Molecular association and the isotropic-nematic transition

Abstract

A mean field theory for polar nematic liquid crystals is presented. The liquid crystalline system is considered to be a mixture of anti-parallel pairs and unpaired molecules. An attempt is made to understand the behaviour of such polar nematogens using an extended version of the generalized Maier-Saupe theory of nematic mixtures which incorporates a version of the dielectric theory of Maier and Meier extended to mixtures. The theory is applied to the case of nCBs (n-cyanobiphenyls). The effect of including the electronic dielectric constant is also considered. Reasonable agreement is obtained with experimental data for the behaviour of the dielectric permittivity as well as the change in the average dielectric permittivity at the transition. As expected, we also obtained a jump in the mole fraction of the ‘associated pairs’ at the I-N transition.