Refractive indices, density and order parameter of two liquid crystals HBT and OBT

Abstract

The temperature variation of refractive indices (n_e n_e), birefringence (δn), density (ρ) and order parameter (S) of two liquid crystals, namely, N-(p-hexyloxybenzylidene)-p-toluidine (HBT) and N-(p-octyloxybenzylidene)-p-toluidine (OBT) are reported in the smectic (S_A in OBT and S_B in HBT), nematic and isotropic phases. For accurate measurement of δn, the wedge method was modified to eliminate the need to measure the wedge angle. Density measurements indicate that the smectic-nematic and nematic-isotropic phase transitions in these materials are of first order. Using refractive index values and the density data, the internal field factors (γ_e, γ_o), the ratio of principal polarizabilities f(=α_eα_o) and the order parameter, S, have been evaluated and their temperature dependence discussed in the light of molecular geometry. The order parameter has been determined using the isotropic internal field model (Vuks approach) and the anisotropic internal field model (Neugebauer's approach). The S values determined using these two models agree in the nematic phase but differ considerably in the smectic phase.