An Empirical Form for the Orientation-Ordering Intermolecular Potential in Para-Azoxyanisole

Abstract

We demonstrate here that the angle-independent factor of the orientation-ordering intermolecular potential (OIP) U in para-azoxyanisole (PAA) can be well represented by an equation of the form: u = A/T - B, where A and B are positive and independent of temperature T but dependent on pressure P; u is defined as U/kT, where k is the Boltzmann constant. This form for u has been derived from experimentally observed temperature dependence of u at a constant P. In such a determination of u, use has been made of the long-range orientational order parameter Q measured by an NMR technique and a universal u-Q relation. This u-Q relation, formally similar to the one that can be derived from the Maier-Saupe theory, has been obtained from a phenomenological theory and the intermolecular interaction treated under the molecular field approximation. We apply this empirical form for u in understanding a few experimental results in PAA, and show that reasonable agreements can be obtained if certain other facts are also taken into proper consideration.