Reaction rate theory approach to thermodynamic state dependence of hydration shell exchange for Li+(Aq)

Hydration shell exchange of Li+(aq) is analyzed from the standpoint of reaction rate theory for a wide set of thermodynamic conditions, with an emphasis on the supercritical regime, viewing the exchange as an association-dissociation process. It is found that the free energy dependence upon the reaction coordinate of the ion-water complex maintains similar features in ambient and supercritical water, in contrast with related activated processes such as ion pair association. The activation free energy increases with decreasing density (with an inflection point ca. 0.3 g/cm(3)) although it does not parallel the strong decrease in dielectric constant that takes place, at variance with continuum theory. The substantial increase in exchange rate from ambient to supercritical conditions cannot be simply ascribed to the temperature difference, but to an interplay of temperature and thermodynamic state dependence of the activation free energy, while the dynamic features of the exchange are substantially independent of bulk properties. The present system provides a first computational test of Transition State Theory in supercritical fluids, showing that it overestimates the rate constant by approximately a factor of 2, being slightly more successful than in ambient water.