Molecular mechanisms of dielectric relaxation of highly polar liquids

Abstract

The dielectric spectra of highly absorbing simple liquids CH₃F, CH₃Cl, CH₃l CHF₃ caused by reorientations of polar molecules have been computed. The calculations were performed over a wide temperature range for the confined rotator (CR) model and for the hybrid confined rotator/extended diffusion model (HM). The mechanisms of absorption and dielectric losses result from periodic librations of molecules and from their orientation in the alternating local electric field E_c(t), respectively.

The calculated dielectric spectra agree with experiment over the frequency range 0 < ν/cm^–1 < 200, covering the Debye relaxation and Poley absorption regions of the spectra. With increasing temperature T, the libration amplitude β increases while the libration time τ(the correlation time of the angular velocity |Ω|) and the barrier height U_Ω decrease. The maximum discrepancy between theory and experiment occurs at temperatures close to the critical value; in this case the HM gives a better description of the Debye frequency spectrum than the CR model. Estimates based on application of the resonance condition, the liquid cell model and the hard sphere model are proposed.

The self-diffusion coefficient D(T) has been calculated; in this case the collision frequency of molecules 1/τ_v(V) was estimated for the hard-sphere model and it was assumed that the self-diffusion is caused by molecules whose translational velocity v is sufficient to surmount the barrier U_v which is dependent on T. It was shown that the barrier U_v is essentially less than U_Ω and for a first approximation, is proportional to U_Ω.