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

Abstract

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.

Accurate Prediction of Absorption Spectral Shifts of Proteorhodopsin Using a Fragment-Based Quantum Mechanical Method

“…It is also worth noting that the 1-ps QM/MM MD trajectory that we use for analysis will not sample long timescale protein fluctuations [ 60 ]; however, the good agreement found between theoretical excitation energies and experimental measurements suggests that >1-ps fluctuations have a minor influence on the average absorption energy. This finding is in contrast to solvated chromophores, which can have large couplings between the excitation energy and >1-ps solvation dynamics [ 61 , 62 , 63 , 64 ]. The origin of this disparity could be due to a relatively conserved environment of the chromophore in the protein matrix, unlike in solvent, although we cannot fully rule out a fortuitous agreement of our results with experiment.…”

Effects of surface contamination on the interfacial properties of CO2/water/calcite systems

“…To confirm that the results discussed above do not depend on the force field implemented to describe calcite, we repeated selected simulations for a system in which the calcite substrate was described following the parameterisation proposed by Shen et al, 34 which we had implemented in a prior study to quantify the structure of hydration water on calcite. 33 The system composition is 4,000 water molecules and 14,000 CO 2 molecules, the temperature is 323 K, and the pressure is 20 MPa. The contact angle for the water-calcite-CO 2 system was compared with and without the ethanol molecules (see Figure 9).…”

“…32 Because of the large variation of contact angles observed experimentally, we could not conclude which, out of the two force fields tested for describing calcite, was more realistic. To further assess the reliability of the two force fields, Ali et al 33 investigated the structure of the hydration layer on calcite. Both force fields consistently yield a dense first hydration layer and predict similar distributions of aqueous electrolytes near the surface.…”

Mechanistic investigation and free energies of the reactive adsorption of ethanol at the alumina/water interface

“…17 Nonreactive adsorptions can be treated with classic molecular mechanics. 14,18,19 Combining this classic description with an ab initio description of the surface/adsorbate interaction im-proves the energetics of adsorption. [20][21][22][23][24] For instance, using our recent MMSolv approach, the adsorption energy at the water/Pt interface of phenol and benzene was predicted semi-quantitatively.…”