The pressure and temperature dependence of the orientational order of a biaxial solute dissolved in a nematic solvent

Abstract

We have determined the principal components of the Saupe ordering matrix for anthracene-d₁₀ dissolved in the nematogen trans-4-n-heptyl(cyanophenylcy-clohexane) as a function of both temperature and pressure, using deuterium NMR spectroscopy. The availability of an equation of state for the solvent has allowed us to extract the temperature variation of the solute ordering matrix at constant volume as well as the usual constant pressure. This provides the first real test of molecular field theories of the ordering of biaxial particles in a liquid crystal solvent, since such theories assume that the system is at constant volume. As we had expected, the predictions of theory are found to be in better accord with the results at constant volume than at constant pressure, although the agreement is not perfect even at constant volume. We have measured the solute equivalents of the thermodynamic parameter Γ⁽¹⁾{≡-(∂ ln T/∂ ln V)_{c2, 0} } suggested by Alben for pure nematogens to gauge implicitly the relative importance of attractive and repulsive forces in stabilising nematic order. The relationship of the solute to the solvent Γ parameters is discussed together with the significance of their values.