Comparing hydroxide and hydronium at the instantaneous air-water interface using polarizable multi-state empirical valence bond models

Abstract

A new molecular model for the hydroxide anion was developed and its propensity for the air-water interface was compared with that of the hydronium ion. The multi-state empirical valence bond approach with polarizable potentials was used for both of these, allowing them to share their charge with adjacent water molecules. The hydroxide anion appeared to have a lower propensity for the air-water interface than that of hydronium when comparing its potential of mean force with respect to the Gibbs dividing surface. However, upon further inspection it was found that the hydroxide and hydronium ions had similar free energies at the instantaneous air-water interface. The reason for this discrepancy is that the hydroxide anion has a greater free energy when it was partially solvated. Or in other words, when the hydroxide anion is one molecular layer from the instantaneous air-water interface, but potentially still near the Gibbs dividing surface, it is less stable than the hydronium in a similar configuration. This shows that using the instantaneous interface can provide additional important insights.