MgO-water interface: structure and surface dissolution depend on flow and pH

Magnesium oxide (MgO) is frequently in contact with water throughout numerous research and industrial applications and in nature. Remarkably, we found that there is a substantial influence on the interfacial structure and dissolution process whether water is flowing or static at the MgO(100) surface. Sum frequency generation spectroscopy revealed that flowing acidic solutions enhance the charging of the MgO surface, which leads to an increased net orientation of water close to the surface. Contrary, the MgO surface resembles a near neutrally charged state when in contact with static liquid for all tested solutions between pH 3 and pH 11. We explain this surprising observation with the dissolution of MgO in aqueous solutions, which effectively removes charge from the interfacial region. The continuous solution exchange due to flowing liquid shifts the equilibrium towards a more charged state in comparison to static liquid. Additionally, by investigating the transition from flowing to static liquid we found a reaction order of around 0.5 for the dissolution reaction with respect to the H⁺ concentration. Furthermore, the significant effect of the MgO surface dissolution on the interfacial structure points out that other solid-liquid interfaces with similar or higher solubility might exhibit similar properties.