The silica/aqueous interface is one of the most widespread interfaces from both an environmental standpoint as well as in commercial applications as sand is a large component of the earth's crust and glass is a very common industrial material. Despite its ubiquity, the influence of common ions on surface charging at this interface as well as ion effects on the interfacial structure of water are still not well understood, as many spectroscopic methods are not amenable to probing insulator/aqueous interfaces. However, owing to their intrinsic surface selectivity and ability to monitor buried insulator interfaces, second-order nonlinear optical techniques like second harmonic generation (SHG) and sum frequency generation (SFG) are very well suited to study silica in the presence of the aqueous phase. Using a combination of SHG and vibrational SFG, we have explored the influence of monovalent and divalent salts on the pH-dependence of surface charging on silica and the interfacial water structure, respectively.
Our work reveals that the presence of high concentrations of monovalent salts like Li+, Na+ and Cs+ leads to structured water at the interface, even near the point of zero charge of silica (~pH 2). We attribute this structured water to the formation of the electric double layer, which leads to ordered water beyond the inner Helmholtz layer. In contrast, we observe that divalent ions promote the destruction of this ordered water layer, especially at high pH. As an important environmental problem facing Alberta involves the slow settling and solidification of the tailings waste from the Athabascan oil sands, our ability to probe interfacial water holds promise for rapid screening of dewatering agents that are used to promote densification of the oil sands processing waste.