Molecular dynamics simulations of threadlike cetyltrimethylammonium chloride micelles: effects of sodium chloride and sodium salicylate salts.
Molecular dynamics simulations of threadlike cetyltrimethylammonium chloride micelles: effects of sodium chloride and sodium salicylate salts.
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DOI：
10.1021/jp901576e
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发表时间：
2009-05
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影响因子：
--
通讯作者：
Zuowei Wang;R. Larson
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作者：
Zuowei Wang;R. Larson
用原子分子动力学模拟方法研究了添加氯化钠和水杨酸钠对十六烷基三甲基氯化铵(CTAC)表面活性剂水溶液中球状到线状胶束形态转变的影响。长的线状胶束在低浓度的氯化钠溶液中是不稳定的，并破裂成球形胶束，但在阈值[氯化钠]约3.0M以上保持稳定20 ns，这大约是发生球形和棒状胶束转变的实验盐浓度的2.5倍。氯化物反离子在CTAC胶束表面的缔合较弱，反离子解离程度随球形胶束上的氯化钠浓度的增加而略有降低，但在较高的氯化钠浓度下，丝状胶束上的反离子解离程度明显下降。这一效应表明，电解质离子通过屏蔽胶束头部基团之间的静电斥力来驱动胶束形状的转变。另一方面，芳香族水杨酸酯反离子进入胶束内部，其亲水基团停留在表面活性剂头基区，疏水基团部分嵌入胶束的疏水核心。水杨酸根离子与表面活性剂头基的强缔合导致表面活性剂分子紧密堆积，有效地减小了每个表面活性剂的表面积，增加了表面活性剂头基的胶束内有序性，有利于形成长线状胶束。模拟预测的球形和线状胶束的几何性质和静电性质与实验符合得很好。
We use atomistic molecular dynamics simulations to probe the effects of added sodium chloride (NaCl) and sodium salicylate (NaSal) salts on the spherical-to-threadlike micelle shape transition in aqueous solutions of cetyltrimethylammonium chloride (CTAC) surfactants. Long threadlike micelles are found to be unstable and break into spherical micelles at low concentrations of NaCl, but remain stable for 20 ns above a threshold value of [NaCl] approximately 3.0 M, which is about 2.5 times larger than the experimental salt concentration at which the transition between spherical and rodlike micelles occurs. The chloride counterions associate weakly on the surface of the CTAC micelles with the degree of counterion dissociation decreasing slightly with increasing [NaCl] on spherical micelles, but dropping significantly on the threadlike micelles at high [NaCl]. This effect indicates that the electrolyte ions drive the micellar shape transition by screening the electrostatic repulsions between the micellar headgroups. The aromatic salicylate counterions, on the other hand, penetrate inside the micelle with their hydrophilic groups staying in the surfactant headgroup region and the hydrophobic groups partially embedded into the hydrophobic core of the micelle. The strong association of the salicylate ions with the surfactant headgroups leads to dense packing of the surfactant molecules, which effectively reduces the surface area per surfactant, and increases intramicellar ordering of the surfactant headgroups, favoring the formation of long threadlike micelles. Simulation predictions of the geometric and electrostatic properties of the spherical and threadlike micelles are in good agreement with experiments.