Ion Adsorption and Desorption at the CaF2-Water Interface Probed by Flow Experiments and Vibrational Spectroscopy

The dissolution of minerals in contact with water plays a crucial role in geochemistry. However, obtaining molecular insight into interfacial chemistry is challenging. Dissolution typically involves the release of ions from the surface, giving rise to a charged mineral surface. This charge affects the interfacial water arrangement, which can be investigated by surface-specific vibrational Sum Frequency Generation (v-SFG) spectroscopy. For the fluorite-water interface, recent spectroscopic studies concluded that fluoride adsorption/desorption determines the surface charge, which contrasts zeta potential measurements assigning this role to the calcium ion. By combining v-SFG spectroscopy and flow experiments with systematically suppressed dissolution, we uncover the interplay of dominant fluoride and weak calcium adsorption/desorption, resolving the controversy in the literature. We infer the calcium contribution to be orders of magnitude smaller, emphasizing the sensitivity of our approach.

Automated summary

The dissolution of minerals in water is important in geochemistry, but studying the interfacial chemistry at the molecular level is challenging. By using surface-specific vibrational sum frequency generation spectroscopy, this study investigates the chemical reaction at the fluorite-water interface and resolves the controversy regarding the role of fluoride and calcium ions in surface charge.