Water structuring and hydroxide ion binding at the interface between water and hydrophobic walls of varying rigidity and van der waals interactions

The 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 about 5 angstrom 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.