Abstract
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.
Original language | English |
---|---|
Pages (from-to) | 1050-1063 |
Number of pages | 14 |
Journal | ACS Earth and Space Chemistry |
Volume | 4 |
Issue number | 7 |
DOIs | |
State | Published - Jul 16 2020 |
Funding
Support for T.L., S.G., and D.R.C. was provided by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, Geosciences Program under grant DESC0006878 to Ohio State University. Work by LMA was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. T.L. also thanks the computing resources provided by the Deep Carbon Observatory cluster hosted by Rensselaer Polytechnic Institute. This work has benefited from the use of Filter Difference Spectrometer (FDS) at the Lujan Center at Los Alamos Neutron Science Center, funded by DOE Office of Basic Energy Sciences. The Los Alamos National Laboratory is operated by the Los Alamos National Security LLC under DOE Contract DE-AC52-06NA25396. The VISION and SEQUOIA INS experiments of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. The material characterization assistance provided by Drs. Julie Sheets and Susan Welch at The Ohio State University is also greatly appreciated. Improvements in the work at different stages also benefited from the insights and comments provided by anonymous reviewers, which are fully acknowledged.
Keywords
- forsterite
- hydroxylated
- inelastic neutron scattering (INS)
- intermolecular motion
- intramolecular motion
- isochoric heat capacity
- molecular dynamics (MD) simulation
- molecular water
- surface water
- vibration