Despite the multiple length and time scales over which fluid-mineral interactions occur, interfacial phenomena control the exchange of matter and impact the nature of multiphase flow, as well as the reactivity of C–O–H fluids in geologic systems. In general, the properties of confined fluids, and their influence on porous geologic phenomena are much less well understood compared to those of bulk fluids. We used equilibrium molecular dynamics simulations to study fluid systems composed of propane and water, at different compositions, confined within cylindrical pores of diameter ∼16 Å carved out of amorphous silica. The simulations are conducted within a single cylindrical pore. In the simulated system all the dangling silicon and oxygen atoms were saturated with hydroxyl groups and hydrogen atoms, respectively, yielding a total surface density of 3.8 −OH/nm2. Simulations were performed at 300 K, at different bulk propane pressures, and varying the composition of the system. The structure of the confined fluids was quantified in terms of the molecular distribution of the various molecules within the pore as well as their orientation. This allowed us to quantify the hydrogen bond network and to observe the segregation of propane near the pore center. Transport properties were quantified in terms of the mean square displacement in the direction parallel to the pore axis, which allows us to extract self-diffusion coefficients. The diffusivity of propane in the cylindrical pore was found to depend on pressure, as well as on the amount of water present. It was found that the propane self-diffusion coefficient decreases with increasing water loading because of the formation of water bridges across the silica pores, at sufficiently high water content, which hinder propane transport. The rotational diffusion, the lifespan of hydrogen bonds, and the residence time of water molecules at contact with the silica substrate were quantified from the simulated trajectories using the appropriate autocorrelation functions. The simulations contribute to a better understanding of the molecular phenomena relevant to the behavior of fluids in the subsurface.

The wettability behavior of reservoir rocks plays a vital role in determining CO2 storage capacity and containment security. Several experimental studies characterized the wettability of CO2/brine/rock systems for a wide range of realistic conditions. To develop a fundamental understanding of the molecular mechanisms responsible for such observations, the results of molecular dynamics simulations, conducted at atomistic resolution, are reported here for representative systems in a wide range of pressure and temperature conditions. Several force fields are considered, achieving good agreement with experimental data for the structure of interfacial water but only partial agreement in terms of contact angles. In general, the results suggest that, at the conditions chosen, water strongly wet calcite, resulting in water contact angles either too low to be determined accurately with the algorithms implemented here or up to ∼46°, depending on the force field implemented. These values are in agreement with some, but not all experimental data available in the literature, some of which report contact angles as high as 90°. One supercritical CO2 droplet was simulated in proximity of the wet calcite surface. The results show pronounced effects due to salinity, which are also dependent on the force field implemented to describe the solid substrate. When the force field predicts complete water wettability, increasing NaCl salinity seems to slightly increase the calcite affinity for CO2, monotonically as the NaCl concentration increases, because of the preferential adsorption of salt ions at the water–rock interface. When the other force field was implemented, it was not possible to quantify salt effects, but the simulations suggested strong interactions between the supercritical CO2 droplet and the second hydration layer on calcite. The results presented could be relevant for predicting the longevity of CO2 sequestration in geological repositories.

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.

The aggregation of clay particles in aqueous solution is a ubiquitous everyday process of broad environmental and technological importance. However, it is poorly understood at the all-important atomistic level since it depends on a complex and dynamic interplay of solvent-mediated electrostatic, hydrogen bonding, and dispersion interactions. With this in mind, we have performed an extensive set of classical molecular dynamics simulations (included enhanced sampling simulations) on the interactions between model kaolinite nanoparticles in pure and salty water. Our simulations reveal highly anisotropic behavior, in which the interaction between the nanoparticles varies from attractive to repulsive depending on the relative orientation of the nanoparticles. Detailed analysis reveals that at large separation (>1.5 nm), this interaction is dominated by electrostatic effects, whereas at smaller separations, the nature of the water hydration structure becomes critical. This study highlights an incredible richness in how clay nanoparticles interact, which should be accounted for in, for example, coarse-grained models of clay nanoparticle aggregation.

This article describes a simple and inexpensive undergraduate-level kinetics experiment that uses magnetic levitation to monitor the progress and determine the activation energy of a condensation reaction on a polymeric solid support. The method employs a cuvette filled with a paramagnetic solution positioned between two strong magnets. The vertical position of the polymeric beads suspended in the paramagnetic solution correlates with the density of the beads and, consequently, with the progress of the chemical reaction within these beads. Varying the temperature of the reaction between the leucine-functionalized support and 2,5-diiodobenzoic acid under pseudo-first-order reaction conditions yields an activation energy of 65.4 ± 9.2 kJ/mol. This value compares well the activation energy of 63.5 ± 4.1 kJ/mol determined using a density analysis. This experiment combines a number of interdisciplinary concepts including organic chemistry, kinetics, and magnetism and, therefore, could be implemented in a number of undergraduate chemistry courses at various levels.