Study on the interfacial properties of binary surfactants at the oil–water interface using molecular dynamics approaches†

Surfactant flooding is an effective method for enhancing oil recovery. Molecular dynamics (MD) simulations were performed to investigate the interfacial behavior and interactions of four types of surfactants, as individual agents and combined surfactant systems at the oil–water interface. The four surfactants were anionic petroleum sulfonate (PS) surfactants, anionic gemini disulfonate surfactant (GS), zwitterionic dodecyl betaine (BS), and aninon–noionic sodium dodecyl-di(oxyethylene) ether sulfate (AES). The simulated interfacial tension values of the four single surfactants and mixed systems were similar to the experimental data. Various interfacial parameters such as interfacial density distribution and interfacial thickness were analyzed via MD simulation. 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⁻¹. By mixing AES with PS, BS, and GS, the interfacial performance of each system was further enhanced. The mixed PS/AES system achieved a maximum oil–water interfacial thickness of 19.51 Å, with a reduced interfacial formation energy of −274.6 kcal mol⁻¹ and a diffusion coefficient of 0.154 Ų ps⁻¹, indicating the formation of a stable interface. MD research on mixed surfactant systems under complex salt conditions revealed that the PS/AES system exhibited excellent salt resistance with the following tolerance level ranking: Ca²⁺ > Na⁺ > Mg²⁺. MD simulations revealed the oil displacement mechanisms of surfactants from a microscopic perspective, thus providing a theoretical basis for practical applications in oil recovery.