A MODEL FOR HOMOGENEOUS AND INHOMOGENEOUS HARD MOLECULAR FLUIDS - ELLIPSOIDAL FLUIDS

The thermodynamic and structural properties of a fluid of hard axially symmetric ellipsoids are studied on the basis of two approximations, namely that (a) the orientations of the molecules can be restricted to a discrete number of directions and (b) the direct correlation functi on can be written as a superposition of three geometrical functions wi th weights which depend on the density of the fluid. The variational p rinciple of Anderson and Chandler is used to determine the weights. Th e equation of state of the fluid is derived for a range of values of t he ratio of axes (elongation) from 0.35 to 5.0. For comparison new sim ulations have been performed on the same systems. Except at the greate st elongation, the theoretical pressure agrees very well with the simu lated pressure of the restricted orientation model as well as with tha t of a fluid of freely rotating ellipsoids. When the elongation is 5.0 , the pressure of the fluid of freely rotating ellipsoids lies somewha t above the pressure of the restricted orientation model, although for this model the theory and simulation are still in accord. Thus far th e theory has not revealed a transition to a nematic state although the re is some evidence from simulation that when the elongation is 5.0, t he isotropic state of the restricted orientation model is not stable. The direct correlation function of the homogeneous phase is used in co njunction with a density functional in order to investigate the densit y profile of the fluid confined to a slit. Except in the neighbourhood of a wall there is very good agreement between the theory and simulat ion. The discrepancy near the wall is to be expected from the simple f orm of functional used. Overall it appears that the approximation deve loped is adequate for describing the thermodynamic properties of the h omogeneous and inhomogeneous isotropic state, but that it needs some i mprovement for studying the properties of the nematic state.