COMPUTER-SIMULATION STUDIES OF ANISOTROPIC SYSTEMS .22. AN EQUIMOLAR MIXTURE OF RODS AND DISKS - A BIAXIAL NEMATIC

In principle, binary mixtures of rod-like and disc-like particles shou ld exhibit a biaxial nematic phase, but in practice phase separation i nto two uniaxial nematic phases prevents this. Here, we report the res ults of a computer simulation study of an equimolar mixture of rods an d discs in which phase separation is not allowed. The particles are co nfined to the sites of a simple cubic lattice in which each rod is sur rounded by six discs and vice versa. Neighbouring particles interact s uch that they prefer to align with their respective symmetry axes orth ogonal to each other. In contrast, the interaction between next neares t neighbours, which are either rods or discs, is such that their symme try axes tend to be parallel. Monte Carlo simulations of this model mi xture show that an orientationally ordered phase exists at low tempera tures. This nematic phase has overall uniaxial symmetry and the partic les have a negative second rank orientational order parameter, indicat ing that they tend to align at right angles to the director. The two i nterpenetrating sub-lattices containing either rods or discs, however, exhibit a biaxial nematic phase. The results of the simulation are fo und to be in reasonable agreement with the predictions of a molecular field theory for this model mixture. We have also investigated the beh aviour of this mixture when the rods and discs are allowed to exchange between their lattice sites. The mixture is found to separate into tw o uniaxial nematic phases composed essentially of either rods or discs , as expected.