复配表面活性剂对油水界面行为和性质影响的模拟研究

doi:10.11949/0438-1157.20220511

摘要/Abstract

摘要：

含复配表面活性剂的油田废水是一种多组分复杂体系，研究其中的分子作用关系有助于后续废水处理方案的确定。采用分子动力学(MD)模拟方法建立界面模型，通过定义表面活性剂的关键扭转点及相应的分子夹角、定义协同作用能，结合界面处的密度分布函数等性能模拟和界面张力测试结果，多角度分析两类阴阳离子表面活性剂复配对油水界面特性的影响。结果表明，与含单组分表面活性剂的油水体系相比，复配表面活性剂的相反电荷极性头基间静电吸引作用提高了油水界面稳定性；相较于十二烷基磺酸钠/十六烷基三甲基溴化铵(SLS/CTAB)复配体系，十二烷基硫酸钠/十六烷基三甲基溴化铵(SDS/CTAB)中分子间的协同作用可更好地提高体系的稳定性；当复配表面活性剂的配比为8/10~12/6时的油/水界面稳定效果较优、12/6时稳定性最好。研究结果可为石油开采及油水分离方案的确定提供依据。

关键词: 乳液, 表面活性剂, 复配体系, 分子模拟, 界面行为, 油水分离

Abstract:

Oilfield wastewater containing compound surfactants is a multi-component complex system, and the study of molecular interaction is helpful to determine the subsequent wastewater treatment plan. An interface model was established by using molecular dynamics (MD) simulation method. A series of calculations of interfacial properties and the experiment of interfacial tension were performed to examine the effects of two types of anionic/cationic surfactant mixtures on oil/water interfacial property with this model. In particular, this paper defines the concepts of the twist points of surfactants, molecular angles and synergistic energy. The results demonstrated that, compared with the single-component surfactant, the electrostatic attraction between the polar head groups with opposite charges present in mixed surfactants could induce these mixtures to improve the oil/water interface stability. In oil-water systems consisting of mixed surfactants, the intermolecular synergy of sodium dodecyl sulfate/cetyltrimethyl ammonium bromide (SDS/CTAB) mixtures can better enhance the stability of the system compared to sodium dodecyl sulfonate/cetyltrimethyl ammonium bromide (SLS/CTAB) mixtures. The compounding ratio range of SDS/CTAB mixtures that could better stabilize the oil-water interface was from 8/10 to 12/6, with 12/6 being the best. The results could provide programmatic guidance for crude oil extraction and oily wastewater treatment.

Key words: emulsion, surfactant, mixed system, molecular simulation, interface behavior, oil-water separation

中图分类号:

TQ 423.1

廖艺, 牛亚宾, 潘艳秋, 俞路. 复配表面活性剂对油水界面行为和性质影响的模拟研究[J]. 化工学报, 2022, 73(9): 4003-4014.

Yi LIAO, Yabin NIU, Yanqiu PAN, Lu YU. Modeling the effects of mixed surfactants on the behaviors and properties of the oil-water interface with molecular dynamics[J]. CIESC Journal, 2022, 73(9): 4003-4014.

图/表 15

图1 模型初始状态示意图

Fig.1 Schematic diagram of the initial state of the model

表1 模拟体系的分子个数

Table 1 The number of molecules in simulated system

体系分类	n₁(ASAA)	n₂(CSAA)	水	油
单一体系(CSAA)	0	16	2000	50
复配体系(Mix)	4	13	2000	50
	8	10	2000	50
	9	9	2000	50
	12	6	2000	50
	16	3	2000	50
单一体系(ASAA)	20	0	2000	50
纯油水体系(Oil/Water)	—	—	2000	50

图2 ASAA/CSAA体系能量随时间的变化

Fig.2 Energy variation of ASAA/CSAA system with time

图3 SLS/CTAB体系YZ平面平衡图[SLS(紫色)、CTAB(蓝色)；反离子为Na+(橙色)、Br-(绿色)]

Fig.3 Equilibrium diagram of SLS/CTAB system in YZ plane [SLS(purple), CTAB(blue); Anti-ion: Na+(orange), Br–(green)]

表2 ASAA/CSAA体系的油水界面吸附截面积减小值ΔA

Table 2 Reduction of adsorption cross-sectional area at the oil-water interface of ASAA/CSAA system

复配比	ΔA_SDS/CTAB/nm²	ΔA_SLS/CTAB/nm²
4/13	0.42	0.38
8/10	0.52	0.48
9/9	0.45	0.38
12/6	0.43	0.37
16/3	0.26	0.23

图4 ASAA/CSAA体系各组分和离子沿Z轴的密度分布

Fig.4 Density distribution of different components and ions along Z-axis in ASAA/CSAA system

图5 SDS各角度的示意图

Fig.5 Schematic diagram of each angle of SDS

图6 ASAA/CSAA体系的倾斜角概率分布

Fig.6 Distribution of the tilt angle of ASAA/CSAA system

图7 ASAA/CSAA体系的头基、尾基倾斜角平均值

Fig.7 Average of head base and tail base tilt angles of ASAA/CSAA system

图8 ASAA/CSAA体系各分子的θHT分布概率

Fig.8 The θHT distribution probability of each molecules in ASAA/CSAA system

图9 表面活性剂分子头基中S、N与水分子中O间的径向分布函数

Fig.9 The radial distribution function between S and N in head group of surfactants and O in water

图10 不同界面区域的水分子扩散系数

Fig.10 Diffusion coefficients of water molecules in different interface regions

图11 界面区厚度示意图

Fig.11 Schematic diagram of interfacial thickness

图12 ASAA/CSAA体系的各界面区厚度

Fig.12 Interfacial thickness of each phase in ASAA/CSAA

图13 ASAA/CSAA体系的界面张力和协同作用能

Fig.13 Interfacial tension and synergistic energy of ASAA/CSAA system

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