多元体系油水界面上常见表面活性剂行为的分子动力学模拟

doi:10.7503/cjcu20170242

摘要/Abstract

摘要：

采用分子动力学模拟研究了以十二烷基苯磺酸钠(SDBS)为代表的阴离子型表面活性剂, 以十二烷基三甲基溴化铵(DTAB)为代表的阳离子型表面活性剂, 以壬基酚聚氧乙烯醚(NPE)为代表的非离子型表面活性剂, 以十二烷基二甲基甜菜碱(Betaine)为代表的两性表面活性剂及空白实验. 模拟了表面活性剂在油水界面上的行为, 考察了表面活性剂分子与石油分子之间的径向分布函数(RDF)、石油分子在竖直方向的均方位移(MSD)、油水界面张力(IFT)、石油层与岩石层之间的相互作用能、石油层的相对浓度在竖直方向的分布及石油分子质心位置随模拟时间的变化关系等, 讨论了不同表面活性剂的洗油性能. 结果表明: (1) SDBS, NPE和Betaine分子初始状态下呈近似的规律排列, 非极性端部分插入油相中, 极性端延伸进入水相中; 随后表面活性剂的极性端表现出聚集趋势, 逐渐形成一个外部亲油内部亲水的一个胶束状粒子, 粒子随模拟的进行逐渐融入到油层当中; DTAB从开始的近似规则排列逐渐变为无规排列, 但是始终保持亲油端插入到油相中, 亲水端位于油水界面上. (2) 表面活性剂分子与石油分子之间的相互作用强弱顺序为Betaine≈DTAB<SDBS<NPE. (3) 由质心高度和动力过程中的图像截图分析, 表面活性剂洗油效果的顺序为Betaine>SDBS>NPE>DTAB>None. 模拟结果与实际的驱油结果一致, 从分子层面上解释了不同表面活性剂洗油的规律.

关键词: 分子动力学, 油水界面, 界面张力, 相互作用, 洗油效果

Abstract:

In order to explore the behavior of common surfactants on oil/water interface in complex systems, five models were designed to study using the molecular simulation method. Except a blank model, four different surfactants were placed in the same complex systems as follows: sodium dodecyl benzene sulfonate(SDBS), dodecyltrimethylammonium bromide(DTAB), nonylphenol ethoxylates(NPE), and betaine. These four surfactants represented the anionic surfactant, the cationic surfactant, the nonionic surfactant, and the zwitterionic surfactant, respectively. For these models, radial distribution function(RDF) of surfactants and the oil molecules, mean square displacement(MSD) along z direction of oil molecules, oil/water interfacial tension(IFT), interaction energy between the oil layer and the mineral stone layer, the distribution of relative concentration of oil molecules along z direction, etc. were analyzed. For the SDBS, NPE, Betaine system, at the beginning, the surfactants arranged regularly at the interface of oil/water, the lipophilic parts of the surfactants were partially inserted into the oil phase, the hydrophilic parts were stretched into the water phase. Then the hydrophilic parts of the surfactants got together and formed to a micelle gradually. The micelle, which of its interior is hydrophilic and its exterior is lipophilic, moved to the oil layer gradually. For DTAB, at first, the surfactants were distributed regularly at the interface of oil/water, they gradually became disorderly as the simulation time increased. But the lipophilic head stretched into oil phase and the hydrophilic head distributed at the oil/water from the beginning to end. The order of the interaction between surfactants and oil molecular were: Betaine≈DTAB<SDBS<NPE; The order of oil displacement efficiency of different surfactants were: Betaine>SDBS>NPE>DTAB>None. Above all, the model systems constructed were rationalized. The results obtained from the simulation were agreement with the experimental data. The research gave partly the illustration of actual oil displacement efficiency of different surfactants at the molecular level.

Key words: Molecular dynamics, Oil/water interface, Interfacial tension, Interaction, Oil displacement efficiency

中图分类号:

O641
O631

TrendMD:

江蓉君, 罗健辉, 白瑞兵, 江波, 周歌. 多元体系油水界面上常见表面活性剂行为的分子动力学模拟. 高等学校化学学报, 2017, 38(10): 1804.

JIANG Rongjun, LUO Jianhui, BAI Ruibing, JIANG Bo, ZHOU Ge. Molecular Dynamics Simulation on Behavior of Common Surfactants at the Oil/Water Interface in Complex Systems^†. Chem. J. Chinese Universities, 2017, 38(10): 1804.

图/表 9

Fig.1 Construction of common surfactants model systems(such as SDBS)

Fig.2 Energy of the DTAB model as the function of simulation time

Fig.3 Model systems for calculating the interfacial tension(such as SDBS)

Fig.4 Behavior variation of different surfactants systems(highlight for surfactants)(A1—A5) SDBS; (B1—B5) NPE; (C1—C5) DTAB; (D1—D5) Betaine. (A1—D1) 0 ps;(A2—D2) 250 ps; (A3—D3) 500 ps; (A4—D4) 1000 ps; (A5—D5) 2000 ps.

Fig.5 RDF for different surfactants and oil layers

Fig.6 Centroid variation(Zoil center) of oil molecules along the simulation time

Fig.7 Snaps of different surfactants systems at 0 ps(A1—E1), 500 ps(A2—E2), 2000 ps(A3—E3), respectively(A1—A3) None; (B1—B3) DTAB; (C1—C3) SDBS; (D1—D3) Betaine; (E1—E3) NPE.

Fig.8 Relative concentrations of oil molecules of different surfactants systems at 500 ps(a) and 2000 ps(b)(A) None; (B) DTAB; (C) SDBS, (D) Betaine; (E) NPE.

Fig.9 MSD of oil molecular in different surfactants systems along z-direction

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