Ionic coupling effects in dynamic electrophoresis and electric permittivity of aqueous concentrated suspensions

Abstract

In the recent past the standard electrokinetic model formulated for a concentrated suspension subjected to a static electric field has been checked in detail against more sophisticated models which correctly compute effects associated to the so-called added or released counterions (i.e., those compensating the surface charge of the particles), or to realistic descriptions of the chemistry of the solution. Important, as it was evidenced in dc fields, is to explore the consequences of neglecting the above-mentioned features in the even more significant case of ac fields, whereby details of the electrokinetic response are observed which go unnoticed at constant fields. In the present contribution, a general ac electrokinetic cell model for a concentrated suspension in an arbitrary electrolyte solution is developed that includes all the effects mentioned. Attention is paid to the frequency spectra of the dynamic electrophoretic mobility and the dielectric dispersion of concentrated suspensions. Different coupling situations are considered between the released counterions and the ions from the background salt solution (the only ones accounted for within the standard model), that have been shown to be responsible for some important deviations in electrokinetic predictions when a comparison is made against standard results. Differences between them are mainly noticeable at high field frequencies regarding the Maxwell-Wagner counterion condensate relaxation, but also have been found at lower frequencies, when Maxwell-Wagner-O’Konski and alpha- or concentration polarization relaxations are concerned, due to overall changes in: released counterion mobilities, screening effects on particles charge or ionic concentration profiles in the double layers. The complementarity of dynamic mobility and dielectric dispersion is well appreciated in distinguishing the different situations. In the paper, the way in which these general features are affected by changes in ionic strength, particle size and charge, or solids concentration, are discussed. It is expected that this contribution can provide a better understanding of the electrokinetic response of suspensions of nanoparticles in realistic aqueous media due to their relevance for many applications in industry and biotechnology.