Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136, USA.
J Phys Chem B. 2009 Oct 22;113(42):13697-710. doi: 10.1021/jp901576e.
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.
我们使用原子分子动力学模拟来研究添加氯化钠（NaCl）和水杨酸钠（NaSal）盐对十六烷基三甲基氯化铵（CTAC）表面活性剂水溶液中球形到线状胶束形状转变的影响。研究发现，长线状胶束在低浓度 NaCl 下不稳定，会断裂成球形胶束，但在[NaCl]约 3.0 M 的阈值以上保持稳定 20 ns，这大约是实验中球形和棒状胶束之间转变的盐浓度的 2.5 倍。氯离子在 CTAC 胶束表面上弱缔合，随着球形胶束上[NaCl]的增加，反离子的离解度略有降低，但在高[NaCl]下的线状胶束上显著降低。这种效应表明，电解质离子通过屏蔽胶束头基之间的静电排斥作用来驱动胶束形状转变。另一方面，芳香水杨酸盐反离子穿透胶束内部，其亲水头基留在表面活性剂头基区域，疏水头基部分嵌入胶束的疏水核内。水杨酸盐离子与表面活性剂头基的强烈缔合导致表面活性剂分子的密集堆积，有效地减少了每个表面活性剂的表面积，并增加了表面活性剂头基的胶束内有序性，有利于长线状胶束的形成。球形和线状胶束的几何和静电性质的模拟预测与实验结果吻合良好。
