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Momentum-Resolved Sum-Frequency Vibrational Spectroscopy of Bonded Interface Layer at Charged Water Interfaces

Interface-specific hydrogen- (H-)bonding network of water next to a substrate (including air) directly controls the energy transfer and chemical reaction pathway at many charged aqueous interfaces. Yet, experimental characterization of such bonded water layer structure is still a challenge due to the presence of the ion diffuse layer. We now develop a sum-frequency (SF) spectroscopic scheme with varying photon momentums as an all-optic solution for retrieving the vibrational spectra of the bonded water layer and the diffuse layer, and hence microscopic structural and charging information about an interface. Application of the method to a charged surfactant-water interface reveals a hidden weakly-donor-H-bonded water species, suggesting an asymmetric hydration-shell structure of fully solvated surfactant headgroups. In another application to a zwitterionic phosphatidylcholine (PC) lipid monolayer-water interface, we find a highly polarized bonded water layer structure associating to the PC headgroup, while the diffuse layer contribution is experimentally proven to be negligible. Our all-optic method offers not only an in situ microscopic probe of the electrochemical and biological interfaces, but also a new opportunity for promoting these researches toward high spatial and temporal resolutions.