Anisotropic Translational Diffusion of Methane and Chloroform in Thermotropic Nematic and Smectic Liquid Crystals

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Abstract
Nuclear magnetic resonance pulsed-gradient spin-echo measurements are reported for the anisotropic translational diffusion coefficients and their temperature dependence, of methane, monodeuteromethane and chloroform dissolved in several thermotropic liquid crystals exhibiting nematic, smectic A and smectic B phases. For methane, in those liquid crystals forming only nematic phases, the diffusion parallel to the director (D ⊥) is greater than the perpendicular diffusion (D); the magnitude of the diffusional anisotropy (D ⊥/D ⊥) depends on the nature of the nematic medium. In the nematic phases of smectic-forming materials, D ⊥/D ⊥ < 1. In the smectic A phases, D ⊥/D ⊥ ≪ 1 with a much larger activation energy for D ⊥. In the smectic B phase, D ⊥/D ⊥ ≪ 1 with similar activation energies. Chloroform exhibits similar behavior with deviations in the nematic phases of a polymorphic liquid crystal. A qualitative interpretation of these results is given in terms of several factors which govern the diffusion behavior: different diffusion characteristics in aliphatic and aromatic regions of the ordered medium, expulsion of methane (and possibly chloroform) from the aromatic cores into the aliphatic end chains as a result of the smectic layering behavior and pronounced pretransitional ordering effects in nematic phases of the polymorphic liquid crystals. In addition to the diffusion measurements, proton dipolar and deuterium quadrupolar splittings in these systems are reported for CH4 and CH3D. These splittings are related to the strength of solute-solvent interaction and consequently to the diffusion patterns. Complementary data is presented for methane proton spin-lattice relaxation measurements in these systems.