Statistical Theory of Surface Tension and Molecular Orientations at the Free Surface in Nematic Liquid Crystals

The surface tension γ is calculated in the mean field approximation in the system of rod-like molecules interacting via attraction as well as hard-core repulsion. It is found that the excluded volume effect favours the normal alignment of molecules at the free surface. The molecular orientations observed at the free surface are explained as an effect of counterbalance between the repulsive and attractive intermolecular forces as in the following: (1) normal to the surface (8CB, 5CB), (2) tilt against the surface, (3) transition between the normal and tilt orientations at a certain temperature T 0 lower than the nematic-isotropic transition temperature T c (MBBA, EBBA). The jump of γ at T c obtained in this work agrees with the experimental results.