Colloid Polym Sci 274:848-853 (1996) 9 Steinkopff Verlag 1996 F. Carrique A.V. Delgado Dielectric relaxation pattern of dilute colloidal suspensions Received: 14 August 1995 Accepted: 30 January 1996 Dr. F. Carrique ( ~ ) Departamento de Fisica Aplicada 1 Facultad de Ciencias Universidad de Mfilaga 29071 MMaga, Spain A.V. Delgado Departamento de Fisica Aplicada Facultad de Ciencias Universidad de Granada 18071 Granada, Spain Abstract An immediate method of analysis of the relaxation charac- teristics of a colloidal suspension, like of any dielectric, is based on the so-called Cole-Cole representation (imaginary part versus real part) of its complex dielectric constant in a wide frequency range. In this work, we show theoretical plots calculated according to the models developed by DeLacey and White (J Chem Soc Faraday Trans 2 77:2007-2039), and by Rosen et al. (J Chem Phys 98: 4183~4194; this model uses the dynamic Stern layer theory). Both theoretical approaches to the dielectric relaxation pattern of a colloidal suspension are compared to each other, and to experimental data obtained on polystyrene suspensions. Although no significant differences are found between the theoretical predictions of the relaxation patterns (except for the values of the dielectric constant; the DSL model yields higher polarizabilities of the suspen- sions), none of the models can exactly reproduce the frequency dependence of the dielectric constant of a colloidal system. We propose a modification of DeLacey and White's model to include the possibility that the ionic drag coefficients depend on the ion position in the double layer. The final results show that the general trends of the frequency dependence of the quantities involved are not modified, irregardless of the changes in ionic drag coefficients. Key words Dielectric relaxation - dielectric constant - standard electrokinetic model - dynamic Stern layer model - position-dependent drag coefficient Introduction Both theoretical [-1-9] and experimental [10-17] data on the dielectric response and conductivity of a dilute col- loidal suspension in the presence of AC fields show signifi- cant relaxation processes at low frequencies (up to the radiofrequency range). The explanation for the existence of such processes relates them to the polarization of the electric double layers surrounding the particles: the ion cloud, due to the finite time constants for the diffusive transport processes occurring in it, cannot follow the changes of the applied field when the oscillation period of the latter is small as compared to the characteristic diffu- sion times of ions in the double layer. As a consequence, the polarization is out of phase with respect to the applied field, and hence the strong dielectric dispersion typical in colloidal systems for the frequency range above mentioned. The quantity of interest is the complex dielectric con- stant of the colloidal suspension, e*(co) = 4(co) - ie~(co)(~ is the angular frequency of the applied field). Although plots of either dr(oo) or e~'(co) vs. frequency are often ana- lyzed, it is also interesting to consider the so-called