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 kBT 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.