On the Propensity of Excess Hydroxide Ions at the Alcohol Monolayer-Water Interface.

Atomistic molecular dynamics simulations have been employed to study the self-ion (H and OH) distribution at the interface between long-chain C-OH alcohol (cetyl alcohol) monolayer and water. It is well known that the free air-water interface is acidic due to accumulation of the hydronium (HO) ions at the interface. In the present study, we have observed that contrary to the air-water interface, at the long-chain alcohol monolayer-water interface, it is the hydroxide (OH) ion, not the hydronium ion (HO) that gets accumulated. By calculating the potential of mean forces, it is confirmed that there is extra stabilization for the OH ions at the interface relative to the bulk, but no such stabilization is observed for the HO ions. By analyzing the interaction of the self-ions with other constituents in the medium, it is clearly shown that the favorable interaction of the OH ions with the alcoholic -OH groups stabilizes this ion at the interface. By calculating coordination numbers of the self-ions it is observed that around 50% water neighbors are substituted by alcoholic -OH in case of the hydroxide ion at the interface, whereas in the case of hydronium ions, only 15% water neighbors are substituted by the alcoholic -OH. The most interesting observation about the local structure and H-bonding pattern is that the hydroxide ion acts solely as the H-bond acceptor, but the hydronium ion acts only as the H-bond donor.