PNNL

Abstract

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 multi-state empirical valence bond (MS-EVB) potential. The latter model was parameterized 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. The MS-EVB model gave a hydroxide anion that approached very close to the interface before it had a sharp increase in free energy at the Gibbs dividing surface. This showed a hydroxide anion that was present at the interface, but strongly repelled from its outer edge near the air. This work was supported by the US Department of Energy's Office of Basec Energy Sciences, Chemical Sciences program. Pacific northwest national Laboratory is operated by Battelle for DOE.