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 $p\mathrm{Hs}$ 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 (${\mathrm{OH}}^{-}$) relative to that in bulk water. This mechanism effectively attracts the ${\mathrm{OH}}^{-}$ to the water-hydrophobic interface with respect to its counterion. The result illustrates the striking difference of proton transfer of hydrated ${\mathrm{OH}}^{-}$ at the interface and in the bulk, which is responsible for the intrinsic charging effect at the hydrophobic interface.

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• Interface & surface thermodynamics