(127d) Effect of the Medium On the Thermodynamics of Ion-Water Clusters

Ion-water clusters bridge the gap between an ion in the gaseous state and a fully solvated ion. Hence thermochemical experiments on ion-water clusters provide a direct route for estimation of single ion hydration free energies. However, inferences about structural aspects of hydration can be drawn only when data from these experiments are interpreted in the light of molecular theories of hydration. Primitive quasichemical theory, an approach based on the regularization of the potential distribution theorem is one such theoretical approach which explicitly considers ion-water clusters. It obtains estimates of the sum of the free energy of forming the ion water cluster from its component ligands and the free energy of its subsequent solvation for different ion-water clusters. The lowest estimate thus obtained, corresponds to the hydration free energy of the ion. The ion-water cluster corresponding to this estimate is thermodynamically the most stable coordination state. In the present study we revisit some of the approximations entailed in the primitive quasichemical approach and elucidate the role of the medium. We find that the medium promotes a tighter binding of the water ligands comprising the cluster to the central ion. Hence the free energy of formation of an ion-water cluster in aqueous phase is lower than the free energy of formation of a similar cluster in gaseous phase. Since similar medium effects are expected to be important in hybrid models of solvation our results are of broader relevance to the cluster community in general.