Effect of temperature on the dielectric relaxation in solvent mixtures at microwave frequencies

The dielectric relaxation behavior of organic liquids was investigated using three representative solvent mixtures between 10 and 70 degrees C in which the effectiveness of ideal free energy relationships for describing the process of dipolar relaxation was examined. The selected solvent mixtures (nitrobenzene-toluene, ethanol-butanol, and butanol-formamide) incorporate different specific molecular interactions, The dielectric spectra of these solvents and solvent mixtures can be fit to the Debye model to obtain dielectric relaxation times and static dielectric constants as a function of temperature and solution composition. The free energy of activation for the dipolar relaxation process, Delta G, and the Kirkwood correlation factor, g, were determined using these fitting parameters for these solvent systems at various temperatures. The results provide useful descriptions of structures and thermodynamic properties of the solvent mixtures. In particular, Delta G behaves ideally in mixing for nitrobenzene-toluene and ethanol-butanol mixtures, and exhibits nonidealities for butanol-formamide mixtures. The nonideal dielectric mixing behavior can be expressed in terms of excess free energies of activation for the dielectric relaxation process, and the excess properties can be described by activity coefficients at infinite dilution using the Margules equation.