Thermodynamics of Solutions with Liquid Crystal Solvents. VIII. Solute Induced Nematic-Isotropic Transitions

An experimental approach has been devised which permits the study of the effect of nonmesomorphic solutes of different sizes and shapes on nematic phase stability. The apparatus consists of a vacuum microbalance system which employs a Cahn RG Electrobalance to determine the weight of volatile solute absorbed by a known weight of nematic material at a given temperature and solute partial pressure. From such measurements, solute activity coefficients (γ2) were determined as a function of solute mole fraction (x 2) at several set temperatures (T) below the nematic-isotropic transition temperature (61.2°C) of the pure nematogen p-methoxybenzylidene-p'-n-propylaniline (MBPA). The solute probes studied were n-heptane, benzene and carbon tetrachloride. For all three systems the observed behavior was: In γ2 decreased sharply with increasing x 2 in the nematic region, became discontinuous at a mole fraction x2 corresponding to the solute induced transition point, and then decreased gradually (approaching zero) with increasing x 2 in the isotropic region. From studies at different T values, dT/d-x2 was determined for all three solutes. These experimental results are discussed in terms of the solute molecular characteristics and in light of recent theoretical results obtained from a statistical mechanical lattice model of a two component mixture of non-interacting rigid rods of different size. This study provides additional evidence of the important role of repulsive forces in governing nematic stability.