Ion hydration: linking self-diffusion and reorientational motion to water structure

Abstract

Ions affect the self-diffusion and relaxation of water. Such ion-induced changes of water dynamics have long been rationalized in terms of the change of the water structure around the ions. The aim of this paper is to establish a link between dynamics and the “water structure” on the basis of the extended jump (EJ) model for the reorientational motion of water, Eyring's transition state theory of the self-diffusion of water, and the statistical thermodynamic preferential solvation theory developed from the Kirkwood–Buff (KB) theory. Through the synthesis of the above three, we formulate a scheme to correlate the ion-induced water dynamics changes to the water structure. With this new formulation, it is shown that chaotropic or negatively hydrated ions preferentially bind the transition state of water motion thereby stabilizing the transition state, whereas the exclusion of kosmotropes or positively hydrated ions suppresses the formation of the transition state. The ion effects on water dynamics are thus analyzed in a unified manner in terms of KB integrals, which represent the (averaged) “structures” of water.