Stabilization of Hydroxide Ion at Interface of Hydrophobic Monolayer on Water via Reduced Proton Transfer

We report a joint study, using sum-frequency vibrational spectroscopy (SFVS) and ab initio molecular dynamics (AIMD) simulations, respectively, on (sub-)monolayer hexane/water interface with varied vapor pressures of hexane and different pHs in water. We show clear evidence that hexane on water revises the interfacial water structure in a way that stabilizes the hyper-coordinated solvation structure and slows down the migration of hydroxide ion (OH−) relative to that in bulk water. This mechanism effectively attracts the OH− to the water-hydrophobic interface with respect to its counter-ion. The result illustrates the striking difference of proton transfer of hydrated OH− at the interface and in the bulk, which is responsible for the intrinsic charging effect at the hydrophobic interface.