Surfactant desorption and scission free energies for cylindrical and spherical micelles from umbrella-sampling molecular dynamics simulations
Surfactant desorption and scission free energies for cylindrical and spherical micelles from umbrella-sampling molecular dynamics simulations
复制标题
伞式采样分子动力学模拟中圆柱形和球形胶束的表面活性剂解吸和自由断裂能
DOI：
10.1016/j.jcis.2021.04.138
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发表时间：
2021
影响因子：
9.9
通讯作者：
Larson, Ronald G.
中科院分区：
文献类型：
--
作者：
Wen, Boyao;Bai, Bofeng;Larson, Ronald G.
假设与单个表面活性剂从胶束中的吸附/脱附和长胶束的融合/断裂有关的自由能可以用来估计胶束动力学的速率常数随表面活性剂和盐浓度的变化。实验我们计算了表面活性剂从胶束中逸出的自由能Gesc值和长胶束的裂解自由能Gscis，并结合伞状取样，计算了十二烷基硫酸钠(SDS)在氯化钠(NaC L)和十六烷基三甲基氯化铵(CTAC)在水杨酸钠(NASIC)中的胶束。对于球形胶束，Gesc值在一定的聚集数时有极大值，盐-表面活性物质的摩尔浓度比接近一致，与实验一致。对于柱状胶束，十二烷基硫酸钠/氯化钠最小，CTAC/鼻腔最大，均为ATR~0.7，CTAC胶束的Gsciss值也在R~0.7时达到峰值，且随着R的增大，十二烷基硫酸钠胶束的Gsciss值单调增加。结合静电屏蔽和胶束半径随R增大而减小的现象，解释了逃逸自由能和断裂自由能对R的非单调依赖关系。从CTAC/NASA的自由能可以推断出从球形胶束到柱状胶束的转变，以及随着盐浓度的变化而在吸附/解吸和熔融/裂解动力学之间的转变。
HypothesisThe 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.ExperimentsWe compute the escape free energies △Gescof surfactant from micelles and the scission free energies △Gscissof 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).FindingsFor spherical micelles, △Gescvalues have maxima at certain aggregation numbers, and at salt-to-surfactant molar concentration ratiosRnear unity, consistent with experiments. For cylindrical micelles, SDS/NaCl shows a minimum, and CTAC/NaSal a maximum in △Gesc, both atR~ 0.7, while △Gscissof CTAC micelles also peaks at aroundR~ 0.7 and that of SDS micelles increases monotonically withR. We explain the non-monotonic dependence of escape and scission free energies onRby a combination of electrostatic screening and the decrease of micelle radius with increasingR. 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.