The Re-entrant Nematic, Enhanced Smectic A Phases and Molecular Composition

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Abstract
Phase diagram and X-ray studies of mixtures of 8OCB (cyanooctyloxybiphenyl) and 4O8 (butyloxybenzilidene octylaniline)and 8OCB and 6OCB (cyanohexyloxybiphenyl) demonstrate: 1. 4O8 molecules inhibit the pairing of the 8OCB molecules. One 4O8 molecule is attributed with the breakup of one 8OCB pair. 2. The maximum pressure at which the smectic A phase is stable increases as the number of 8OCB pairs decreases. 3. The temperature at which the smectic A phase is stable is maximum when the mixture is most polar [no 8OCB pairs at all]. For these mixtures, there is no nematic phase. 4. There is no smectic A phase when all of the 8OCB molecules are paired. 5. When the nematic-smectic A transition temperature of the mixture approaches the nematic-isotropic transition temperature of the pure compound, the nematic phase is completely suppressed. Assuming that the smectic A order parameter is coupled to the nematic order parameter and the coupling depends on temperature and composition, a Landau description is formulated which qualitatively summarizes all of these results.