Firetto Vanessa, Floriano Michele A, Panagiotopoulos Athanassios Z
Dipartimento di Chimica Fisica F. Accascina, Università di Palermo, Viale delle Scienze, Parco d'Orleans, Ed. 17, 90128 Palermo, Italy.
Langmuir. 2006 Jul 18;22(15):6514-22. doi: 10.1021/la060386c.
The micellization behavior of a series of model surfactants, all with four head and tail groups (H4T4) but with different degrees of chain stiffness, was studied using grand canonical Monte Carlo simulations on a cubic lattice. The critical micelle concentration, micellar size, and thermodynamics of micellization were examined. In all cases investigated, the critical micelle concentration was found to increase with increasing temperature as observed for nonionic surfactants in apolar or slightly polar solvents. At a fixed reduced temperature and increasing chain stiffness, in agreement with previous observations, it was found that the critical micelle concentration decreased and the average micelle size increased. This behavior is qualitatively consistent with that experimentally observed when comparing hydrogenated and homologous fluorinated surfactants. Thermodynamic considerations based on the analysis of the temperature dependence of the critical micelle concentration indicated that both effects could be interpreted as arising from an increased number of heterocontacts between hydrophobic portions of stiff surfactants and a lower entropic cost on packing rigid chains. Structural analysis that was also based on considering the inner micellar radial dependence of the surfactant head and tail site fraction distributions suggested that, on stiffening the molecular backbone, the resulting micellar aggregates grew, without appreciable deviations from spherical symmetry. Stiffer surfactants led to a slightly denser micellar core because of better packing.
利用立方晶格上的巨正则蒙特卡罗模拟，研究了一系列均具有四个头基和尾基（H4T4）但链刚性程度不同的模型表面活性剂的胶束化行为。考察了临界胶束浓度、胶束尺寸和胶束化热力学。在所有研究的情况下，均发现临界胶束浓度随温度升高而增加，这与非离子表面活性剂在非极性或微极性溶剂中的情况一致。在固定的对比温度下且链刚性增加时，与先前的观察结果一致，发现临界胶束浓度降低，平均胶束尺寸增加。当比较氢化和同系氟化表面活性剂时，这种行为在定性上与实验观察结果一致。基于对临界胶束浓度温度依赖性分析的热力学考虑表明，这两种效应都可以解释为刚性表面活性剂疏水部分之间异质接触数量增加以及刚性链堆积时熵成本降低所致。同样基于考虑表面活性剂头基和尾基位点分数分布的胶束内部径向依赖性的结构分析表明，在分子主链变硬时，形成的胶束聚集体会生长，且与球对称性无明显偏差。由于堆积更好，刚性更强的表面活性剂会导致胶束核心密度略高。
