Unveiling Microscopic Structures of Charged Water Interfaces by Surface-Specific Vibrational Spectroscopy.

A sum-frequency spectroscopy scheme is developed that allows the measurement of vibrational spectra of the interfacial molecular structure of charged water interfaces. The application of this scheme to a prototype lipid-aqueous interface as a demonstration reveals an interfacial hydrogen-bonding water layer structure that responds sensitively to the charge state of the lipid headgroup and its interaction with specific ions. This novel technique provides unique opportunities to search for better understanding of electrochemistry and biological aqueous interfaces at a deeper molecular level.