Rate Theory of Ion Pairing at the Water Liquid–Vapor Interface

Abstract

There is overwhelming evidence that certain ions are present near the liquid–vapor interface of aqueous salt solutions. Despite their importance in many chemical reactive phenomena, how ion–ion interactions are affected by interfaces and their influence on kinetic processes is not well understood. We carried out molecular simulations to examine the thermodynamics and kinetics of small alkali halide ions in the bulk and near the water liquid–vapor interface. We calculated dissociation rates using classical transition-state theory and corrected them with transmission coefficients determined by the reactive flux method and Grote–Hynes theory. Our results show that in addition to affecting the free energy of ions in solution, the interfacial environments significantly influence the kinetics of ion pairing. Results on the relaxation time obtained using the reactive flux method and Grote–Hynes theory present an unequivocal picture that the interface suppresses ion dissociation.