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 16 -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 (H 3 O + ) 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 (H 3 O + ) 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 H 3 O + 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.