PRETRANSITIONAL BEHAVIOR OF NEMATOGENIC MIXTURES - H-2 NMR INVESTIGATION

Molecular held theory predicts that the field-induced orientational or der in the isotropic phase of a nematogenic mixture should diverge as the nematic phase is approached. However, as soon as the biphasic regi on, in which nematic and isotropic phases coexist, is entered, the pre transitional divergence is predicted to be quenched and the held-induc ed order should then decrease slightly with decreasing temperature. In an earlier experimental study of this pretransitional behaviour, it w as found that the divergence of the held-induced order is not quenched immediately the biphasic region is entered. Here, we describe experim ents to investigate this unexpected disagreement between theory and ex periment. The reason for the disagreement is identified as the presenc e of concentration gradients within the sample and a method for the el imination of such gradients is demonstrated. Since molecular diffusion on the scale created by phase separation is relatively slow, it is es sential to establish the reproducibility of data, to confirm that the system is at thermodynamic equilibrium.