Dielectric Relaxation in the Electric Double Layer

Abstract

There are two groups of phenomena that can cause frequency dispersion of capacity and resistivity at a perfectly polarizable interface. The first comprises artifacts which lead to the observation of an apparent frequency dispersion: Of these, penetration of the electrolyte to shielded parts of the electrode and the resulting transmission line phenomena are discussed. The second group comprises processes which lead to an inherent frequency dispersion of the double-layer impedance: among these are, in particular, the relaxation phenomena of polar double-layer constituents. Experimental results have been obtained for the interface: Hg (dropping) /6NHCl aq., in terms of parallel capacity, Cpd1, and resistivity, Rpd1, of the electric double layer as a function of frequency, v. These results are consistent with the occurrence of dielectric relaxation of the water dipoles adsorbed at the interface, and characterized by a certain distribution of relaxation times around τ0≃10−6 sec. The results are not consistent with the values ∂ lnRpd1/∂ lnv expected from the effect of a possible penetration of electrolyte into the capillary. On the basis of a molecular model of the relaxation process, a rough estimate of the energy of activation for the process which results in a change of direction of the water dipole is made which is consistent with the mean relaxation time, τ0, estimated on the basis of the dielectric properties of double-layer water. It is shown that the frequency variation of the double-layer capacity is less than the detection limits of most ac bridges up to very high frequencies, as long as the high-atom polarization of water remains unchanged, and only dipole polarization is gradually reduced to zero with increase of frequency.