Phase diagrams from spin probe studies for binary mixtures of non-mesomorphic solutes in 5CB

The effect of low concentrations of a non-mesomorphic solute impurity on the nematicto-isotropic transition of 4-cyano-4′-n-pentylbiphenyl (5CB), has been investigated by ESR spectroscopy, using predeuterated-2,2,6,6-tetramethyl-4-piperidone-N-oxide (PD-Tempone), at mole fractions of the order of 10^-5, as a probe. It has been found that at such low mole fractions PD-Tempone simply probes the nematic-to-isotropic transition without observably perturbing it. This fast approach relies on the fact that a smaller coupling constant is obtained for the spin probe in the orientationally ordered nematic phase relative to the isotropic phase. The method has been further refined in this study by the use of an especially designed sample holder which, in conjunction with a temperature control and monitoring system, allowed control of the temperature of the sample to within ±0.02°C. Additionally measurements were made, for each solute, at several solute mole fractions within the range from 0.01 to 0.07. The results were used to obtain β_N and β₁, the moduli of the slopes of the nematic and isotropic boundary lines respectively in the reduced nematic-to-isotropic transition temperature (T^*) versus solute mole fraction (x₂) diagrams. The non-mesomorphic solutes used in this study are n-nonane, 3,3-diethylpentane, tetrapropyltin and tetrabutyltin.