Influence of dimerization on the nematic-isotropic phase transition in strongly polar liquid crystals

A molecular theory of the nematic-isotropic phase transition is developed for the fluid composed of rod-like particles with large longitudinal dipoles. The equilibrium between the monomers and dimers with antiparallel dipoles is explicitly taken into account and the concentration of dimers is determined self-consistently together with the nematic order parameters. We show that for small central dipole moments the nematic-isotropic transition temperature increases with the increasing dipole. This is in accordance with the results of previous approaches. By contrast, for large values of the dipole the transition temperature decreases due to the growing concentration of dimers. This result enables one to explain the results of recent computer simulations that reveal a destabilization of the nematic phase in a system of hard rods with large central longitudinal dipoles. The temperature variation of the concentration of dimers is also analysed and the sign of the concentration discontinuity at the transition point is correlated with the qualitative influence of dimers on the transition temperature.