ON THE ALIGNMENT OF THERMOTROPIC NEMATIC AND SMECTIC LIQUID-CRYSTALS ON LECITHIN COATED SURFACES

The orientation of thermotropic nematic and smectic liquid crystals at monomolecular films of lecithin on glass as well as germanium substrates has been studied by polarizing microscopy and FTIR spectroscopy. Nematic liquid crystals with only a weak tendency to form smectic phases exhibit a dependence of their orientation on the temperature and the packing density of the monolayer which can be explained by a sterical model. Liquid crystals in the smectic A phase as well as nematics with a strong tendency to build up smectic layers, however, align homeotropically, independent on the packing density of the lecithin film and the temperature. This can be understood as a screening of the lecithin film properties by the smectic-like boundary layers of the nematic liquid crystal.