THE ISOTROPIC-NEMATIC TRANSITION OF DIPOLAR SPHEROCYLINDERS - COMBINING THERMODYNAMIC PERTURBATION WITH MONTE-CARLO SIMULATION

We attempt to predict the phase behaviour of elongated molecules posse ssing longitudinal point dipoles by treating the dipolar interaction a s a perturbation on a reference system consisting of hard spherocylind ers. Thermal averages of up to the fourth power of the total dipolar i nteraction are evaluated by Monte Carlo simulation in the NVT ensemble of the nonpolar spherocylinders over a range of densities spanning th e isotropic-nematic transition density. From the evaluation of the fir st two terms in the high-temperature perturbation expansion of the fre e energy we find that the nematic phase is destabilized relative to th e isotropic phase (when compared with the non-polar system) as one swi tches on a weak central dipolar interaction. In contrast, off-centre d ipoles appear to have a stabilizing effect on the nematic phase for we ak dipoles, but as the dipole strength increases the nematic is seen t o be destabilized with respect to the isotropic phase.