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Salt Effects on the Structure and Dynamics of Interfacial Water on Calcite Probed by Equilibrium Molecular Dynamics Simulations

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journal contribution
posted on 29.10.2020, 14:38 by Azeezat Ali, Tran Thi Bao Le, Alberto Striolo, David R. Cole
It is important to understand the properties of interfacial water at mineral surfaces. Since calcite is one of the most common minerals found in rocks and sedimentary deposits, and since it represents a likely phase encountered in reservoirs dedicated to carbon sequestration, it is crucial to understand the behavior of fluids on its surface. In this study, the impacts of sodium chloride (NaCl), potassium chloride (KCl), and magnesium chloride (MgCl2) on the structure and dynamics of water on the calcite interface were investigated using equilibrium molecular dynamics simulations. Two force fields were compared to model calcite. The resultant properties of interfacial water were quantified and compared in terms of atomic density profiles, surface density distributions, radial distribution functions (RDFs), hydrogen bond (HB) density profiles, angular distributions, and residence times. Our results show the formation of distinct interfacial molecular layers, with water molecules in each layer having slightly different orientations, depending on the force field implemented. The fluid behavior within the first interfacial layers differs from that observed in bulk water. There was a tendency for water molecules in adjacent layers to form HBs between each other or the surface, as opposed to the formation of HBs within each hydration layer. The addition of ions disrupts the well-organized structure of oxygen atoms in the first and second hydration layers, with KCl having the biggest effect. Conversely, far from the interface, MgCl2 leads to the lowest number of HBs per water, out of the salts considered. The residence time of water within the second hydration layer follows a biexponential decay, suggesting the simultaneous presence of two dynamic mechanisms, one characterized by shorter time scales than the other. The time scale associated with the former mechanism decreases as the salt concentration is increased, whereas the opposite is observed for the slower mechanism. In general, the results obtained with the two force fields used to simulate calcite are similar in terms of the features of the hydration layers and HB network but differ significantly in their predictions for the residence times. Although experimental results are not available to identify which of the two force fields yields predictions that more closely resemble reality, the results highlight the contributions of surface–water, water–water, and ion–water interactions on the wetting properties of calcite, which are especially important for calcite–water–electrolyte interactions commonly observed in nature.

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