Molecular-Optical and Structural Anisotropy of the Nematic Mixture E7

Abstract
The relation of the refractive indices nj of the nematic mixture E7 consisting of cyanobiphenyl derivatives for the light-wave polarization along (j = ||) and across (j = ⊥) the optical axis with the polarizability parameters (components γ_j^α, mean value γ_m^α = (γ_||^α + 2γ_⊥^α)/3, anisotropy δγ_α = γ_||^α - γ_⊥^α) and orientational order of α-type molecules comprising the mixture was investigated. The local-field tensor with components f_j =1+L_j(n_j² – 1) was introduced for the mixture and the relationship between the Lorentz-tensor components L_j and the components L_j^α was established. The temperature dependences L_j(Т) for the E7 mixture were obtained using experimental dependences n_j(λ,T). The relationship L_j(L_j^α) was confirmed. The temperature dependence of the material order parameter S(T) was obtained. The change of the mean value γ_m(Т) = ((γ_|| + 2γ_⊥)/3 for the studied mixture due to changing of S(Т) was observed. The dependence γ_m(S) was shown to be caused by two opposite-sign contributions due to changing conformation of molecular biphenyl cores and intermolecular interactions. The obtained results were compared with the consequences of using model tensors f_j, which literature data are known for mixtures.

Keywords: nematic mixtures, orientational order of nematic mixtures, local-field anisotropy in nematic mixtures, molecular properties in nematic mixtures