A study of orientational ordering in a fluid of dipolar Gay-Berne molecules using density-functional theory

We present a density-functional approach to describe the orientational ordering of nonpolar and dipolar Gay-Berne fluids. The first-order perturbation theory developed by Velasco [J. Chem. Phys. 102, 8107 (1995)] for a Gay-Berne fluid is simplified and tested for molecules with a length to breath ratio of kappa=3 and energy anisotropies of kappa'=1, 1.25, 2.5, and 5. The theory is found to be in fair agreement with existing simulation data for the location of the isotopic-nematic phase transition, but it overestimates the vapor-liquid critical point of the fluid due to a description of the free energy at the mean-field level. The effect on the phase behavior of including a central longitudinal point dipole within the Gay-Berne molecule is studied using a correct treatment of the long-range dipolar contribution at the level of a second-order virial theory [B. Groh and S. Dietrich, Phys. Rev. E 50, 3814 (1994)]. For a given energy anisotropy of kappa'=5 and reduced dipole moment mu*=0.5 we search for a stable ferroelectric nematic phase by changing the length to breath ratio kappa. We do not find any evidence of ferroelectric nematic ordering for kappa>1.5; the system only exhibits vapor-liquid and isotropic-nematic phase transitions for these values of the aspect ratios. For a slightly elongated and oblate shaped potential (e.g., kappa=0.5), regions of stable isotropic-ferroelectric nematic and nematic-ferroelectric nematic phase coexistences are observed. The results of the theory indicate that a ferroelectic nematic fluid phase may be stabilized with respect to the positional ordering in the fluid of oblate dipolar particles. Comparison are made, where appropriate, with the existing results of Monte Carlo simulations for dipolar Gay-Berne fluids (Rull and co-workers, Molec. Phys. 94, 439 (1998); J. Chem. Phys. 109, 9529 (1998)). (C) 2002 American Institute of Physics.