Vibrational Behavior of Water Adsorbed on Forsterite (Mg2SiO4) Surfaces

The dynamics of water on or in a mineral substrate plays an important role in interfacial processes. This is because the structure and dynamics of interfacial water deviate from those of bulk water due to the change of interactions between surface water molecules and the interactions between the surface water and the substrate. This work presents a study of the vibrational behavior of water on a forsterite (Mg- end member of olivine) surface using inelastic neutron scattering (INS) and molecular dynamics (MD) simulations as complementary tools. The synthetic nano-forsterite used (dominated by the (010) crystal face), i.e., "dry" sample, inherently has a partial hydration/hydroxylation layer on the surface, as shown by previous studies and TGA. In the INS experiments, three water loadings (0.5, 1, and 2 monolayers) were added to the nano-forsterite surfaces. For samples with lower water loadings, i.e., dry and 0.5 monolayers, the INS spectra exhibited a red shift (lower frequency) of the water libration band and strengthening (blue shift, higher frequency) of the O-H stretching modes, implying weakening of the hydrogen bonding acting on the water molecules. In the simulations, we modeled the forsterite (010) surface and titrated it with two water loadings representing the lower and higher experimental water loadings. The lower loading in the simulation is equivalent to the dry and 0.5 monolayer samples in the experiment, thus suggesting weak hydrogen bonding between water molecules. The higher-loading simulation emulates the multilayer adsorption experiment. This produced a more significant shift of the vibrational bands, implying increased hydrogen-bonding strength and disorder between water molecules. The MD simulations complement the INS study by providing a detailed interfacial structure, and the combination of the two approaches provides a fundamental understanding of how the presence of the olivine surface impacts the vibrational behavior of water under different degrees of hydration-a phenomenon widely associated with terrestrial and extraterrestrial surfaces and near-surface processes.