Short-range molecular interactions governing the orientational ordering of apolar molecules dissolved in nematic solvents

The present study of the ordering of apolar molecules dissolved in uniaxial mesophases has been carried out by a model where the interactions between the solute and the local medium are described on a molecular scale. On the contrary, the mutual interactions between the solvent molecules are not explicitly taken into account, but a mean description of the macroscopic anisotropy of the bulk of the solvent is provided. This is done by defining a "virtual" mesophase director whose instantaneous orientation in the solute molecular frame is randomly given step-by-step during the numerical calculation of the solute order parameters. This model has been applied to apolar molecules biphenylene, 1,4-dicyanobenzene, 1,4-dinitrobenzene, and p-benzoquinone. The predicted order parameters have been compared with the H-1 NMR experimental data obtained for the same compounds dissolved in nematic solvents EBBA and ZLI1132 and in the 55 wt % ZLI1132 + EBBA magic mixture, where long-range effects are believed to be very small. A nearly perfect matching of the simulated solute-ordering matrices with the data in the magic mixture has been found, thus strengthening the common assumption that long-range effects can be neglected in that mixture. The model has also been tested on so-called magic solutes cyclohexane, 1,4-trans-dimethylcyclocyclohexane, and trans-decalin (whose electronic structure should preclude significant anisotropic long-range interactions) dissolved in ZLI2452. Once more, we obtained excellent results in reproducing the experimental order parameters (Terzis, A. F.; Poon, C.-D.; Samulski, E. T.; Luz, Z.; Poupko, R.; Zimmermann, H.; Muller, K.; Toriumi, H.; Photinos, D. J. J. Am. Chem. Soc. 1996, 118, 2226) over a very large range of temperatures, and this is a further evidence of the validity of the approach.