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

Shanshan Yang, Mohan Chen, Yudan, Su, Jianhang Xu, Xifan Wu, and Chuanshan Tian

Submitted to Phys. Rev. Lett. (2020)

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.