Determination of Orientational Order Parameter from Optical Studies for a Homologous Series of Mesomorphic Compounds

We have measured the refractive indices (no , ne ) and densities of four members (nonyl, decyl, undecyl and dodecyl) of a homologous series cyanobiphenyl alkyl ether over their entire mesomorphic range and in their isotropic phases. The molecular polarizabilities have been calculated from refractive indices using both Vuks' and Neugebauer's relations. The additive rule of bond polarizabilities has also been employed to calculate the molecular polarizabilities of the four compounds assuming two different configurations of the alkyl chains. The orientational order parameter values at different temperatures for each compound have been calculated from these polarizability data. Since all the four compounds possess smectic A phases, we have compared our experimental order parameters with those calculated from the McMillan's model. Our experimental (P 2)values for these four compounds show a pronounced odd-even effect.