The Effect of Quasispherical Solutes on the Nematic to Isotropic Transition in Liquid Crystals.

摘要

When nonmesomorphic solutes are added to nematic liquid crystals, the nematic-isotropic transition temperature is depressed an a two-phase region is observed. The slopes Beta(n) and Beta(i) of the nematic and isotropic phase boundary lines in the reduced nematic-isotropic transistion temperature (T*) vs solute mole fraction (x) diagrams are reported for binary mixtures of quasispherical solutes with nematic liquid crystals. The systems studied are Me4Sn, Et4C, Et4Sn, Pr4Sn, Bu4Pb, and Bu4Sn (Me = methyl, Et -ethyl, Pr = propyl, and Bu = butyl), mixed with p-methoxybenzylidene-p-n-butylaniline (MBBA), and CCl4 mixed with p-p-di-n-hexyloxyazoxybenzene(DHAB). Also reported are infinite dilution solute activity coefficients for several mixtures involving Et4C and CCl4. The experimental results are compared with predicted values from statistical-mechanical theories based on (a) a lattice model of hard rectangular parallelpipeds and cubes, (b) a virial expansion treatment of hard spherocylinders and spheres, and (c) a proposed modification of a mean field treatment (Maier-Saupe type) of axially symmetric and spherical molecules. All three models correctly predict the observed trend of increasing Beta(n) and Beta(i) with increasing solute size and yield predicted slopes which are at least within a factor of 2 of the experimental slopes.