Optical Studies of the Anisotropic Solute-Solvent Interaction in Liquid Crystals

Abstract

Abstract “Compensatcd” nematic mixtures of cholesteryl derivatives are well suited as anisotropic solvents for optical experiments in the ultraviolet. The average orientations of a great number of aromatic molecules in a nematic mixture of cholesteryl chloride and cholesteryl laurate at 30°C have been determined by absorption polarization studies. Our attempts to correlate the elements of the Saupe order matrix of these molecules with their physical properties yielded the following results: For some elongated molecules a linear relation holds between the average orientation of the long molecular axis and the molecular dimensions, while large deviations from this correlation are observed for molecules with easily polarizable substituents. The order parameters are directly related to the principal solute polarizabilities of the solute molecules which can be determined with good accuracy from the order matrix elements. We conclude from these results that repulsive forces do not contribute considerably to the anisotropic solute-solvent interaction potential. It is further shown that relative order parameters in cholesteric solvents can be determined by measurement of the circular dichroism caused by the solute absorption.