Nonideality in the binary phase diagrams of homologous 4'-n-alkyl-4-cyanobiphenyls

CHED 1013

Joshua M. Cobb, jcobb@hmc.edu and Gerald R. Van Hecke. Department of Chemistry, Harvey Mudd College, 301 Platt Blvd, Claremont, CA 91711
Binary mixtures of homologous 4'-n-alkyl-4-cyanobiphenyls (nCB, n=# of alkyl carbons), exhibit smectic and nematic liquid crystalline phases. Phase diagrams were constructed using polarizing microscopy for the mixtures 5CB+8CB, 5CB+7CB, and 7CB+8CB to discover any systematic trends in the phase properties that correlate with molecular structure. The 7CB+8CB binary mixture exhibited isotropic-nematic (N-I) transition temperatures that were greater than the N-I temperatures for the pure materials, proving this mixture thermodynamically to be nonideal. The 5CB+7CB binary mixture exhibited two distinct nematic phases, one of which has a local smectic ordering. Nonideality and two nematic phases were observed in 7CB mixtures and may be related to the fact the 7CB is the first member of this series to exhibit a smectic phase. The nonideality of the mixtures will be analyzed and discussed using the equal Gibbs analysis.