CEA, IRAMIS, SIS2M, LIONS (Laboratoire Interdisciplinaire sur l'Organisation Nanométrique et Supramoléculaire), F-91191 Gif-sur-Yvette cedex, France.
J Phys Chem B. 2010 Feb 11;114(5):1932-8. doi: 10.1021/jp910267v.
Mixtures of ionic surfactants of opposite charge ("catanionic" mixtures) show strongly nonideal behaviors, for example, in terms of evolution of surface tension, critical micelle concentration, or morphology with respect to composition in each surfactant. In several catanionic systems, it has been proposed that the interaction between both surfactants is so strong that lateral phase segregation occurs within bilayers, with crystallites of preferential composition demixing from the excess of the other surfactant. Here, we investigate the temperature-composition phase diagram of the myristic acid/cetyltrimethylammonium mixtures. Combining microcalorimetry, X-ray diffusion, and solid-state deuterium NMR, we demonstrate that no separation is observed in the gel (L(beta)) state. The catanionic mixtures therefore behave like two-dimensional solid solutions with a negative azeotrope: the existence of a composition at which a maximum in melting temperature is observed does not imply the existence of a preferential crystal of this composition, but results from the preferential attraction between unlike amphiphilic molecules. Additionally, this study reveals the presence of a so-called intermediate phase, that is, a phase that shows dynamic properties intermediate between that of the L(beta) and the L(alpha) phases.
相反电荷的离子型表面活性剂混合物（“反离子型”混合物）表现出强烈的非理想行为，例如，表面张力、临界胶束浓度或各表面活性剂组成的形态随组成的变化。在几种反离子型系统中，有人提出两种表面活性剂之间的相互作用如此强烈，以至于双层内发生横向相分离，优先组成的微晶从过量的另一种表面活性剂中分离出来。在这里，我们研究了豆蔻酸/十六烷基三甲基铵混合物的温度-组成相图。结合微量热法、X 射线扩散和固态氘 NMR，我们证明在凝胶（L(beta)）状态下没有观察到分离。因此，反离子型混合物表现为具有负共沸物的二维固溶体：在观察到熔融温度最大值的组成处存在并不意味着存在这种组成的优先晶体，而是由于不同两亲分子之间的优先吸引力所致。此外，这项研究揭示了存在所谓的中间相，即表现出介于 L(beta)相和 L(alpha)相之间的动态特性的相。
