Thirmotropic Smectic Liquid Crystals of Ionic Amphiphilic Compounds: A General Discussion

Abstract

Starting from a reinvestigation of the long chain Na n-alkanoates in the molten state, the mesomorphism of amphiphilic molecules comprising an alkyl chain and an ionic endgroup is generally discussed. Two different smectic mesophases of the ‘neat’ type can occur for these compounds: smectic I, viscous and birefringent, with regular layer stacking; smectic II (often described in the literature as a conventional liquid), fluid and opticallyisotropic, due to the small dimensions of the liquid crystalline domains, and with fluctuations in the interlayer distances, as indicated by its X-ray diffraction pattern. Possible structural arrangements of the ionic end-groups in the smectic layers are presented, and the differences experimentally observed between short chain and long chain compounds explained in terms of a simple electrostatic model. In agreement with the experimental finding, the mesomorphic state is evaluated from the model to be stable relative to the isotropic state up to temperature well above those at which thermal decomposition occurs.