The temperature dependence of biaxiality of solutes dissolved in nematic solvents

The predictions of several models present in the literature, using the common second rank two-terms spherical harmonics expansion to express the mean-torque potential experienced by molecules dissolved in nematic solvents, are often contradicted by experimental evidence of temperature dependence of the solute orientational biaxiality (related to the ratio between the coefficients of the expansion). In this paper, from orientational data obtained by proton NMR technique for four rigid solutes dissolved in six nematic solvents, a linear dependence of the biaxiality parameter against the reduced temperature is observed in a central region of temperatures (quite far from phase transitions). The results are rationalized relating the slopes of the curves to the sign and magnitude of the dielectric anisotropies of the solvents.