Departament d'Enginyeria Química, ETSEQ, Universitat Rovira i Virgili, Av. dels Països Catalans 26, 43007 Tarragona, Spain.
Langmuir. 2012 Feb 28;28(8):3730-43. doi: 10.1021/la204132c. Epub 2012 Feb 16.
We present a microscopic analysis of shape transitions of micelles of model linear nonionic surfactants. In particular, symmetric H(4)T(4) and asymmetric H(3)T(6) surfactants have been chosen for the study. In a previous work, it has been observed that symmetric surfactants have a strong tendency to prefer spherical micelles over a wide range of chemical potentials, while asymmetric surfactants undergo shape transitions between a spherical micelle at low concentration to other forms, mainly finite cylindrical micelles. This study combines the application of a two-dimensional single-chain mean-field theory (SCMFT) with Monte Carlo (MC) simulations of exactly the same systems. On the one hand, the characteristics of the SCMFT make this method suitable for free energy calculations, especially for small surfactants, due to the incorporation of relevant microscopic details in the model. On the other hand, MC simulations permit us to obtain a complete picture of the statistical mechanical problem, for the purpose of validation of the mean-field calculations. Our results reveal that the spherical shape for the symmetric surfactant is stable over a large range of surfactant concentrations. However, the asymmetric surfactant undergoes a complex shape transition that we have followed by calculating the standard chemical potential as a function of the aggregation number. The results indicate that the system forms prolate spheroids prior to developing short capped cylinders that gradually grow in length, with some oscillations in the energy of formation. The most important result of our work is the evidence of a bifurcation where, together with the elongated objects, the system can develop oblate aggregates and finally a torus shape similar to a red blood cell.
我们展示了模型线性非离子表面活性剂胶束形状转变的微观分析。特别是，选择了对称的 H(4)T(4)和不对称的 H(3)T(6)表面活性剂进行研究。在之前的工作中，已经观察到对称表面活性剂具有强烈的倾向，即在广泛的化学势范围内优先选择球形胶束，而不对称表面活性剂在浓度较低时会从球形胶束转变为其他形式，主要是有限的圆柱状胶束。这项研究结合了二维单链平均场理论（SCMFT）的应用和完全相同系统的蒙特卡罗（MC）模拟。一方面，SCMFT 的特点使得该方法适用于自由能计算，特别是对于小分子表面活性剂，因为模型中包含了相关的微观细节。另一方面，MC 模拟使我们能够获得统计力学问题的完整图景，以便验证平均场计算。我们的结果表明，对称表面活性剂的球形形状在很大的表面活性剂浓度范围内是稳定的。然而，不对称表面活性剂经历了复杂的形状转变，我们通过计算标准化学势作为聚集数的函数来跟踪这种转变。结果表明，该体系在形成短帽状圆柱形之前形成长形的扁球体，在形成过程中能量会出现一些波动。我们工作的最重要结果是证据表明存在分岔，其中除了伸长的物体外，系统还可以形成扁球形聚集体，最终形成类似于红细胞的环形状。
