Hydronium Behavior at the Air-Water Interface with a Polarizable Multistate Empirical Valence Bond Model

Molecular dynamics simulations were carried out to understand the propensity of the hydronium ion for the air-water interface with a polarizable multistate empirical valence bond (MS-EVB) model. Reasonable agreement with experiment for radial distribution functions and very good agreement for hydronium diffusion were found for the model. The polarizable MS-EVB model had no free energy minimum at the air-water interface. However, when polarizability on the hydronium ion alone was removed, a free energy of around -1.5 kcal/mol was calculated at the air-water interface. This discrepancy was found to be due to the behavior of water molecules in the first solvation shell of a hydronium ion. These water molecules contained a moderate amount of hydronium character, resulting in the delocalization of the hydronium ion. For the system with polarizable hydronium ions, this delocalization was the same at the interface as in the bulk, but for the system without polarizable hydronium ions, the delocalization increased as the hydronium approached the air-water interface. This delocalization results in a stabilization of the hydronium charge and moves it more toward the bulk, increasing its propensity for the air-water interface when hydronium ion polarizability is removed.