Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109, United States; State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China.
State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China.
J Colloid Interface Sci. 2021 Oct;599:773-784. doi: 10.1016/j.jcis.2021.04.138. Epub 2021 May 1.
The free energies associated with adsorption/desorption of individual surfactants from micelles and the fusion/scission of long micelles can be used to estimate the rate constants for micellar kinetics as functions of surfactant and salt concentration.
We compute the escape free energies △G of surfactant from micelles and the scission free energies △G of long micelles from coarse-grained molecular dynamics simulations coupled with umbrella sampling, for micelles of both sodium dodecylsulfate (SDS) in sodium chloride (NaCl) and cetyltrimethylammonium chloride (CTAC) in sodium salicylate (NaSal).
For spherical micelles, △G values have maxima at certain aggregation numbers, and at salt-to-surfactant molar concentration ratios R near unity, consistent with experiments. For cylindrical micelles, SDS/NaCl shows a minimum, and CTAC/NaSal a maximum in △G, both at R ~ 0.7, while △G of CTAC micelles also peaks at around R ~ 0.7 and that of SDS micelles increases monotonically with R. We explain the non-monotonic dependence of escape and scission free energies on R by a combination of electrostatic screening and the decrease of micelle radius with increasing R. Transitions from predominantly spherical to cylindrical micelles, and between adsorption/desorption and fusion/scission kinetics with changing salt concentration can be inferred from the free energies for CTAC/NaSal.
可以将单个表面活性剂从胶束中吸附/解吸以及长胶束融合/断裂的自由能用于估计胶束动力学的速率常数，作为表面活性剂和盐浓度的函数。
我们通过粗粒化分子动力学模拟与伞状采样相结合，计算了表面活性剂从胶束中逃逸的自由能△G和长胶束从粗粒化分子动力学模拟中断裂的自由能△G，用于研究十二烷基硫酸钠（SDS）在氯化钠（NaCl）和十六烷基三甲基氯化铵（CTAC）在水杨酸钠（NaSal）中的胶束。
对于球形胶束，△G 值在某些聚集数处具有最大值，并且在盐与表面活性剂摩尔浓度比 R 接近 1 时，与实验结果一致。对于圆柱形胶束，SDS/NaCl 在 R0.7 处显示出最小的△G 值，而 CTAC/NaSal 在 R0.7 处显示出最大的△G 值，同时 CTAC 胶束的△G 值也在 R~0.7 左右达到峰值，而 SDS 胶束的△G 值随 R 的增加单调增加。我们通过静电屏蔽和胶束半径随 R 增大而减小的组合来解释逃逸和断裂自由能对 R 的非单调依赖性。从 CTAC/NaSal 的自由能可以推断出从主要为球形胶束到圆柱形胶束的转变，以及随着盐浓度变化的吸附/解吸和融合/断裂动力学之间的转变。
