Effects of temperature and polydispersity on the dielectric relaxation of dilute ethylcellulose suspensions

The role of temperature on the low-frequency dielectric dispersion is analyzed for moderately polydisperse suspensions of spherical ethylcellulose latex particles. The study is carried out in the 10-50 degrees C temperature range for two different electrolyte concentrations, namely, 5 x 10(-5) and 10(-4) M NaCl. It is found that the relaxation frequency increases with temperature, whereas the amplitude of the dielectric dispersion decreases when temperature is raised. This agrees qualitatively with predictions based on the classical electrokinetic theory (DeLacey, E. H. B., and White, L. R., J. Chem. Soc, Faraday Trans. 2 77, 2007 (1983)). However, the quantitative agreement is very far from being satisfactory. To try to overcome these differences, we have applied a more complete model in which tangential motions of ions in the inner part of the electric double layer is allowed for (DSL model, Mangelsdorf, C. S., and White, L. R., J. Chem. Sec., Faraday Trans. 86, 2859 (1990)). Although in most situations DSL models considerably improve the agreement between theory and experiment, in our case the dynamic Stern layer correction does not seem to be enough to bring much closer experimental data and predictions. It is for this reason that we also consider the fact that our suspensions are not strictly monodisperse. Keeping polydispersity in mind (this can be done by simply taking the volume average particle radius as a representative size parameter) and introducing it in the DSL model, it is shown that a much better description of the main features of the dielectric dispersion, that is, the amplitude of the dielectric increment, and the characteristic relaxation frequency of the suspensions can be reached. (C) 1999 Academic Press.