A model dipolar ellipsoidal fluid

Abstract

A theoretical method previously introduced in order to study the structure and thermodynamics of a charged ellipsoidal fluid is here applied to the study of a dipolar ellipsoidal fluid. To enable the strong-coupling (large dipole) form of the direct correlation function to be written analytically, the model employs ellipsoids with charges distributed over their surfaces. This implies that the electric field at a large distance from one molecule results from many multipoles although the dipole is the dominant one. A geometric ansatz for the direct correlation function containing a small number of parameters is then constructed and the parameters determined through the means of a variational principle based upon the mean spherical approximation. From the resultant direct correlation function it is possible to determine thermodynamic properties, including the relative permittivity of the fluid and the electrostatic energy. Within the model no ferroelectric transition is observed. Further the resultant direct correlation function is in a suitable analytic form for applications to the structure of an inhomogeneous fluid. The model is applied to fluids of ellipsoids with elongations ranging between 1/5 (oblate) and 5 (prolate). The range of dipole moments and densities allows relative permitivities up to approximately 80. The results are consistent with other studies of hard dipolar fluids although the model is not precisely the same except in the special case of hard spheres.