NONSPHERICAL MOLECULES WITH SPHERICAL SQUARE-WELLS REVISITED - SOME POSSIBLE NEW PHASE-DIAGRAMS FROM DENSITY-FUNCTIONAL THEORY

A simple model fluid, in which molecules consist of hard non-spherical cores embedded in spherical square-wells, has been re-examined. Withi n the Gaussian approximation of Onsager-Parsons-Lee theory, for very n arrow attractive square-wells, ordinary liquid-vapour coexistence is p reempted by a direct transition between a low density disordered fluid phase and a nematic phase. This is consistent with recent simulation results on fluids of Gay-Berne particles, and of hard spherocylinders with generalized square-wells which probed the effect of changing the ratio of attraction range to molecular elongation. Moreover, coexisten ce between two nematic phases is found on further decreasing the width of square-well, analogous to the isostructural solid-solid transition s observed in fluids characterized by very short range isotropic inter actions.