Probing the Mineral–Water Interface with Nonlinear Optical Spectroscopy

Abstract

The interaction between minerals and water is manifold and complex: the mineral surface can be (de)protonated by water, thereby changing its charge; mineral ions dissolved into the aqueous phase screen the surface charges. Both factors affect the interaction with water. Intrinsically molecular‐level processes and interactions govern macroscopic phenomena, such as flow‐induced dissolution, wetting, and charging. This realization is increasingly prompting molecular‐level studies of mineral–water interfaces. Here, we provide an overview of recent developments in surface‐specific nonlinear spectroscopy techniques such as sum frequency and second harmonic generation (SFG/SHG), which can provide information about the molecular arrangement of the first few layers of water molecules at the mineral surface. The results illustrate the subtleties of both chemical and physical interactions between water and the mineral as well as the critical role of mineral dissolution and other ions in solution for determining those interactions.

Role of Ions on the Surface-Bound Water Structure at the Silica/Water Interface: Identifying the Spectral Signature of Stability

“…The spectrum of χ (3) is similar to that of bulk water, 19 which is consistent with the diffuse layer exhibiting bulk-like hydrogen bonding with a small amount of net alignment due to the presence of the static electric field. This static electric field emanating in the z-direction (along the surface normal) can be related to the electrostatic potential 46 Although the potential at the silica surface is often invoked in the χ (3) technique, [47][48][49] according to the Gouy-Chapman-Stern-Grahame model of the electric double layer, the potential which aligns water in the diffuse layer is that outside of the outer Helmholtz plane (OHP), rather than the surface. This diffuse layer potential, which we argue is the potential that contributes to the χ (3) term, is often approximated as the zeta potential (ζ), which is experimentally determined based on the electrokinetic or electrophoretic properties of a system.…”

Water at charged interfaces

“…The systems described above have been chosen as representative examples to discuss water at charged interfaces and because they are systems that the authors have extensively investigated, but by no means are the only possible examples of charged interfaces 6,17,[29][30][31][32][33][34][35] . All the systems considered in this Review feature a charged interface in contact with an aqueous solution containing ions.…”

The Role of Ions on the Surface-Bound Water Structure at the Silica/Water Interface: Identifying the Spectral Signature of Stability

“…is similar to that of bulk water, 19 which is consistent with the diffuse layer exhibiting bulk-like hydrogen bonding with a small amount of net alignment due to the presence of the static electric field. This static electric field emanating in the z-direction (along the surface normal) can be related to the electrostatic potential 46 Although the potential at the silica surface is often invoked in the χ (3) technique, [47][48][49] according to the Gouy-Chapman-Stern-Grahame model of the electric double layer, the potential which aligns water in the diffuse layer is that outside of the outer Helmholtz plane (OHP), rather than the surface. This diffuse layer potential, which we argue is the potential that contributes to the χ (3) term, is often approximated as the zeta potential (ζ), which is experimentally determined based on the electrokinetic or electrophoretic properties of a system.…”