Interfacial behavior of hydroxide ions has been investigated by means of molecular
dynamics simulations of aqueous KOH solutions between hydrophobic carbon-like walls. In
agreement with previous calculations we show that a rigid, attractive wall strongly structures
water molecules in neighboring hydration layers, leading to a concentration peak of hydrated
OH- ions located at about 5 . from the wall. However, allowing for thermal motion of the
wall atoms, as well as suppressing the van der Waals interactions between the wall and water
hydrogen atoms strongly reduces both water structuring and the anionic peak in the interfacial
region. We infer that soft hydrophobic environments with weak dispersion interactions with
water are not expected to exhibit an appreciable structuring effect on interfacial water
molecules. Hence, the mechanism for OH- adsorption operative near a hard attractive wall
may not be applicable to soft aqueous interfaces including the limiting case of the water/air
interface.