Molecular theory of hexb-sma-isotropic transitions in ultrathin liquid crystal films

A microscopic theory is developed to treat the ultrathin film of liquid crystals of molecules that have no cylindrical symmetry. The Hamiltonian is derived from the basic electrostatic interaction among electrons by considering the dipole-dipole and dipole-quadrupole interactions between nonchiral molecules. It exhibits the in-plane sixfold symmetry. From a unified model with the same interaction constants we are able to explain simultaneously the layer-thinning SmA-I transition, the anomalous multiplex heat capacity, the strong singularity in the HexB-SmA transition, and the coexistence of different phases. The theoretical calculations agree quantitatively with recent experimental results for the free-standing 54COOBC films.