The Effect of Salt on the Water Structure at a Charged Solid Surface: Differentiating Second- and Third-order Nonlinear Contributions

We have used visible-infrared sum-frequency generation spectroscopy to reveal fundamental characteristics of water structure at the fused silica surface. By studying a wide range of ionic strengths, from 0.05 mM to 4 M, we are able to comment on the contributions of second- and third-order nonlinearities to the spectroscopic response. 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.