The Role of Ions on the Surface-Bound Water Structure at the Silica/Water Interface: Identifying the Spectral Signature of Stability

Abstract

We explore the influence of salt addition on the structure of water interacting closely with a charged silica surface. Isolating these surface effects is challenging, even with surface-specific techniques like sum frequency generation (SFG), because of the presence of aligned water nanometers to microns away from the charged silica. Here we combine zeta potential and SFG intensity measurements with the maximum entropy method and reported heterodyne second harmonic and sum frequency generation results to deconvolute from the total signal intensity the SFG spectral contributions of the waters adjacent to the surface. Deconvolution reveals that at very low ionic strength the surface water structure is similar to that of a neutral silica surface near the point of zero charge with waters oriented in opposite directions. This result suggests the known metastability of silica near the PZC and the stability of silica in low ionic strength solutions may originate from the same source, these oppositely oriented surface-bound waters. Orientation changes are induced upon adding salt, which lead to a decrease in the total amount of aligned water at the surface.

NaCl, MgCl2, and AlCl3 Surface Coverages on Fused Silica and Adsorption Free Energies at pH 4 From Nonlinear Optics

“…The latter two techniques circumvent the strong absorber problem of water in the ultraviolet and midinfrared regions, which prevents the probing of most inorganic ions at surfaces using conventional electronic or vibrational spectroscopic techniques (notable exceptions are oxyanions like arsenate, [10][11][12] chromate, 13 phosphate, 14 or sulfate.) 15 Interface-specific vibrational spectroscopies such as sum frequency generation probe the response of water's O-H oscillators in the interfacial region to the presence of oxy- 16 and non-oxy ions, [17][18][19][20][21] from which metal cation number densities, or even relative surface coverages, have not yet been quantified due to absorptive-dispersive mixing of the Stern and diffuse layer contributions to the nonlinear optical response. [22][23][24] Electronic spectroscopies are challenging as most common inorganic ions do not possess strong enough electronic transitions that are readily accessible at buried aqueous interfaces.…”