Anisotropic Translational Diffusion of Methane in N-(4-n-pentyloxybenzyl dene)-4-n-hexylaniline (50.6)

Abstract

The temperature dependence of the translational diffusion coefficients is reported for methane dissolved in the thermotropic liquid crystal N-(4-n-pentyloxybenzylidene)-4-n-hexylaniline (50.6). The general features of methane diffusion behavior in the N, S_A and crystal S_B phases of 50.6 are quite similar, with respect to the magnitude of the rates and corresponding anisotropies, to those found in the homolog 40.6. The mutual characteristics are: that considerable smectic character (D _|| < D _||) exists in the nematic phase, that D _|| ≪ D _|| with E _|| ≫ E _|| in the S_A phase because of increasing layer definition and of methane expulsion from the cores to the chains and that D _⊥/D_|| ∼ 30 (with E _|| ∼ E _⊥) in the crystalline S_B phase. Methane behavior in the S_F phase of 50.6 is very unusual, showing a slight decrease in D_⊥ and large increases in D _|| upon decreasing temperature with D _⊥/D_|| values of from 10 to 5. This is possibly the result of the decrease in the close-packed core thickness coupled to increase in the disorder (loss of 3-d correlation) within the layer in the S_F phase. The diffusion behavior in the S_G phase is similar to that found in the B_B phase. The factor of three larger D _|| values in S _G over S _B is thought to be the result of persistance of disordered regions remaining from the Sr phase.