Ion Solvation in Water–Acetonitrile Mixtures

The results of a preliminary molecular dynamics study of ion (Na+ and Cl&) solvation and association in water, acetonitrile, and aqueous acetonitrile are dis- cussed. These mixtures are compositionally heterogeneous on the scale of a few molecular sizes, and this heterogeneity is expected to be an important factor governing ion solvation and association. Results for ions in pure water and pure acetonitrile are preseInted for ambient conditions. The degree of association is low in water, a high-density fluid, and high in acetonitrile, a low-density fluid. A separate set of simulations for water at supercritical temperatures demon- strates that the fluid density is more important than the temperature in deter- mining the degree of ion association. Preliminary results for the mixtures are inconclusive except that they indicate the existence of an important sampling issue related to the compositional heterogeneity of the mixtures. An approach to resolving the sampling issue is suggested.