Pretransitional behaviour of nematogenic mixtures - H-2 NMR investigation

Molecular held theory predicts that the field-induced orientational order in the isotropic phase of a nematogenic mixture should diverge as the nematic phase is approached. However, as soon as the biphasic region, in which nematic and isotropic phases coexist, is entered, the pretransitional divergence is predicted to be quenched and the held-induced order should then decrease slightly with decreasing temperature. In an earlier experimental study of this pretransitional behaviour, it was found that the divergence of the held-induced order is not quenched immediately the biphasic region is entered. Here, we describe experiments to investigate this unexpected disagreement between theory and experiment. The reason for the disagreement is identified as the presence of concentration gradients within the sample and a method for the elimination of such gradients is demonstrated. Since molecular diffusion on the scale created by phase separation is relatively slow, it is essential to establish the reproducibility of data, to confirm that the system is at thermodynamic equilibrium.