Investigating the Water Organization at the Calcite (10.4)-Water Interface at High pH

Calcite, the most abundant carbonate mineral in Earth’s crust, is at the heart of many environmental and technological processes. As part of the geological carbonate-silicate cycle, calcite dissolution and precipitation are central for the regulation of atmospheric carbon dioxide levels on geological timescales. Moreover, calcite is involved in buffering of sea- and freshwater pH values and biomineralization of marine organisms. Important technological applications of calcite are the use in concrete and as a storage material for anthropogenic carbon. Since most processes on calcite take place in an aqueous environment, they are governed by the structure and properties of the calcite-water interface. Consequently, there has been a large body of research establishing a detailed understanding of the interface between the most-stable calcite (10.4) surface and water. This includes the development of so-called surface complexation models describing the surface speciation and properties of the calcite-water interface in thermodynamic equilibrium with aqueous solutions of varying composition. An important part of these models is the description of the species at the interface, which heavily depends on the protonation and deprotonation of surface-bound water and interfacial carbonate groups. However, the de-/protonation of calcite is difficult to quantify experimentally due to calcite dissolution and carbonate buffering. Here, we apply interface-sensitive vibrational sum frequency generation (SFG) spectroscopy to directly assess the water species present at the calcite-water interface at high pH. With SFG spectroscopy, we can measure the vibrational spectrum of interfacial species, providing insights into the molecular organization and chemical environment at the interface. We aim to quantify the change of hydroxyl species present at the interface with increasing pH to quantify the deprotonation constant of surface-bound water contributing to the development of more-accurate surface complexation models.