球状胶束中表面活性剂脱附的离子调控机制

doi:10.19596/j.cnki.1001-246x.8701

摘要/Abstract

摘要：

建立耦合伞状采样的粗粒度分子动力学方法，研究球状胶束中表面活性剂分子的脱附过程，揭示表面活性剂聚集数、盐种类及浓度对表面活性剂脱附过程的影响机制。发现球状胶束半径及偏心率均随聚集数增加而增大，盐浓度的影响主要取决于抗衡离子的半径和吸附特性，半径更大、吸附更强的水杨酸根离子对胶束结构的影响更为显著；基于伞状采样方法获得了表面活性剂脱附自由能、脱附时间等关键参数，发现球状胶束中表面活性剂脱附自由能和脱附时间均随聚集数和盐浓度呈非单调变化，揭示其主要机制为离子吸附引起的静电屏蔽作用；发现自由能在表面活性剂脱附过程中起主导作用，结合胶束热力学理论发展了临界胶束浓度预测方法，获得了临界胶束浓度下胶束尺寸的分布范围。

关键词: 胶束, 表面活性剂脱附, 离子调控, 分子动力学, 自由能计算

Abstract:

A coarse-grained molecular dynamics method coupled with umbrella sampling is developed to study the desorption process of SDS and CTAC surfactants from spherical micelles. The influence mechanisms of aggregation number, salt species and concentrations on the desorption process of surfactant from spherical micelles are revealed. We find that the radius and eccentricity of spherical micelles both increase with aggregation number rising, and the effects of salt concentration depend on the ionic radius and adsorption characteristics of the counterions. Larger salicylate ions with stronger adsorption ability have more significant effect on the micellar structures. Based on umbrella sampling method, we obtain the desorption free energy and time of surfactants from micelles. we find that the desorption free energy and desorption time of surfactant both show a non-monotonic variation with the increase of aggregation number and salt concentration, whose mechanisms are attributed to the electrostatic shielding induced by ion adsorption on micelle surface. Moreover, we reveal that the free energy plays a leading role in surfactant desorption process. Combing the theory of micelle thermodynamics, we further develop the critical micelle concentration prediction method, and obtain the distribution range of micelle size under the critical micelle concentration of surfactants.

Key words: micelle, surfactant desorption, ionic regulation, molecular dynamics, free energy calculation

中图分类号:

O469

温伯尧, 高根英, 路熙, 关松涛, 骆政园, 白博峰. 球状胶束中表面活性剂脱附的离子调控机制[J]. 计算物理, 2024, 41(2): 193-202.

Boyao WEN, Genying GAO, Xi LU, Songtao GUAN, Zhengyuan LUO, Bofeng BAI. Ionic Regulation Mechanisms of Surfactant Desorption from the Spherical Micelles[J]. Chinese Journal of Computational Physics, 2024, 41(2): 193-202.

图/表 8

图1 球状胶束模型、分子粗粒化及Martini珠子类型

Fig.1 Spherical micelle model and coarse-grained representations of molecules and their Martini bead types

图2 球状胶束中表面活性剂吸附/脱附过程的PMF曲线及采样直方图(CTAC/NaSal体系，N = 80，R = 1.0)

Fig.2 PMF profiles and sampling histogram of surfactant adsorption and desorption processes at or from the spherical micelle (CTAC/NaSal system, N = 80, R = 1.0)

表1 SDS球状胶束结构性质(N = 60，R = 0)

Table 1 Structural properties of SDS spherical micelles (N = 60, R = 0)

	R_g/nm	R_s/nm
Our simulation results	1.55±0.05	2.00±0.06
Ref.[30]	1.55±0.30	1.98±0.27
Ref.[31]	1.50	1.96
Ref.[32]	1.58	1.90
Ref.[33]	1.64	1.99
Ref.[34]	1.60±0.06	1.97±0.08
Ref.[35]	1.54	2.00

图3 不同聚集数和盐-表面活性剂浓度比的球状胶束半径 (a) SDS/NaCl体系; (b) CTAC/NaSal体系

Fig.3 Micelle radius of spherical micelle with different aggregation number and salt-to-surfactant concentration ratio (a) SDS/NaCl system; (b) CTAC/NaSal system

图4 不同聚集数和和盐-表面活性剂浓度比的球状胶束偏心率 (a) SDS/NaCl体系; (b) CTAC/NaSal体系

Fig.4 Eccentricities e of spherical micelles with different aggregation number and salt-to-surfactant concentration ratio (a) SDS/NaCl system; (b) CTAC/NaSal system

图5 不同聚集数和盐-表面活性剂浓度比的球状胶束中表面活性剂分子的脱附自由能 (a) 聚集数; (b) 盐-表面活性剂浓度比对脱附自由能及胶束净电荷值

Fig.5 Desorption free energy of surfactant from spherical micelle with different aggregation number and salt-to-surfactant concentration ratio (a) aggregation number; (b) salt-to-surfactant concentration ratio on desorption free energy and net charge of micelle

图6 球状胶束表面活性剂分子脱附过程的平均首次通过时间及脱附时间 (a) 相同聚集数下CTAC和SDS分子脱附的平均首次通过时间; (b) 不同聚集数下表面活性剂分子的脱附时间; (c) 不同盐-表面活性剂浓度比下表面活性剂分子的脱附时间

Fig.6 Mean first-passage time profiles and desorption time of SDS molecules from the interfaces (a) mean first-passage time of SDS molecules from the oil-water interfaces with different adsorbed numbers; (b) desorption time of SDS molecules from interfaces with different adsorbed numbers; (c) effects of ionic species and concentrations on the desorption time of SDS molecules

图7 不同聚集数下SDS和CTAC表面活性剂CMC预测值

Fig.7 Predicted CMC values of SDS and CTAC surfactants with different aggregation numbers

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