THE EFFECT OF THE CONCENTRATION OF DISPERSED PARTICLES ON THE MECHANISMS OF LOW-FREQUENCY DIELECTRIC-DISPERSION (LFDD) IN COLLOIDAL SUSPENSIONS

There are two mechanisms which are currently used to explain the low-f requency (kHz range) dispersion of the dielectric permittivity of susp ensions in electrolyte solutions (LFDD). The first, the surface diffus ion mechanism (SDM), associates the LFDD with the diffusion of bound i ons along the particle surface caused by the applied electric field. T he second, the volume diffusion mechanism (VDM), follows from the gene ralization for alternating fields of the classical theory of the relax ation effect in electrophoresis and associates the LFDD with the diffu sion of free ions in the diffuse double layer. It has been found that VDM is much more strongly dependent on particle concentration than SDM , opening new possibilities for the investigation of each of these two mechanisms separately. The reason is that when the concentration of p articles in suspension increases, the characteristic length for the pr opagation of volume diffusion processes decreases together with the de crease of the free electrolyte volume, whereas the characteristic leng th for the surface diffusion remains constant. Correspondingly, when p article concentration is raised, the relaxation time of the VDM effect must decrease, whereas it must remain constant for the SDM mechanism. Thus, by varying the concentration of particles in suspension, one ca n separate the dispersion curves of SDM and VDM. A simple model is ela borated which can be useful to predict the volume fraction dependence of the parameters of LFDD; in particular, its amplitude and critical f requency. The results are compared with experimental data obtained wit h polymer latex dispersions of volume fractions ranging from 3 to 16%. It is found that the dielectric behaviour (the volume fraction depend ence of both the amplitude and critical frequency of LFDD) of the disp ersions is reasonably well explained with our model, thus demonstratin g that VDM prevails in the systems studied. Experimental data previous ly found by other authors are also discussed in the light of the model presented. (C) 1998 Elsevier Science B.V. All rights reserved.