Throwing Salt into the Mix: Altering Interfacial Water Structure by Electrolyte Addition.

Interfacial water commonly distinguishes itself from the bulk phase by adopting a polar, ordered structure. It is well-established that electrolytes can act to perturb this structure; however, the nature of this perturbation remains a topic of interest. In this study, surface- and structure-sensitive nonlinear vibrational spectroscopy is used to monitor electrolyte-induced changes in interfacial water structure. Solution ionic strength was varied over 5 orders of magnitude, and spectra were collected from two mineral surfaces (fused silica and calcium fluoride) and two polymer surfaces (polystyrene and poly(methyl methacrylate)). Analysis of the spectra reveals striking similarities and differences between these four aqueous interfaces; trends in overall intensity do not always follow changes in the spectral shape. Our results reveal the complex interplay between surface charge, ion adsorption, and hydrophobicity in determining interfacial water structure in the presence of dissolved ions.