Phase Diagram and Component Order Parameters in a Nematic Mixture: A Combined Deuterium and Proton NMR Study

Deuterium and proton magnetic resonance results on mixtures of nematic perdeuterated para-azoxy-anisole (PAAd14) and potential nematic 4-4’ diacetoxy-2-2’ dimethyl azoxybenzene (Ac9Ac) are reported. The phase diagram is established and the order parameters of both components are determined. These data are compared with existing theories. The molecular mean field theory of Pallfy-Muhoray et al. is found to be adequate to describe the behaviour of the order parameters, implying that the nematic interaction parameters satisfy the geometric mean rule. The virtual clearing temperature and the molecular volume of Ac9Ac are estimated. It is also shown that the concept of pseudo-clearing temperature may be useful to predict the component order parametrs in (dilute) solutions of Ac9Ac in terms of the order parameter of the pure solvent PAAAd14. The same theory is found to be rather poor to describe quantitatively the phase diagram. The molecular field/lattice model of Brochqard et al. predicts that in dilute solutions of Ac9Ac, the number of lattice sites occupied by one Ac9Ac molecule is much larger than its physcial volume. Using a simple geometrical model to analyze the magnitude of the measured order parameteres, we suggest that this number of sites should be identified with the average volume in which the orientational order of the solvent is perturbed by the presence of one solute molecule.