The effect of quasispherical solutes on the nematic to isotropic transition in liquid crystals

Abstract

When nonmesomorphic solutes are added to nematic liquid crystals, the nematic–isotropic transition temperature is depressed and a two‐phase region is observed. The slopes, β_n and β_i, of the nematic and isotropic phase boundary lines in the reduced nematic–isotropic transition 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 parallelepipeds 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 β_n and β_i with increasing solute size and yield predicted slopes which are at least within a factor of 2 of the experimental slopes. The modified mean field approach is found to give the best agreement with the activity coefficient data. A simple experimental method is proposed for estimating the effect of potential impurities (x?0.01) on the nematic–isotropic transition, and possible directions for future research in this area are suggested.