Viscous Behavior of Binary Mixtures of Mesogenic Solvent/Non-Mesogenic Solute

The paper presents results of the shear viscosity measurements performed on diluted binary mixtures of mesogenic solvent n-hexylcyanobiphenyl (C$\text{}_{6}$H$\text{}_{13}$PhPhC≡N,6CB) and two non-mesogenic admixtures: (i) n-heptylcyanophenyl (C$\text{}_{7}$H$\text{}_{15}$PhC≡N,7CP), composed of the molecules of the same polarity as the solvent molecules but of a slightly shorter length, and (ii) 4-n-propylcyclohexyl-4'-n-pentylphenyl (C$\text{}_{3}$H$\text{}_{7}$CyHxPhC$\text{}_{5}$H$\text{}_{11}$,3CyP5), composed of the non-polar molecules but of a length very close to that of the mesomorphic solvent molecules. The experiment showed that the concentrational depression of the clearing temperature and the temperature extent of the isotropic + nematic (I + N) two-phase region in the mixtures are significantly smaller, i.e. the nematic phase is more thermodynamically stable, when the admixture molecular length is compatible to that of the mesogenic solvent, regardless of the polarity of the admixture molecules. The activation energy for freely flowing mixtures in the isotropic, nematic, and two-phase I + N regions was determined and discussed.