Hydroxide anion at the air-water interface

Whether aqueous interfaces are acidic or basic has implications for interfacial chemistry, but the question remains open. We employ first-principles molecular dynamics simulations to determine the intrinsic propensity of OH^- for the air-water interface and find that OH^- is stabilized by roughly k_BT at the interface vs. the bulk. We predict, therefore, that the surface population OH^- is slightly enhanced. Our simulations suggest that the solvation of OH^- at the interface is similar to that observed in small water clusters, and they reveal changes in the orientation of solvating water molecules that are consistent with surface-sensitive vibrational spectra.