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

TL;DR

This paper introduces a novel sum-frequency spectroscopic technique with variable photon momenta to analyze the microscopic structure of bonded water layers at charged interfaces, revealing hidden water species and polarization effects.

Contribution

The study develops an all-optic, momentum-resolved SF spectroscopy method to characterize interfacial water structures and charge distributions with high spatial and temporal resolution.

Findings

Detection of weakly-donor-H-bonded water species at surfactant-water interfaces

Observation of highly polarized bonded water layers at lipid monolayers

Negligible diffuse layer contribution in certain zwitterionic interfaces

Abstract

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.