. Can. J. Chem. 68, 875 (1990). The orientational order parameters of benzonitrile and o-, m-, and p-dicyanobenzene, monobromo-and p-dibromobenzene, and p-benzoquinone, dissolved in two nematic solvents, 1132 and EBBA, have been measured as functions of temperature, and used to determine the potential energy parameters for each solute-solvent pair. These parameters have been correlated with a short-range interaction based upon a shape and size function of the solute molecule and a long-range contribution due to the interaction between the solute molecular quadrupole moment and the average electric field gradient. Introduction When solutes are dissolved in nematic liquid crystal solvents, the interaction between solute and solvent molecules results in an orientational ordering of the solute. An understanding of solvent-solute interactions in these anisotropic systems is desirable in view of their importance, for instance, as analogues of biological membranes and in various technological applications. Recently, the orientational behaviour of mono-and dichlorobenzenes in two nematic solvents, EBBA (a Schiff-base) and 11 32 (a mixed phenylcyclohexylcyanide solvent), was studied as a function of temperature and of concentration (1,2). The results showed that chlorobenzene molecules were more highly oriented in 1132 than in EBBA. This difference was attributed to a change in sign of the electric field gradient in the two solvents and the resulting effect upon the interaction between the molecular quadrupole moment of the solute and the average electric field gradient from the solvent. The importance of this interaction in determining the solute orientation was first demonstrated in studies of D2 and HD dissolved in nematic solvents by Burnell and co-workers (3-6), who also showed that a mixture of the two solvents in which the field gradient is zero could be used to eliminate the long-range interaction and limit the potential to a short-range interaction. This latter contribution was interpreted using a Hooke's law force constant for the displacement of the solvent molecules with the molecular circumference of the solute defining this displacement (6). Diehl and co-workers (7) have studied a series of eleven chlorobenzenes and interpreted the orientation using a bond interaction model. They concluded that a molecular shape function must be included and adopted the force constantmolecular circumference model. In our earlier studies on
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