Vibrational Behavior of Water Adsorbed on Forsterite (Mg₂SiO₄) Surfaces

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.

Oxide– and Silicate–Water Interfaces and Their Roles in Technology and the Environment

“…Similar adsorbed water structures are found for a variety of important minerals. For forsterite, MD and incoherent neutron scattering (INS) show that two H 2 O molecules adsorb per Mg atom, and the oxygen in the water molecule bonds to the metal ion, and forms H-bonds to the oxygen atoms of the solid . Water structures at numerous other mineral surfaces, including hematite, rutile, cassiterite, − and clay surfaces, have also been studied.…”

Theory-Guided Inelastic Neutron Scattering of Crystalline Alkaline Aluminate Salts Bearing Principal Motifs of Solution-State Species

“…(iii) The higher energy losses are dominated by multiphonon processes and other backgrounds. 45 In Figure 3, the features in the measured INS spectra have been labeled (black numbers), and putative assignments of the corresponding features in the simulated spectra have been made (corresponding yellow numbers). The simulated INS spectra (Figure 3) are the hydrogen-projected VDOSs from frozen phonon calculations, arbitrarily broadened by a Gaussian point spread function with a full width at half-maximum of 15.7 cm −1 (eq 2).…”

“…(ii) The librational region (Figure b), containing the most prominent features between ∼500 and ∼1250 cm –1 , is composed of water librational motions (rotational oscillations) and the motions of H atoms that are covalently bonded to the oxygens of aluminate anions (hereafter referred to as aluminate OH ligands). (iii) The higher energy losses are dominated by multiphonon processes and other backgrounds . In Figure , the features in the measured INS spectra have been labeled (black numbers), and putative assignments of the corresponding features in the simulated spectra have been made (corresponding yellow numbers).…”

Physics and chemistry on the surface of cosmic dust grains: a laboratory view

“…Several experimental techniques have been developed to measure diffusion on flat surfaces [144,145]. Such techniques allow measurements on a variety of length and timescales from the looking at individual adsorbate hops on video frame rate timescales at low coverage [146][147][148][149] corresponding to diffusion rates of 10 -19 to 10 -16 cm 2 s -1 ; through intermediate length scales of tens of nanometre [150,151] to the micrometre scale [152][153][154][155][156] corresponding to diffusion rates from 10 -15 cm 2 s -1 to about 10 -5 cm 2 s -1 and providing key information on the energetic barriers for diffusion on larger length scales at a wide range of coverage. Several of these techniques have the potential to be adapted to powdered materials though specific techniques have been developed by the catalysis community to address diffusion on fractal surfaces [157].…”