The silica/aqueous interface is of great importance owing to its relevance to the environment and use in many industrial applications. Accordingly, there have been many studies including those using second-order nonlinear optical methods to probe the interfacial structure at the silica/aqueous interface in the presence of different salt concentrations, salt identities, and pH. One challenge that has remained is distinguishing the response of water immediately adjacent to the surface from water in the diffuse layer, particularly at lower salt concentrations when the Debye length extends some distance from the surface. Here we measure vibrational sum frequency generation (SFG) at the silica/aqueous interface as a function of salt concentration and at varying pH. We use the maximum entropy method with referencing to phase-sensitive sum frequency generation spectra as well heterodyne-second harmonic generation measurements to determine the complex SFG spectra. Complementary zeta potential measurements were also performed under the same experimental conditions to allow us to separate the diffuse layer (chi(3)) response from the interfacial structure of water at the surface (the chi(2) response). Using this experimental and analytical approach, we observe distinct behavior of water near the surface and further in the diffuse layer. For example, we observe that near neutral pH the surface water structure exhibits similarity to the surface water structure at the point of zero charge of silica (pH 2), which is not obvious from the SFG spectrum at pH 6 prior to the analysis, as the distinct contribution of the diffuse layer to the total spectrum is substantial at pH 6 and very low salt concentration. We will also discuss how the water in the Stern layer evolves with varying pH and constant salt concentration. All of this work builds on the historic, foundational contributions of Professor Ken Eisenthal and the development of his chi(3) technique, which has proven to be a powerful method to study the structure at mineral oxide/aqueous interfaces.

Separating the interfacial structure of water at the silica surface and in the diffuse layer with nonlinear optical spectroscopy

ACS Fall 2021

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