MONTE-CARLO SIMULATIONS OF SOLUTE ORDERING IN NEMATIC LIQUID-CRYSTALS- SHAPE ANISOTROPY AND QUADRUPOLE-QUADRUPOLE INTERACTIONS AS ORIENTING MECHANISMS

Monte Carlo computer simulations were used to investigate the effects of shape anisotropy and electrostatic interactions as mechanisms for o rientational ordering of solutes in nematic liquid crystals. The simul ation results were analyzed in terms of two theories of solute orderin g which derive mean-field orientational potentials from the intermolec ular pair potential. In the calculations, solute and solvent molecular shapes were approximated by hard ellipsoids. Most simulations also in corporated the interaction between point quadrupole moments placed at the centers of the ellipsoids. In the hard-core systems, orientational order parameters and distribution functions were calculated for a col lection of different solutes under a variety of conditions. A theory d ue to Terzis and Photinos [Mol. Phys. 83, 847 (1994)] was found to und erestimate the effect of shape anisotropy on orientational ordering dr astically. The introduction of an effective solvent packing fraction w as unable to improve the predictive power of the theory significantly. The quadrupolar systems were used to investigate a mean-field model f or solute ordering which considers an interaction between the solute m olecular quadrupole moment with an average electric-field gradient. Th e simulations indicate that the electric-field gradient sampled by the solute is highly dependent on the properties of the solute, contrary to some experimental evidence. Further, the effects of the intermolecu lar quadrupolar interactions on orientational ordering and the electri c-field gradient were analyzed using a mean-field potential derived he re and based on the theory due to Emsley, Palke, and Shilstone [Liq. C ryst. 9, 649 (1991)]. This model was found to provide a qualitatively correct but quantitatively imprecise prediction of orientational order ing.