Solute-Solvent Interactions within the Nematic Mesophase

This paper reports the temperature dependence of the orientational order of a spin probe dissolved in the nematic mesophases of four di-n-alkoxyazoxybenzenes, determined with the aid of electron resonance spectroscopy. The solute orientational order can provide valuable information concerning the pseudo-potential for the solute in the mesophase and hence the nature of the anisotropic solute-solvent interaction. Techniques for obtaining details of this interaction from our results are discussed in terms of extensions of the Maier-Saupe theory to include a general anisotropic intermolecular potential and to multicomponent mixtures of rod-like molecules. Our analysis suggests that the pseudo-potential for the solute, like that for the solvent, must include interactions in addition to London dispersion forces employed in the Maier-Saupe theory. The investigations of solute-solvent interactions were extended to an L-shaped spin probe to see if, as has been suggested, its arms are preferentially aligned with respect to the director in a nematic mesophase. No such preferential alignment could be detected.