Interactions at charged surfaces are a major part of the chemistry of many naturally occurring, solid-liquid interfaces.  Detailed structural information at these interfaces may be revealed by vibrational sum-frequency spectroscopy.  However, it has been shown that the addition of a surface charge complicates the interpretation of the spectroscopic results.  We present a model of the second- and third- order nonlinear response at charged surfaces based upon measurements taken at the aqueous NaCl-fused silica interface over a wide range of ionic strengths, from 0.05 mM to 4 M.  Spectra obtained from extremely dilute salt concentrations provide evidence of the previously sought increasing surface charge with ionic strength. This is followed by a screening regime where the extent to which the surface field penetrates into the bulk is limited by the electrolyte. Data from intermediate salt concentrations reveal a few strongly ordered layers of water immediately adjacent to the surface. At high concentrations, we observe a significant disruption of solvent ordering. Together, the observation of these four distinct regimes provides a unified understanding of interfacial water structure in the presence of salt that consolidates previous reports in the literature.

Interfacial water structure at the aqueous salt-fused silica interface

243rd National Meeting (2012)

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