Stabilization of Hydroxide Ions at the Interface of a Hydrophobic Monolayer on Water via Reduced Proton Transfer

We report a joint study using surface-specific sum-frequency vibrational spectroscopy and ab initio molecular dynamics simulations, respectively, on a pristine hydrophobic (sub)monolayer hexane-water interface, namely, the 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 hypercoordinated 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 counterion. 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.