ELECTROSTATIC INTERACTIONS IN LIQUID-CRYSTALS - ORDERING OF RIGID SOLUTES IN NEMATIC SOLVENTS

The orientational distribution of a probe molecule in a nematic solven t is derived in terms of the intermolecular potential describing steri c, dispersional, and electrostatic interactions with the solvent molec ules. Orientational correlations among the latter molecules are neglec ted in the derivation, whereas probe-solvent correlations are treated explicitly. The theory is used to analyse NMR measurements of order pa rameters of small solute molecules in nematic solvents. Tenser compone nts of the solute potential of mean torque up to fourth rank are evalu ated. The various electrostatic contributions (dipole-dipole, dipole-q uadrupole, quadrupole-quadrupole, etc.) are assessed quantitatively. I t is found that the residual overall electrostatic interaction is sens itive to the configuration of the permanent electrostatic moments with in the molecular frame and to the shapes of both the solute and the so lvent molecules. Contributions from the induced moments are rather sma ll. An accurate and consistent description of the measured order param eters is obtained. The results of the calculations are used to elucida te the limitations and discrepancies of models which describe electros tatic interactions by attributing average properties (e.g., electric f ield gradient) to the nematic solvent.