Dielectric and optical studies of a nematogen (4,4-n-heptyl-cyanobiphenyl)

The permittivity, refractive index, molecular dipole moment, dielectric relaxation parameters and molecular Kerr constant are reported for the nematogen 4,4-n-heptyl-cyanobiphenyl (HCB) in the nematic phase, in the pure isotropic phase and in benzene solutions. The anisotropy of molecular polarizability is less than in benzene. Kirkwood–Bordewijk g-factors show significant dipole antiparallelism in the isotropic and nematic phases: in the latter it is much larger parallel to the director axis than perpendicular to it. The two major (orthogonal) component activation enthalpy and entropy terms for dipole reorientation (Bauer) in the nematic phase (including benzene solutions) are compared with those in the isotropic media: in the pure nematic ΔHB(‖)= 60 ± 8 kJ mol–1; ΔHB(⊥)= 6 ± 2 kJ mol–1. The current theoretical representations of the nematic phase are examined using the experimental data. The occurrence of pre-transitional molecular interaction in benzene solutions (above 3 mol dm–3) and in the pure isotropic phase can be detected.