The Role of Supercritical/Dense CO 2 Gas in Altering Aqueous/Oil Interfacial Properties: A Molecular Dynamics Study

One of the main mechanisms contributing to enhanced oil recovery processes using compressed (supercritical) carbon dioxide (sc-CO 2 ) 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-CO 2 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-CO 2 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-CO 2 density at the interface to sc-CO 2 bulk density decreases as the sc-CO 2 mole fraction increases. This interesting behavior is owed to the difference in the interaction between CO 2 and water and between CO 2 and hydrocarbon, which diverges as the CO 2 mole fraction increases in the system. Moreover, our investigation indicated that sc-CO 2 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 CO 2 . We also found that, as the CO 2 content increases, the interfacial width increases, which contributes negatively to the IFT. Furthermore, it was found that, as the sc-CO 2 mole fraction increases, the hydrocarbon diffusion coefficients increase. The diffusivity response to CO 2 addition was determined by the molecular weight and polarity of the hydrocarbon.