Departamento de Física, Universidad Autónoma Metropolitana-Iztapalapa, Apartado Postal 55-534, México 09340, Distrito Federal, México.
Nature. 2012 May 2;485(7396):86-9. doi: 10.1038/nature11084.
Technological applications of liquid crystals have generally relied on control of molecular orientation at a surface or an interface. Such control has been achieved through topography, chemistry and the adsorption of monolayers or surfactants. The role of the substrate or interface has been to impart order over visible length scales and to confine the liquid crystal in a device. Here, we report results from a computational study of a liquid-crystal-based system in which the opposite is true: the liquid crystal is used to impart order on the interfacial arrangement of a surfactant. Recent experiments on macroscopic interfaces have hinted that an interfacial coupling between bulk liquid crystal and surfactant can lead to a two-dimensional phase separation of the surfactant at the interface, but have not had the resolution to measure the structure of the resulting phases. To enhance that coupling, we consider the limit of nanodroplets, the interfaces of which are decorated with surfactant molecules that promote local perpendicular orientation of mesogens within the droplet. In the absence of surfactant, mesogens at the interface are all parallel to that interface. As the droplet is cooled, the mesogens undergo a transition from a disordered (isotropic) to an ordered (nematic or smectic) liquid-crystal phase. As this happens, mesogens within the droplet cause a transition of the surfactant at the interface, which forms new ordered nanophases with morphologies dependent on surfactant concentration. Such nanophases are reminiscent of those encountered in block copolymers, and include circular, striped and worm-like patterns.
液晶的技术应用通常依赖于对表面或界面处分子取向的控制。这种控制是通过形貌、化学和单层或表面活性剂的吸附来实现的。衬底或界面的作用是在可见长度尺度上赋予有序性，并将液晶限制在器件中。在这里，我们报告了一个基于液晶的系统的计算研究结果，在该系统中，情况正好相反：液晶用于赋予表面活性剂界面排列的有序性。最近在宏观界面上的实验表明，体相液晶和表面活性剂之间的界面耦合可以导致表面活性剂在界面处的二维相分离，但分辨率不足以测量所得相的结构。为了增强这种耦合，我们考虑纳米液滴的极限，其界面用促进介晶在液滴内垂直取向的表面活性剂分子修饰。在没有表面活性剂的情况下，界面处的介晶都与该界面平行。随着液滴的冷却，介晶经历了从无序（各向同性）到有序（向列或近晶）液晶相的转变。当这种情况发生时，液滴内的介晶会导致界面处表面活性剂的转变，这会形成新的有序纳米相，其形态取决于表面活性剂浓度。这些纳米相让人联想到嵌段共聚物中遇到的那些，包括圆形、条纹状和蠕虫状图案。
