Birefringene of Smectic Modifications of the Homologous Thallium Soaps

Abstract

The birefringence has been studied of the smectic high temperature modification (neat phase) of the anhydrous thallium soaps independece on temperature and at different wavelength. The measurements were carried out on uniaxially orientated liquid crystalline layers on the principle of Abbé's double prism. The lower homologues exhibit negative double refraction which decreases with increasing chain length and decreasing temperature. In the middle members of the homogous series a change in the sign of double refraction is observed. The higher members exhibit positive birefringence, which increases with increasing alkyl chain length and with decreasing temperature. An attempt will be made to to explain this behaviour on the basis of the structure of the smectic modification of the thallium salts.