Electrostatic interactions in liquid crystals: ordering of rigid solutes in nematic solvents

Abstract

The orientational distribution of a probe molecule in a nematic solvent is derived in terms of the intermolecular potential describing steric, dispersional, and electrostatic interactions with the solvent molecules. Orientational correlations among the latter molecules are neglected in the derivation, whereas probesolvent correlations are treated explicitly. The theory is used to analyse NMR measurements of order parameters of small solute molecules in nematic solvents. Tensor components of the solute potential of mean torque up to fourth rank are evaluated. The various electrostatic contributions (dipole-dipole, dipole-quadrupole, quadrupole-quadrupole, etc.) are assessed quantitatively. It is found that the residual overall electrostatic interaction is sensitive to the configuration of the permanent electrostatic moments within the molecular frame and to the shapes of both the solute and the solvent molecules. Contributions from the induced moments are rather small. An accurate and consistent description of the measured order parameters is obtained. The results of the calculations are used to elucidate the limitations and discrepancies of models which describe electrostatic interactions by attributing average properties (e.g., electric field gradient) to the nematic solvent.