Interaction of Water with the Gypsum (010) Surface: Structure and Dynamics from Nonlinear Vibrational Spectroscopy and Ab Initio Molecular Dynamics

Abstract

Water–mineral interfaces are important for several environmental, industrial, biological, and geological processes. Gypsum, CaSO4·2H2O, is a widespread mineral of high technological, medical, and environmental relevance, but little is known about its surface structure and its interaction with water. A molecular-level understanding of gypsum/water interface is given here by a combined experimental/theoretical study. We investigate the structure and dynamics of water adsorbed from vapor on the gypsum (010) single-crystal surface at room temperature, combining sum-frequency generation (SFG) vibrational spectroscopy experiments and ab initio molecular dynamics (AIMD) simulations. The SFG spectra of gypsum at low relative humidity (RH) show an anisotropic arrangement of structural water molecules and the presence of dangling OH groups. The AIMD simulations allow a detailed assignment of the SFG spectra and show that the cleaved (010) surface rearranges to have only 25% of the OH groups pointing away from the surface. At higher RHs, the first adsorbed water layer binds to these OH groups and forms an anisotropic arrangement, but with the amount of free OH groups significantly suppressed and without any significant diffusion. Upon adsorption of a second water layer, although the topmost layer of molecules is more disordered and dynamic than the previous one, its structure is still influenced by the gypsum surface underneath because it has a much reduced amount of free OH groups with respect to the free surface of water, and a slower surface diffusion with respect to bulk water. The theoretical results corroborate the experimental ones and provide an accurate atomic characterization of the surface structure.

Does the Structural Water within Gypsum Remain Crystalline at the Aqueous Interface?

“…14 In contrast to this intense research effort mainly addressing bulk properties of gypsum, the molecular structure at the surface, 15,16 and in particular the interaction of the surface with water, have only rarely been studied. 17 An interesting question in this respect is whether or not the crystal water of the surface remains intact upon immersion in water, as has been previously found for the first layer of crystal water in brushite (CaHPO 4 •2H 2 O). 18,19 Gypsum possesses a monoclinic crystal structure that is composed of alternating bilayers of calcium sulfate and water.…”

“…Sum-frequency generation (SFG) vibrational spectroscopy at the gypsum (0 1 0) surface has provided insights into the rearrangement of the crystal (i.e., structural) water molecules upon cleavage and the structure of adsorbed (i.e., hydration) water when increasing the relative humidity. 17 From the OH vibrations, the SFG data indicate that the first and second hydration water layers atop the crystal water form a rather ordered structure that is greatly influenced by the presence of the gypsum surface. While SFG provides unsurpassed insights into the water orientation and dynamics at the interface, the spatial information is limited as the signal is averaged over a certain volume at the interface where the structure is locally no longer centrosymmetric.…”

“…Using infrared spectroscopy, the water dynamics inside gypsum crystals can be studied, indicating that the crystal water in bulk gypsum is constrained to a single conformation . In contrast to this intense research effort mainly addressing bulk properties of gypsum, the molecular structure at the surface, , and in particular the interaction of the surface with water, have only rarely been studied . An interesting question in this respect is whether or not the crystal water of the surface remains intact upon immersion in water, as has been previously found for the first layer of crystal water in brushite (CaHPO 4 ·2H 2 O). , …”

Hydration mechanisms of scheelite from adsorption isotherms and ab initio molecular dynamics simulations

“…In particular, fluorite is the most problematic mineral for the selective scheelite flotation [25], which currently constitutes a technical and scientific challenge. Fortunately, this issue can be overcome by finely tuning the collector and depressants formulations [29,30,45], which should be based on a thorough and accurate description of the adsorption mechanisms at play at the liquid/solid interface [46][47][48][49][50][51]. Before the addition of the surfactant, minerals are milled and conditioned in the aqueous phase, which means that water molecules are already adsorbed on the surfaces.…”

“…In this study, we investigated the adsorption of water molecules on scheelite using manometric gas sorption experiments combined with ab initio molecular dynamics (AIMD) simulations, including a correction for dispersive interactions. AIMD simulations are currently among the most powerful methods used for the description of surface phenomena, in terms of efficiency, accuracy, and computational cost [48,[61][62][63][64][65]. The isosteric enthalpy of adsorption has been used as the value to link experimental and theoretical results, since it can be determined by both methods.…”