The silica/aqueous interface not only plays a prominent role in the environment, but is also common in industry owing to the use of silica as substrate or stationary phase in biotechnology and separation science, respectively.  Our lab is interested in how the structure of water at this interface varies with pH and composition of the aqueous phase.  We utilize second-order nonlinear optical spectroscopies like vibrational sum frequency generation to monitor the interfacial molecular structure owing to the sensitivity of these techniques to noncentrosymmetric molecular assemblies.  Our work has revealed that adding monovalent salts at high millimolar concentrations (100 mM and higher) can greatly influence the interfacial water structure even at low pH values that approach the point-of-zero charge of silica.  Distinct ion specific trends are observed in the water structure, but these trends are highly pH dependent indicating that changing the charge density of the surface with varying pH influences the silica’s relative affinities for specific cations versus water.  This sensitivity of cation affinity to pH has implications for cation-specific electric double layer models of the interface. We also observe a striking change in affinity for organic molecules like acetonitrile depending on the pH exposure of the surface.  Finally, I will discuss how the combination of divalent ions and pH influences the water structure at silica, of relevance to environmental remediation of the tailings waste products of mining processes.

Exploring the interfacial structure of water at buried silica/aqueous interfaces with nonlinear optical spectroscopy

2019 Fall National Meeting

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