Ab Initio Study of Water Polarization in the Hydration Shell of Aqueous Hydroxide: Comparison between Polarizable and...

Ab initio simulations of aqueous hydroxide are performed to study the structure and polarization of water molecules in the first solvation shell. Polarization is found to depend on the configuration of the hydrogen-bond (HB) donors. In the most common case of four HB donors, the dipole moment of water molecules is much larger than those in the first shell of monovalent ions. When there are only three HB donors, the water dipole moment exceeds even those in the first shell of a divalent cation. We also show that the dipole fluctuations in the first hydration shell of hydroxide are reduced compared to bulk water, which can provide a rationale for the propensity of hydroxide for interfaces with hydrophobes. Because of its unique properties, hydroxide provides a nontrivial test for benchmarking classical models. Comparison of the ab initio results with those obtained from the classical models indicates that the latter need to be further improved in order to yield reliable results.