Nature 485, 86–89 (2012)

Credit: © 2012 NPG

Liquid crystals, elongated molecules formed by a rigid core bearing flexible terminal chains, are intrinsically anisotropic. Controlling their orientation over large domains at surfaces or interfaces — through mechanical or chemical treatment — leads to a variety of phases with different properties. This behaviour has been widely exploited for technical applications. Now, using computational simulations, Juan de Pablo at the University of Wisconsin–Madison and co-workers have shown that liquid crystals themselves are able to cause other species to orient at an interface.

The researchers modelled spherical nanodroplets consisting of ellipsoidal mesogens — the rigid units in liquid crystals that drive their ordering — surrounded by surfactant and water molecules. The model is set up so that the mesogens are fairly strongly anchored to the water and surfactant molecules. The latter two species do not interact with each other, but favour different orientations of the mesogens within the droplet.

On cooling, the initially disordered mesogens order into a nematic then a smectic liquid-crystal phase. This induced the formation of highly organized surfactant domains at the interface, which adopted different morphologies depending on the mesogens' ordering and the water/surfactant ratio. The surfactants formed, for example, circular or striped patterns around the droplet. Changing the spherical droplets to planar or cylindrical shapes also led to ordered surfactant domains, albeit organized to a lesser extent. This demonstration that anisotropic molecules can cause surfactant species to organize at liquid–liquid interfaces may make it possible in future to functionalize liquid droplets in a specific, segregated manner, and further assemble them into complex architectures.