Exploration of the interfacial tension variation behavior in different CO2/oil systems based on experimental and molecular dynamics simulation studies

•IFTs in various CO2/n-alkane oil systems were studied based on measurement and MD simulations;•Differences on IFT variation laws were distinguished;•Molecule interchange at the interface and the interaction energy attribute to the difference on IFT variation laws. The trend of decreasing interfacial tension (IFT) with increasing pressure varies in different CO2/oil systems. To explore the mechanisms behind these phenomena, measurements and molecular dynamics (MD) simulations were performed in this study. First, the IFTs of four CO2/n-alkane pairs, including CO2/n-C10H22, CO2/n-C14H30, CO2/n-C15H32 and CO2/n-C16H34, were measured in the pressure range of 1–19 MPa and temperature range of 37.3–81.9 °C. It is observed the IFTs in the CO2/n-C10H22 and CO2/n-C14H30 systems decrease monotonically with increasing pressure, whereas those of the CO2/n-C15H32 and CO2/n-C16H34 systems exhibit nonlinear changes after reaching a certain pressure. The MD simulations were carried out on CO2/n-C14H30 and CO2/n-C16H34 systems under the corresponding measurement conditions. Good agreement was found between the MD simulation results and the measurement results, not only for the IFT values but also for the particular IFT trend transition pressures in the CO2/n-C16H34 system. Finally, the molecule interchange behavior at the interface region as well as the interaction energy between CO2 and oil molecules were scrutinized on the basis of the MD results. The change patterns of the number of CO2 molecules that penetrate into the oil phase and the system van der Waals energy match perfectly with the IFT trends thus reveal the mechanisms that lead to the different IFT behaviors in the CO2/n-C14H30 and CO2/n-C16H34 systems. This work can be used to understand the IFT characteristics of CO2/oil systems.