Interfacial properties of binary surfactants at oil-water interface from molecular dynamics approaches

Abstract

Surfactant flooding is an effective method to enhance oil recovery. Molecular dynamics (MD) simulations were performed to investigate the interface behavior and interactions of four types of surfactants, both as individual agents and in combined surfactant systems at the oil-water interface. The four surfactants were anionic petroleum sulfonate (PS) surfactants, gemini anionic disulfonate surfactant (GS), zwitterionic dodecyl betaine (BS), and aninon-ionic sodium dodecyl-di(oxyethylene) ether sulfate (AES). The simulated interfacial tension values of four single surfact and the mixed systems were similar with the experimental data. Various interfacial parameters such as interfacial density distribution and interfacial thickness, were analyzed by MD simulation. The simulation results indicated that the single AES exhibited the best interfacial performance among the single systems, with the lowest interfacial formation energy of -229.4 kcal/mol. By mixing AES with PS, BS, and GS, the interfacial performances were further enhanced. The mixed PS/AES system achieved the maximum oil-water interfacial thickness of 19.51 Å, with a reduced interfacial formation energy of -274.6 kcal/mol and a diffusion coefficient of 0.154 Å2/ps, indicating the formation of a stable interface. MD research of the mixed surfactants system in complex salt conditions revealed that the PS/AES system exhibited excellent salt resistance, with tolerance levels ranking as Ca2+>Na+>Mg2+. MD simulations revealed the oil displacement mechanisms of surfactants from a microscopic perspective, providing a theoretical basis for practical applications in oil recovery.