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.

Geological carbon dioxide sequestration in deep saline aquifers can play a key role in the successful mitigation of greenhouse gas emissions. Several conditions have been identified that affect the solubility of CO 2 in water, including temperature, pressure, pH, and salinity. At similar conditions, the solubility in bulk fluids differs from the solubility in confined porous media. We conducted equilibrium molecular dynamics (MD) simulations to investigate the solubility of CO 2 in water confined in slit-shaped calcite pores. We studied the effects of brine (NaCl and MgCl 2 ). Compared to bulk water/brine, the solubility of CO 2 is lower in calcite pores and decreases as the pores narrow. Adsorption energy calculations were performed to compare our results to CO 2 solubility in water-filled silica pores. These results indicate that narrower calcite pores are less attractive for the adsorption of CO 2 . In addition, the simulation results suggest that the difference in the positions where the ions adsorb also affects whether salts increase or decrease the solubility of CO 2 in confined water. Confinement and ions also reduce the mobility of CO 2 in water. These observations contribute to the design of long-term CO 2 sequestration strategies as they provide boundary constraints for the amount of CO 2 that can dissolve in hydrated pores, as well as the timing of CO 2 transport in such systems.

The solubility of asphaltenes in hydrocarbons changes with pressure, composition, and temperature, leading to precipitation and deposition, thereby causing one of the crucial problems that negatively affects oil production, transportation, and processing. Because, in some circumstances, it might be advantageous to promote asphaltene agglomeration into small colloidal particles, molecular dynamics simulations were conducted here to understand the impacts of a chemical additive inspired by cyclohexane on the mechanism of aggregation of model island and archipelago asphaltene molecules in toluene. We compared the results in the presence and absence of a kaolinite surface at 300 and 400 K. Cluster size analyses, radial distribution functions, angles between asphaltenes, radius of gyration, and entropic and energetic calculations were used to provide insights on the behavior of these systems. The results show that the hypothetical additive inspired by cyclohexane promoted the aggregation of both asphaltenes. Structural differences were observed among the aggregates obtained in our simulations. These differences are attributed to the number of aromatic cores and side chains on the asphaltene molecules as well as to that of heteroatoms. For the island structure, aggregation in the bulk phase was less pronounced than that in the proximity of the kaolinite surface, whereas the opposite was observed for the archipelago structure. In both cases, the additive promoted stacking of asphaltenes, yielding more compact aggregates. The results provided insights into the complex nature of asphaltene aggregation, although computational approaches that can access longer time and larger size scales should be chosen for quantifying emergent meso- and macroscale properties of systems containing asphaltenes in larger numbers than those that can currently be sampled via atomistic simulations.