A note on surface spectroscopic studies of charged water interfaces

This article assesses the value of sum-frequency vibrational spectroscopy (SFVS) as a technique to interrogate charged water interfaces. Despite extensive studies using this technique in the past two decades, our knowledge gleaned on the structure of these interfaces is still very limited. Recently, the development of dual phase-sensitive (PS) SFVS and the combined PS SFVS and difference frequency vibrational spectroscopy (DFVS) techniques have greatly enhanced the sensitivity for detecting the scale of the interfacial depth profile, but the detailed structural variation of the depth profile is still not susceptible to being explored except for simple cases of low bulk ion concentrations. We suggest here that by extending the technique of combined PS-SFVS and DFVS to several different pairs of sum and difference frequencies, we could extract, through spectral analysis, a set of vibrational spectra in connection with a set of consecutive individual sub-layers of a charged water interface. Theoretical fitting of these spectra could then reveal the approximate depth variation of the average microscopic structure of the interface.