Salt Effects on the Structure and Dynamics of Interfacial Water on Calcite Probed by Equilibrium Molecular Dynamics Simulations
Azeezat Ali, Tran Thi Bao Le, Alberto Striolo, David R. Cole
Abstract
It is important to understand the properties of interfacial water at mineral surfaces. Since calcite \nis one of the most common minerals found in rocks and sedimentary deposits, and since it \nrepresents a likely phase encountered in reservoirs dedicated to carbon sequestration, it is \ncrucial to understand the behaviour of fluids on its surface. In this study, the impacts of sodium \nchloride (NaCl), potassium chloride (KCl) and magnesium chloride (MgCl2) on the structure \nand dynamics of water on the calcite interface were investigated using equilibrium molecular \ndynamics simulations. Two force fields were compared to model calcite. The resultant \nproperties of interfacial water were quantified and compared in terms of atomic density \nprofiles, surface density distributions, radial distribution functions, hydrogen bond density \nprofiles, angular distributions, and residence times. Our results show the formation of distinct \ninterfacial molecular layers, with water molecules in each layer having slightly different \norientations, depending on the force field implemented. The fluid behaviour within the first \ninterfacial layers differs from that observed in bulk water. There was a tendency for water \nmolecules in adjacent layers to form hydrogen bonds between each other or the surface, as \nopposed to the formation of hydrogen bonds within each hydration layer. The addition of ions \ndisrupts the well-organized structure of oxygen atoms in the first and second hydration layers, \nwith KCl having the biggest effect. Conversely, far from the interface, MgCl2 leads to the \nlowest number of hydrogen bonds per water, out of the salts considered. The residence time of \nwater within the second hydration layer follows a bi-exponential decay, suggesting the \nsimultaneous presence of two dynamic mechanisms, one characterized by shorter time scales \nthan the other. The time scale associated with the former mechanism decreases as the salt \nconcentration is increased, whereas the opposite is observed for the slower mechanism. In \ngeneral, the results obtained with the two force fields used to simulate calcite are similar in \nterms of the features of the hydration layers and hydrogen bond network but differ significantly \nin their predictions for the residence times. Although experimental results are not available to \nidentify which of the two force fields yields predictions that more closely resemble reality, the \nresults highlight the contributions of surface-water, water-water, and ion-water interactions on \nthe wetting properties of calcite, which are especially important for calcite-water-electrolyte \ninteractions commonly observed in nature.