Dipolar and dispersion interactions in nematogens A computer simulation study

Abstract

A classical system, consisting of identical cylindrically symmetric particles, with centres fixed on a simple-cubic lattice, may or may not support nematic—like orientational order depending on the interaction potential. For example, a Lebwohl-Lasher potential model produces orientational order, as does an anisotropic London—de Boer dispersion potential restricted to nearest neighbours, although increasing its range destroys the orientational order and brings about a staggered configuration of the system. In consequence, a Maier-Saupe molecular field treatment is appropriate in the first two cases, but not in the last. On the other hand, according to computer simulation results, a purely dipolar interaction produces a low temperature antiferroelectric phase which can be regarded as an extreme case of a nematogen. We carried out Monte Carlo simulations on a potential model defined by a linear combination of dipolar and full ranged London-de Boer dispersion terms, in order to study their competition. To save computer time and yet monitor the physically relevant changes, we have varied their relative weights, while keeping fixed both the temperature and the anisotropy parameter in the dispersion potential. Simulation results show that in energetic terms the two interactions are cooperative, whereas in structural terms their interplay brings about first a weak but recognizable increase of nematic ordering, and then its collapse with the onset of the staggered configuration.