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

Abstract

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 Et₄C(tetraethylmethane) and R ₄Sn (R = C₂H₅, n-C₃H⁷ and n-C₄H₉) 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 β₁, the moduli of the slopes of the nematic and isotropic boundary lines in the reduced NI transition temperature (T^*) vs solute mole fraction (x₂) 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 x₂ diagram are discussed.