Influence of head group and chain length of surfactants used for stabilising liquid crystal shells

Abstract

ABSTRACTWe investigate the stability and textural development in nematic liquid crystal (LC) shells, with aqueous interior and exterior, as a function of the type and concentration of surfactant stabiliser of the shell interfaces. The LC is the common thermotropic nematic 5CB and the surfactants are commercial, of cat- as well as of anionic type, with varying alkyl chain length. In addition to stabilising the shell interfaces, surfactants are generally assumed to promote radial (homeotropic) LC alignment, based on prior studies where the surfactant concentration was well above the critical micelle concentration (CMC). Here, we focus on the low-concentration range, below CMC. We find that both cat- and anionic surfactants can stabilise shells, although the higher water solubility of cationics can render stabilisation more difficult. We also conclude that surfactants do not necessarily impose homeotropic alignment; if the surfactant concentration is very low, the director may adopt planar alignment at the 5CB–water interface. Interestingly, the threshold concentration, where the surfactant takes control of alignment, is different for the shell inside and outside. Shells stabilised by solutions of surfactant with concentration near the threshold may therefore adopt a hybrid configuration, with homeotropic inside and planar outside.