The Importance of the Water Molecular Quadrupole for Estimating Interfacial Potential Shifts Acting on Ions Near the Liquid-Vapor Interface

Interfacial electrostatic potential gradients arise from nonuniform charge distributions encountered crossing the interface. The charges involved can include the molecular charges predominantly bound to each neutral solvent molecule and distributions of ions (or electrons) free to move in the interfacial region. This paper focuses on the solvent contribution to the interfacial potential. Quasichemical theory (QCT) provides a physical framework for the analysis of near-local (chemical) and far-field contributions to ion solvation free energies. Here, we utilize QCT to analyze cavity net potentials that contribute to the single-ion real solvation free energy. In particular, we discuss the results of molecular dynamics simulations of water droplets large enough to exhibit bulklike behavior in the droplet interior. A multipolar analysis of the cavity potential illustrates the importance of the solvent molecular quadrupole due to the near-cancellation of the dipolar contributions from the cavity-liquid and liquid-vapor interfaces. The results reveal the physical origin of the previously observed strong classical model dependence of the cavity potential.