown that the selective solvation process can be well described by a simple phenomenological model based on the ideas of elementary ch emical 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 equ ilibrium value, and a slower spatial redistribution process during whi ch the composition of the first shell achieves equilibrium. The redist ribution phase depends on the rate of mutual ion-solvent diffusion and also on the rate of particle exchange between the first and second sh ells. A detailed analysis of the exchange process indicates that excha nges occur on virtually a one-to-one basis with the insertion of a str onger dipole into the first shell being mirrored by an almost immediat e ejection of a weaker one. (C) 1997 American Institute of Physics.