Thermodynamics of Solutions with Liquid Crystal Solvents. IV. GLC Determination of the Degree of Order in a Nematic Mesophase

Abstract

Abstract Owing to thermal motion, molecular orientation is never entirely complete in a nematic mesophase. The commonly used methods of obtaining a measure of the extent of parallelism in this mesophase are ESR and NMR. Proposed here is a gas-liquid chromatography (GLC) approach for determining the degree of order, S. Our working model regards the nematic phase as composed of a fraction of completely aligned molecules and a fraction of completely isotropie molecules. Further, solute molecules dissolved in the nematic state are assumed to distribute themselves between these two arrangements. Based on the assumptions of this “two phase” model and on exact thermodynamic arguments, a set of simultaneous equations is derived for evaluating S from GLC-determined solute infinite dilution activity coefficients and partial molar excess enthalpies. From meassurements on two nematogenic substances, P-azoxyanisole and 4, 4′-dihexoxyazoxybenzene, S values are obtained which are independent of the nature of the solute probe. The advantages and limitations of this method and the reason for the minor discrepanies between our S values and those determined from resonance experiments (ours are slightly lower) are discussed. The thermodynamic quantities corresponding to solution in completely aligned solvent are determined and discussed. The potential utility of this approach (e.g., studying order within the cholesteric mesophase) is also described.