Abstract:

Surfactants have an important influence on the properties of oil-water interface. The high temperature and high salinity reservoir environment seriously affects the interfacial chemical properties and oil displacement effect of surfactants. In order to study the effect of different surfactant structures on the properties of oil-water interface. The microscopic behavior and mechanism of anionic surfactant sodium dodecyl sulfate (SDS) and group-modified surfactant SDS-B at the oil-water interface were studied by molecular dynamics simulation. The results show that the introduction of alkane in the hydrophobic tail chain of SDS surfactant changes the arrangement of surfactant molecules at the oil-water interface. Compared with the single hydrocarbon chain surfactant, the double hydrocarbon chain structure makes the surfactant still closely perpendicular to the oil-water interface under high temperature and high salt environment, and SDS-B has good molecular interface behavior. At the same time, the increase in the number of alkane groups causes the SDS molecules to show slight bending, causing the surfactant molecules to form multiple aggregates, which is conducive to the formation of multilayer adsorption. The repulsive effect of SDS-B head group on Ca²⁺ was significantly stronger than that of SDS, and the first peak value of radial distribution function decreased by 0.89. The oil-water interface thickness of SDS-B in Ca²⁺ environment was improved compared with that of SDS, and the thickness increased from 1.13 nm to 1.52 nm, which significantly enhanced the interface stability, indicating that the introduction of hydrocarbon chain improved the Ca²⁺ salt resistance of surfactants. The head group of SDS-B is easy to form an intramolecular hydrogen bond structure with the hydrocarbon chain group, and the hydration ability of the head group is improved. The cations in the brine are greatly bound. The diffusion coefficients of Ca²⁺, Mg²⁺ and Na⁺ are reduced by 0.027×10^-4, 0.065×10^-4 and 0.064×10^-4 cm²/s, respectively. The hydrophilic and interfacial behavior of SDS-B head groups is superior to that of SDS in complex salt environments and higher ionic concentrations. This study has important guiding significance for the design of new surfactants in tertiary oil recovery.

Key words: surfactant, hydrocarbon chain, salt resistance, molecular dynamics, oil-water interface, simulation

摘要：

表面活性剂对油水界面性质具有重要影响，高温高盐油藏环境严重影响表面活性剂的界面化学特性和驱油效果。为研究不同表面活性剂结构对油水界面性质的影响。采用分子动力学模拟方法研究了阴离子表面活性剂十二烷基硫酸钠（SDS）和进行基团修饰的表面活性剂SDS-B在油水界面上的微观行为和作用机理。结果表明，在SDS表面活性剂的疏水尾链中引入链烷烃改变了表面活性剂分子在油水界面的排列方式，相较于单烃链表面活性剂，双烃链结构使表面活性剂在高温高盐环境下依旧能紧密垂直于油水界面，SDS-B具有良好的分子界面行为。同时，链烷烃基团数目的增加导致SDS分子表现出轻微的弯曲，使表面活性剂分子形成多处聚集体，有利于形成多层吸附。SDS-B头基对Ca²⁺的排斥作用明显强于SDS，径向分布函数第一峰值降低0.89，而且SDS-B在Ca²⁺环境下的油水界面厚度较SDS得到改善，厚度从1.13 nm升高到1.52 nm，显著增强了界面稳定性，表明烃链的引入提高了表面活性剂的抗Ca²⁺盐特性。SDS-B头基易与烃链基团形成分子内氢键结构，头基水化能力提高，盐水中的阳离子受到较大的束缚力，Ca²⁺、Mg²⁺、Na⁺扩散系数分别降低了0.027×10^-4、0.065×10^-4、0.064×10^-4 cm²/s。在复杂盐环境及更高离子浓度下SDS-B头基亲水性及界面行为均优于SDS。本研究对三次采油中新型表面活性剂的设计具有重要的指导意义。

关键词: 表面活性剂, 烃链, 耐盐性, 分子动力学, 油水界面, 模拟