Molecular Dynamics Study on CO2 Foam Films with Sodium Dodecyl Sulfate: Effects of Surfactant Concentration, Temperature, and Pressure on the Interfacial Tension

CO2 foam flooding technology can be used for geological storage of greenhouse gas CO2 while enhancing oil recovery. In this paper, we performed molecular dynamics simulations for the CO2 foam film systems stabilized with an anionic surfactant of sodium dodecyl sulfate (SDS). Compared to other factors of a foam system, interfacial tension was used as the primary indicator to evaluate the stability of the SDS foam film. The effects of the concentration of the SDS surfactant, temperature, and pressure on the interfacial tension were studied. Based on the calculated results, the stability of the CO2 foam film was discussed. It is found that under the high-concentration condition, SDS molecules can form a dense and thick molecular layer at the interface, blocking the contact of CO2 molecules and water molecules and reducing the interfacial tension. Consequently, the stability of the foam liquid film can be improved. Low temperature and high pressure lead to high density of the CO2 phase, which enables strong interactions between CO2 and the hydrophobic tails of SDS molecules. The interfacial structure thus formed can reduce the contact probability between CO2 and water molecules, generating lower interfacial tension. Therefore, high SDS concentration, low temperature, and high pressure are beneficial to the stability of the CO2 foam. We show that the adsorption of CO2 molecules at the interface, the interfacial thickness, and solvent accessible surface area of the surfactant to the CO2 phase are affected by the density of CO2 bulk phase, which are important interfacial properties affecting the CO2/water interfacial tension and the stability of the CO2 foam.