Enhancement of the nematic order of polar liquid crystals in thin cells

Experimental observations show that the nematic order parameter (S) is significantly enhanced as the thickness of the cell is reduced. However, calculations based on earlier theories are not in good agreement with the experimental data. We earlier developed a molecular mean field model that assumes that the medium with polar molecules consists of inter-converting anti-parallel and parallel pairs, to explain the molecular origin of the 'two lengths' model. The model has been used to explain effects such as double re-entrance and the effect of electric field on the nematic-isotropic transition temperature, T-NI. We have recently extended the Maier-Saupe (M-S) theory using an empirical variation of surface potential, to account for the enhancement in which a molecule near the surface is assumed to feel the mean field potential (M-S type) and also the surface-induced potential. Variation in the density of the medium due to variation of S has been ignored. In the present paper, the change in the relative proportion of the parallel pairs due to changes in the density of the medium is included in the M-S theory with the surface potential. This results in improved agreement with the experimental data.