One of the main mechanisms contributing to enhanced oil recovery processes using compressed (supercritical) carbon dioxide (sc-CO2) is alterations in the oil–water interfacial properties. However, it has been a challenge to experimentally investigate such effects. In our investigation presented here, we performed molecular dynamics simulations to explore these changes. We studied the role of sc-CO2 in changing the interfacial and transport properties of systems composed of water and pure hydrocarbons, namely, hexane, octane, benzene, and xylene. The simulations were performed at 100 bar and 350 K. It was observed that sc-CO2 accumulates at the interface, which leads to a reduction in the interfacial tension (IFT) of water–oil systems. Our further analysis of such accumulation showed that the ratio of sc-CO2 density at the interface to sc-CO2 bulk density decreases as the sc-CO2 mole fraction increases. This interesting behavior is owed to the difference in the interaction between CO2 and water and between CO2 and hydrocarbon, which diverges as the CO2 mole fraction increases in the system. Moreover, our investigation indicated that sc-CO2 forms a film between the two phases, which displaces oil molecules away from the interface. This film was stabilized by hydrogen bonds between water and CO2. We also found that, as the CO2 content increases, the interfacial width increases, which contributes negatively to the IFT. Furthermore, it was found that, as the sc-CO2 mole fraction increases, the hydrocarbon diffusion coefficients increase. The diffusivity response to CO2 addition was determined by the molecular weight and polarity of the hydrocarbon.

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We conducted molecular dynamics (MD) simulations to investigate the effect of supercritical carbon dioxide (sc-CO2) on the interfacial and transport properties of water–oil systems. The oil phase was resembled by employing different binary hydrocarbons (paraffin + aromatic), namely, benzene + hexane, benzene + octane, xylene + hexane, and xylene + octane. Furthermore, we added an asphaltene to the system composed of xylene and hexane to study the interfacial behavior of the heaviest fraction of oil (asphaltene) in the presence of CO2. The simulations were performed under the operating conditions of 100 bar and 350 K. The results showed that aromatics, CO2, and asphaltenes accumulated at the interface at low CO2 mole fractions (x CO2 ). However, when x CO2 increased, it displaced the aromatics away from the interface and toward the bulk. At very high x CO2 , the aromatics accumulated at the oil bulk. Similarly, asphaltene molecules stacked at the interface at low x CO2 , and as x CO2 increased, some of the asphaltene molecules dissolved and aggregated in the oil bulk. CO2 forms a film between water and oil phases, and as the thickness of the film increases, it displaces the hydrocarbons away from the interface. The addition of sc-CO2 diluted the interface, formed hydrogen bonds (H bonds) with water, which stabilize the CO2 film, and reduced the interfacial tension in all systems. Furthermore, the addition of sc-CO2 increased the diffusivity of the oil phase in all systems. However, it significantly affected the diffusivity of systems that have less polar aromatics.

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The lack of reliable predictive modeling methods and robust experimental techniques has hindered the rational design of hierarchical materials with desired structure−property−performance attributes suitable for extreme environments. With this context in mind, we explore the utility of ReaxFF reactive molecular dynamics (MD) simulations in combination with in-operando wide-angle X-ray scattering (WAXS) and X-ray pair distribution function (PDF) analyses. To demonstrate the method, we consider kaolinite, a natural hierarchical material, as the candidate and determine thermally induced chemical and structural transformations when heated from 298 to 1673 K. We first compare the key structural features from the PDF data and WAXS peaks obtained experimentally to those calculated from MD simulations. Upon observing excellent agreement, we proceed to elucidate the underlying chemical reaction mechanisms associated with dehydroxylation and sintering, identify intermediate and transition states, and also estimate energy barriers of individual reactions and their effects on the structural organization of kaolinite obtained using MD simulations. On heating from 298 to 873 K, dehydroxylation reactions lead to the transformation of crystalline kaolinite with octahedrally coordinated aluminum atoms to semicrystalline metakaolin with ∼90% tetrahedrally coordinated aluminum atoms. Sintering reactions and the subsequent emergence of mullite (a high-temperature phase of kaolinite) are observed on heating metakaolin from 1055 to 1673 K. We also find that heating rates have a significant effect on the onset temperature of dehydroxylation and sintering reactions. A rapid heating rate leads to early dehydroxylation (425 K) and sintering (1055 K), whereas a 10 times slower heating rate delays dehydroxylation (622 K) and sintering reactions (1100 K). An outcome of our method is a regime map that illustrates the degree of agreement between the experimental data and simulation results in describing the thermally induced onset of atomic-level reorganizations in materials. Herein, quantitative agreement between simulation predictions and experimental data is noted at lower temperatures (T < 1000 K), and minor deviations between these methods is noted for T > 1000 K. The remarkable agreement between the methods observed in our study reiterates the reliability of the combined ReaxFF approach in predicting material properties under chemically reactive and extreme conditions.

With increasing interest in using or displacing confined water for CH 4 recovery or CO 2 storage in nanoporous environments, understanding the organization and diffusion of gases is confined water environments is essential. In this study, the effect of hydration on the structure and diffusivity of confined carbon dioxide (CO 2) and methane (CH 4) in 2 nm slit-shaped calcite nanopore was studied using classical molecular dynamics simulations. The absence of confined water and the effect of different water concentrations including one layer of confined water composed of 150 water molecules, 500 water molecules, and 1,296 water molecules that correspond to the density of bulk water of 1 g/cm 3 on the structural arrangement and diffusivity of confined CO 2 and CH 4 were investigated. Water molecules were found to influence the anisotropic distribution and mobility of confined CO 2 and CH 4 significantly by altering the structures of the adsorbed gas layers onto the calcite surfaces. The preferential adsorption of water on calcite surface over CO 2 and CH 4 resulted in the displacement of the adsorbed gas molecules toward the center of the pore. This water-induced displacement impacts the diffusivity of the confined gases by enabling transport through the center of the pore where there are fewer intermolecular collisions and less steric hindrance for transporting the molecules. Therefore, the diffusivity of CO 2 and CH 4 is higher in the presence of a single water layer as opposed to in pores without water. Energetic calculations showed that van der Waals and electrostatic interactions contributed to the affinity of CO 2 for calcite surfaces, while van der Waals interactions dominate CH 4 interactions with calcite and the surrounding water molecules. The anisotropic variations in the diffusivities of confined fluids emerge from changes in the organization of confined fluids and potential differences in the free energy distributions as a function of the orientation of the calcite surface. These findings suggest that any efforts to potentially engineer the nano-scale pore environment in calcite for enhanced gas recovery or storage will require us to consider the organization and anisotropic transport behaviors of confined fluids.