The Effect of Quasispherical Solutes on the Nematic to Isotropic Transition in 7CB

Density as a function of temperature was measured in the nematic and isotropic phases of p-n-heptyl-p'-cyanobiphenyl (7CB). Similar measurements were made on dilute solutions of the quasispherical solutes Et 4 C(tetraethylmethane) and R 4 Sn (R = C 2 H 5 , n-C 3 H 7 and n-C 4 H 9 ) in 7CB. Addition of these solutes to 7CB led, as expected, to a depression in its nematic-isotropic (NI) transition temperature and to the formation of a two-phase region. From these density measurements, β N and β 1 , the moduli of the slopes of the nematic and isotropic boundary lines in the reduced NI transition temperature (T * ) vs solute mole fraction (x 2 ) diagrams, were obtained. These thermodynamic results are contrasted with values for the same solutes in p-n-pentyl-p'-cyanobiphenyl (5CB). Indications that a curvature may exist at very low solute mole fractions in the T * vs x 2 diagram are discussed.