Momentum-dependent sum-frequency vibrational spectroscopy of bonded interface layer at charged water interfaces

Interface-specific hydrogen (H)-bonding network of water directly controls the energy transfer and chemical reaction pathway at many charged aqueous interfaces, yet to characterize these bonded water layer structures remains a challenge. We now develop a sum-frequency spectroscopic scheme with varying photon momenta as an all-optic solution for retrieving the vibrational spectra of the bonded water layer and the ion diffuse layer and, hence, microscopic structural and charging information about an interface. Application of the method to a model 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 lipid monolayer-water interface, we find a highly polarized bonded water layer structure associating to the phosphatidylcholine headgroup, while the diffuse layer contribution is experimentally proven to be negligible. Our all-optic method offers an in situ microscopic probe of electrochemical and biological interfaces and the route toward future imaging and ultrafast dynamics studies.

Automated summary

The hydrogen-bonding network of water at charged aqueous interfaces plays a direct role in controlling energy transfer and chemical reactions. However, characterizing these water layer structures is challenging. In this study, we develop an all-optic solution using sum-frequency spectroscopy to retrieve the vibrational spectra of the bonded water layer and ion diffuse layer, providing microscopic structural and charging information about the interface. The method is applied to model surfactant-water and lipid-water interfaces, revealing hidden water species and polarized water layer structures. This all-optic method offers a promising approach for studying electrochemical and biological interfaces.