A statistical theory of liquid crystalline mixtures: phase separation

Abstract

The molecular field theory of multicomponent liquid crystalline mixtures, developed by Humphries, James & Luckhurst, has been extended to investigate the possibility of phase separation in binary mixtures of rods and spheres. This extension indicates that the addition of a spherical solute to a liquid crystal depresses the transition from the isotropic to the nematic phase. The solute induced phase transition is first order, although the nematic and isotropic phases are found to be separated by a two-phase region consisting of both nematic and isotropic phases. These qualitative conclusions of the theory are in complete accord with experiment and there is also reasonable agreement with certain quantitative predictions. The same problem has been tackled using a lattice model with purely repulsive anisotropic interactions; this theory would appear to be marginally less successful than our own which is based on a weaker anisotropic potential. Consequently it is not possible to use experimental studies of phase separation in binary liquid crystal mixtures to demonstrate the rôle of repulsive forces in liquid crystal formation.