The effect of quasispherical solutes on the smectic-A-nematic and nematic-isotropic phase equilibria in p-n-alkyl-p′-cyanobiphenyl liquid crystals

Density as a function of temperature was measured in the nematic and isotropic phases in the immediate vicinity of the nematic-to-isotropic transition temperature (T_NI) of p-n-hexyl-p′-cyanobiphenyl (6CB) and p-n-octyl-p′-cyanobiphenyl (8CB) and in the smectic-A and nematic phases in the immediate vicinity of the smectic-A-to-nematic transition temperature (T_AN′) of 8CB. Similar measurements were made on dilute solutions of the quasispherical solutes Et₄C (tetraethylmethane) and R₄Sn (R = C₂H₅, n-C₃H₇, and n-C₄H₉) in 6CB and 8CB. Addition of these solutes led, as is expected for first-order phase transitions, to a nematic-isotropic two-phase region bounded by temperatures T_N and T_I, which are lower than T_NI, and in the case of the 8CB systems to a smectic-A-nematic two-phase region bounded by temperatures T_A and T_N', which are lower than T_AN'. Volume changes at the transitions of the liquid crystals confirm that they are weakly first-order, β_N and β_I, the moduli of the slopes of the nematic and isotropic boundary lines, respectively, of the nematicisotropic two-phase regions in the 6CB and 8CB SYstems were obtained assuming that the respective reduced temperatures T_N* (= T_N/T_NI) and T_I* (= T_I/T_NI versus solute mole fraction (x₂) diagrams are linear. β_A and β_N', similarly defined for the smectic-A-nematic two-phase regions in the 8CB systems, were obtained from the respective linear T_A* (= T_A/T_AN′ and T_N′* (= T_N′ / T_AN′) versus x₂ diagrams. The dependencies of β_N and β_A on solute and solvent sizes and the nature of the liquid crystalline phase are discussed.