Dynamics of glass-forming liquids. IV. True activated behavior above 2 GHz in the dielectric α-relaxation of organic liquids

We have measured the dielectric relaxation of butylbenzene and of the glass-former propylbenzene in the frequency range 10−2 Hz to 2×1010 Hz in order to characterize the variation of relaxation times with temperature for these low loss liquids. Additionally, salol has been remeasured above 1 GHz with improved resolution. Using the sensitive data representation [−dlog10(fmaxHz)/d(1/T)]−1/2 vs 1/T we find demarcation temperatures TA, at which the temperature dependence changes from a Vogel–Fulcher type law within the limits TB⩽T⩽TA to Arrhenius behavior for T>TA, corresponding to a position of the loss peak fmax>2 GHz. The activation energies derived from dielectric relaxation data for T>TA are associated with the energy of vaporization, Eη∝ΔEvap. A comparison of dielectric relaxation times τD to viscosity data in this wide range of temperatures suggests the relation τD∝η/T rather than τD∝η.