On the Molecular Kinetic Theory of Cooperative Relaxation Processes in Dipolar Liquid and Crystalline Phases

Abstract

The correlation‐function method is used to describe the dielectric relaxation behavior of liquids and directionally isotropic crystalline phases. The physical model used is that of a regular mixture of dipole molecules and vacancies on a lattice, i.e., the equivalent of the Bragg—Williams approximation. To calculate the cooperative transition probabilities, which govern the relaxation correlation function, the effect of vacancies in the neighborhood of the representative dipole is taken into account. The effect of vacancy diffusion is neglected. The result is a discrete distribution of relaxation times corresponding to cooperative regions with none, one, two, etc., vacancies. This relaxation spectrum is shown to be narrow in the case of liquids and broad in the case of molecular crystals. Upon application of the theory to measurements of the dielectric relaxation of crystalline hydrogen and deuterium halides and of liquid bromoalkanes, the fraction of vacancies and an interaction parameter for these substances were obtained. The values of both quantities were shown to be of the order to be expected from the energy of molecular interactions.