On the temperature dependence of the equal frictions itinerant oscillator model

Abstract

A detailed comparison of the dielectric loss deduced from the equal frictions itinerant oscillator (IO) model has been carried out with experimental observations on methyl chloride made by Gerschel and co-workers in the frequency range 10 GHz-4·8 THz. We find that in the small oscillations approximation this model can provide a very close fit to data in the microwave (MW) and far infrared (FIR) regions but at the expense of a good fit in the intermediate frequency region (300 GHz-1 THz in the case of methyl chloride). Alternatively, the data may be fitted closely at all frequencies up to FIR maximum but at the expense of the fitting in the FIR fall-off region. It is suggested that the difference in the measured absorption over and above that predicted by the model in the intermediate frequency region may arise, in part, from non-linearity in the interaction potential between the dipoles—especially the barrier crossing of dipoles due to a finite barrier height. Alternatively, this excess absorption may be caused by short-range interactions and depend on the shape of the molecule and other structural aspects of the liquid not incorporated into the present model.

Nevertheless, the comparison of the experimental data with the model reveals that the magnitude of the interaction potential between the dipoles decreases with an increase in temperature. The mass of the cage of dipoles, which corresponds indirectly to the number of neighbouring dipoles involved in the co-operative orientational process, decreases with increase in temperature to the extent that, above a certain temperature, such co-operative processes may be considered to have ceased. The system can then be described adequately by the Langevin equation incorporating the moment of inertia of a molecule and its potential energy of interaction with the neighbouring molecules.