Water Structuring and Hydroxide Ion Binding at the Interface between Water and Hydrophobic Walls of Varying Rigidity and van der Waals Interactions

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 Å 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