Abstract: In enhanced oil recovery, the use of the binary mixture of anionic/cationic surfactants could lower the interfacial tension and thus results in high oil recovery. A deep understanding of the underlying mechanisms would help the molecular design of the surfactant molecules used in enhance oil recovery and other related usage. In this work, molecular dynamics simulations were used to study the molecular behaviors and properties of the binary mixtures of nonyl-phenol-ethoxylated-carboxylate (PECNa) surfactants and octadecyl trimethyl ammonium chloride (OTAC) with different molar ratios at the oil?water interface to enrich the oil displacement theory with microscopic understanding. The density distribution of characteristic groups/radicals perpendicular to the interface, the characteristic length and the angle of the characteristic groups of the surfactants molecules perpendicular to the interface, the radical distribution analysis of the charged polar groups, and the distribution of counter-ions in the systems for different PECNa/OTAC ratio were compared. The results showed that the mixture of anionic?cationic surfactant molecules possessed better capacity to reduce the oil?water interfacial tension than that of anionic or cationic surfactants alone. With different molar ratios of the two types of surfactants, due to the attraction between the opposite charged head groups of anionic and cationic surfactant molecules, the electrostatic attraction between the charged groups of the surfactant molecules to their respective counter-ions was weakened, especially for the system with equal molar concentration of anionic and cationic surfactants. Meanwhile, the head group of the anionic surfactant had little effect on the orientation of water molecules in the hydration layer of the head group of the cationic surfactants, and vice versa. By adjusting the molar ratio of the surfactant mixture, a more compact adsorption layer can be obtained, which was promising for reducing the oil?water interfacial tension and enhancing oil recovery.

Key words: anionic and cationic surfactants, binary mixture, molecular dynamics simulations, different molar ratios, interface

摘要： 采油过程中阴/阳离子型表面活性剂复配使用可显著增强驱油效果，对其微观机理的深入研究有助于驱油用表面活性剂的结构优化设计及使用。采用分子动力学方法研究了不同摩尔比的阴离子表面活性剂聚醚羧酸钠(PECNa)和阳离子表面活性剂十八烷基三甲基氯化铵(OTAC)复配体系在油水界面上的分子行为和物理性质。结果表明，复配体系比单种表面活性剂体系更有利于降低油水界面张力。不同复配比体系中，两种表面活性剂头基相反电荷间的吸引作用使表面活性剂之间对各自反离子的静电吸引作用减弱，且等摩尔比体系尤为明显。阴离子表面活性剂的亲水头基对阳离子表面活性剂亲水头基形成的水化层内水分子的结构取向无显著影响，反之亦然。通过调节两种离子型表面活性剂的复配比例，可调整油水界面吸附层微观结构，有望降低油水界面张力，提高采收率。

关键词: 离子型表面活性剂, 二元复配, 分子动力学模拟, 不同摩尔比, 油水界面