Mica-aqueous interface is a canonical model system for interfacial science ranging from molecular adsorption and diffusion in biology and chemistry, cloud seeding in atmospheric chemistry, mineral dissolution and contaminant fate in geochemistry, and many more. Yet, a detailed molecular-level understanding of this interface is still lacking. By using vibrational sum frequency generation (vSFG) spectroscopy supported by density functional theory simulations, we investigate the structure of aqueous solution next to two types of mica: a dioctahedral muscovite mica that contains sub-surface hydroxyls, and a trioctohedral fluorophlogopite mica that lacks such groups. Our results indicate that the vSFG signal response from the muscovite-water interface is rotationally anisotropic which is due to bulk infrared absorption caused by the sub-surface hydroxyl groups. In the case of hydroxyl absent fluorophlogopite, the vSFG signal is found to be rotationally isotropic. Moreover, we find spectral signature of non-hydrogen bonded water at the muscovite-water interface which arises due to proton transfer between sub-surface hydroxyl groups and the first water adlayer. Such species is absent at the fluorophlogopite-water interface, consistent with our assignment. Lastly, we also study the effect of ion identity on the water structure and find direct correlation with site-specific adsorption of ions on the mica surface.

Probing mica-aqueous interface using vibrational sum frequency generation spectroscopy

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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