Formation and migration of H3O+ and OH− ions at the water/silica and water/vapor interfaces under the influence of a static electric field: a molecular dynamics study

Abstract

The atomistic mechanisms of proton transport under the influence of a static electric field at various angles to the water/silica glass interface were simulated using a reactive, all-atom potential. The fields were shown to change the structure of the 20 Å water film significantly, as well as the concentrations and distributions of H₃O⁺ and OH⁻ ions in the film. The field was less than that needed for the dissociation of the water molecule, so the presence of these ions was caused by the interactions with the silica surface. While excess protons at certain silica surface sites can be highly unstable (rattling between adjacent surface sites), protons attached to surface sites that only sample other surface sites are shown to be less mobile in comparison to H₃O⁺ and OH⁻ ions in the water film. After creation of H₃O⁺ and OH⁻ at the silica surface, these ions were observed to have greater mobility away from the glass surface compared to near it. Fields parallel to the glass surface were shown to greatly enhance mobilities of OH⁻ ions. Very high ion mobilities were observed at the water–vapor interface under field orientations of −45° and +45° (relative to the surface plane) respectively. These field orientations are able to pin charges to the vapor interface in addition to dragging them along it. Both vehicular and structural diffusion of the H₃O⁺ and OH⁻ ions were determined as a function of location in the water relative to the silica and vapor interfaces. The results indicate the importance of the orientation of a field to a glass surface and the water vapor interface on proton and ion transport in unsaturated pores.