Statistical Theory of Surface Tension and Molecular Orientations in Nematic Liquid Crystals.II. The Nematic-Isotropic Interface

The surface tension of the nematic-isotropic ( N - I ) interface is calculated in a mean field approximation for the system of rod-like molecules interacting via attraction as well as hard-core repulsion. It is found that the excluded volume effect favors the planar orientation of molecules at the N - I interface in contrast with the case of the nematic free surface at which the effect favors the normal alignment. Experimental behavior of nematogens such as 4- n -pentyl-4'-cyanobiphenyl (5CB) and 4-methoxybenzylidene-4'-butylaniline (MBBA) are analysed. Orientations of a molecule at the N - I interface and at the nematic free surface are explained consistently as effects of a counterbalance between the repulsive and attractive intermolecular forces.