SHORT-RANGE AND LONG-RANGE CONTRIBUTIONS TO THE ORDERING OF RIGID PLANAR SOLUTES DISSOLVED IN A 55WT-PERCENT ZLI1132+EBBA NEMATIC MIXTURE

Nine well studied (polar and non-polar) planar solutes have been used as probes to investigate the mechanism for orientational ordering in t he nematic phase. Second-rank orientational order parameters have been derived from proton dipolar coupling constants, obtained from the NMR spectra of these molecules dissolved in a 55 wt% ZLI1132 + EBBA nemat ic mixture. The experimental data are interpreted in terms of an orien ting potential experienced by molecules in a liquid crystal environmen t. A model for the orienting potential is presented which includes, in addition to short-range repulsion interactions described by the surfa ce tensor model, long-range contributions treated according to a gener alized reaction field approach. Least-squares fits of solvent dependen t parameters to the experimental data have been performed to estimate the weight of the various contributions to the solute orientational or der, and the results obtained in this way are discussed. Finally, the results obtained with the surface tensor model are compared with those from the size-shape potential suggested by other workers.