Some dielectric studies / by Brian James Cooke
Cooke, Brian James
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Two methods of approach are current in the literature for the interpretation of dielectric relaxation. One is that due to Debye which assumes that the relaxation process has its origin in the retardation of molecular reorientation due to frictional forces acting on the molecule. The other treats dipole rotation as a rate process in which the dipole must acquire a certain amount of energy in order to surmount a barrier separating two equilibrium positions of orientation. The dielectric relaxation times of some large ketones have been determined at four temperatures using a cell which does not appear to have been used up to this time for measuring the dielectric constant and loss of low loss liquids. The molecules measured were selected because of their size and shape, five were ellipsoidal and one was disc-like. For the ellipsoidal molecules^ the position of the dipole within the molecule was varied to investigate its effect on the relaxation time. A number of equations, based on the Debye model, which attempt to, account for the size of molecular relaxation time are examined. It is found that only the Fischer equation is satisfactory in predicting the effects of dipole direction within the molecule. The experimentally measured activation energies for all the large molecules were found to be similar and only a little higher than those observed for smaller molecules. In an attempt to understand these values a model is proposed based on the energy expended by the molecule during its reorientation process. The approach leads to a method for predicting the effect of solvent on dielectric relaxation time. It is found that the relaxation time depends exponentially on the internal pressure of the medium surrounding the relaxing species, and the activation energy can be accounted for in terms of the product of an activation volume and the internal pressure. From the activation volume an estimate is obtained of the angle through which the dipole rotates. For small molecules it is found that the angle is of the order of 20 degrees which indicates a fairly large jump accompanying the reorientation. For the larger molecules, however, the angle is much smaller, hence, the behaviour resembles Brownian rotational diffusion.