General electrokinetic model for concentrated suspensions in aqueous electrolyte solutions: electrophoretic mobility and electrical conductivity in static electric fields

In recent years different electrokinetic cell models for concentrated colloidal suspensions in aqueous electrolyte solutions have been developed. They share some of its premises with the standard electrokinetic model for dilute colloidal suspensions, in particular, neglecting both the specific role of the so-called added counterions (i.e., those released by the particles to the solution as they get charged), and the realistic chemistry of the aqueous solution on such electrokinetic phenomena as electrophoresis and electrical conductivity. These assumptions, while having been accepted for dilute conditions (volume fractions of solids well below 1 %, say), are now questioned when dealing with concentrated suspensions. In this work, we present a general electrokinetic cell model for such kind of systems, including the mentioned effects, and we also carry out a comparative study with the standard treatment (the standard solution only contains the ions that one purposely adds, without ionic contributions from particle charging or water chemistry). We also consider an intermediate model that neglects the realistic aqueous chemistry of the solution but accounts for the correct contribution of the added counterions. The results show the limits of applicability of the classical assumptions and allow one to better understand the relative role of the added counterions and ions stemming from the electrolyte in a realistic aqueous solution, on electrokinetic properties. For example, at low salt concentrations the realistic effects of the aqueous solution are the dominant ones, while as salt concentration is increased, it is this that progressively takes the control of the electrokinetic response for low to moderate volume fractions. As expected, if the solids concentration is high enough the added counterions will play the dominant role (more important the higher the particle surface charge), no matter the salt concentration if it is not too high. We hope this work can help in setting up the real limits of applicability of the standard cell model for concentrated suspensions by a quantitative analysis of the different effects that have been classically disregarded, showing that in many cases they can be determinant to get rigorous predictions.Financial supports for this work by MICINN, Spain (projects FIS2010- 18972, FIS2013-47666-C3-1R, 2R, 3R) and Junta de Andalucía, Spain (project P2012-FQM-694), co-financed with FEDER (European Fund for Regional Development) funds by the EU

Análisis de la conductividad y propiedades dieléctricas de suspensiones coloidales : comparación entre resultados experimentales y predicciones teóricas

La medida y el análisis de los espectros de relajación dieléctrica están encontrando cada vez mayor aplicación en la investigación de procesos de formación de estructuras en las interfases de separación de las partes que constituyen un sistema, así como de cambios estructurales bajo influencias externas. en particular, es de gran interés el análisis de los espectros dieléctricos en el estudio de la forma y tamaño de las partículas coloidales, y, sobre todo, en la caracterización superficial de dichos sistemas. en esta tesis, se realiza primero una integración numérica de las ecuaciones diferenciales del modelo mas general (dw) de calculo de las magnitudes dieléctricas de suspensiones, analizándose exhaustivamente la información implícita en la teoría, pero no accesible directamente. se presta especial atención a la búsqueda de la función de distribución de tiempos de relajación asignable a esta teoría, análisis nunca realizado hasta la fecha. desde el punto de vista experimental, se desarrolla en este trabajo una nueva célula de medida y técnica experimental, que permiten obtener los parámetros dieléctricos y de conductividad de las suspensiones analizadas, todas ellas basadas en látex de poliestireno esférico monodisperso. los resultados experimentales se comparan finalmente con las predicciones teóricas, encontrándose en todos los casos un excelente acuerdo cualitativo entre ambos tipos de datosTesis de Granada. Departamento de Física Aplicad

Conductivity of a concentrated colloidal suspension of spherical particles in an alternating electric field

In this paper the complex (ac) conductivity of a concentrated suspension of spherical colloidal particles is considered in the light of a cell model. Previous works have dealt with the study of the conductivity of a concentrated colloidal suspension for general conditions, including arbitrary zeta potential, particle volume fraction, double-layer thickness, and ionic properties of the solution, but only the static case (dc electric fields) was addressed. In this contribution, the complex conductivity of a concentrated suspension is studied for the same general conditions as in the static case. The numerical data presented in this paper cover a wide range of typical situations including the special case of overlap of double layers of adjacent particles. Like in the static case, the treatment is based on the use of a cell model to account for hydrodynamic and electrical interactions between particles. The two relaxation processes occurring in the frequency range of interest (alpha and Maxwell-Wagner- O’Konski) are analyzed for different values of the ionic strength, particle radius, zeta potential and particle concentration. Roughly speaking, these two relaxations tend to overlap in frequency as the volume fraction of solids increases for otherwise general conditions; in such cases, no clear distinction can be established between them. On the other hand, considerable attention has also been devoted to the numerical analysis of the complex conductivity for those special situations where overlapping between double layers is nonnegligible. Finally, a comparison between theoretical predictions and some experimental results is shown, revealing a general good agreement.Financial support for this work by MCyT, Spain (Project MAT 2004-00866), and FEDER funds is gratefully acknowledged