Investigating hydroxide anion interfacial activity by classical and multistate empirical valence bond molecular...

Molecular dynamics simulations were carried out to understand the propensity of the hydroxide anion for the air-water interface. Two classes of molecular models were used, a classical polarizable model and a polarizable multistate empirical valence bond (MS-EVB) potential. The latter model was parametrized to reproduce the structures of small hydroxide-water clusters based on proton reaction coordinates. Furthermore, nuclear quantum effects were introduced into the MS-EVB model implicitly by refitting its potential energy function to account for them. The final MS-EVB model showed reasonable agreement with experiment and ab initio molecular dynamics simulations for dynamical and structural properties. The free-energy profiles for both the classical and MS-EVB models were mapped out across the air-water interface, and the classical model gave a higher free energy at the interface with respect to bulk. However, the MS-EVB model gave little free-energy difference between when the hydroxide anion was in the bulk and when it was present at the air-water interface with its oxygen fully solvated and its hydrogen pointing toward the vapor. When the hydroxide oxygen started to desolvate, the free energy increased dramatically, suggesting that the hydroxide anion can be found in the interfacial region.