Ion size effects on the electrokinetics of salt-free concentrated suspensions in ac fields

We analyze the influence of finite ion size effects in the response of a salt-free concentrated suspension of spherical particles to an oscillating electric field. Salt-free suspensions are just composed of charged colloidal particles and the added counterions released by the particles to the solution, that counterbalance their surface charge. In the frequency domain, we study the dynamic electrophoretic mobility of the particles and the dielectric response of the suspension. We find that the Maxwell-Wagner-O'Konski process associated with the counterions condensation layer, is enhanced for moderate to high particle charges, yielding an increment of the mobility for such frequencies. We also find that the increment of the mobility grows with ion size and particle charge. All these facts show the importance of including ion size effects in any extension attempting to improve standard electrokinetic models.Comment: J. Colloid Interface Sci., in press, 13 pages, 9 figure

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

AC Electrokinetics of Salt-Free Multilayered Polymer-Grafted Particles

Interest in the electrical properties of the interface between soft (or polymer-grafted) nanoparticles and solutions is considerable. Of particular significance is the case of polyelectrolyte-coated particles, mainly taking into account that the layer-by-layer procedure allows the control of the thickness and permeability of the layer, and the overall charge of the coated particle. Like in simpler systems, electrokinetic determinations in AC fields (including dielectric dispersion in the 1 kHz–1 MHz frequency range and dynamic electrophoresis by electroacoustic methods in the 1–18 MHz range) provide a large amount of information about the physics of the interface. Different models have dealt with the electrokinetics of particles coated by a single polymer layer, but studies regarding multi-layered particles are far scarcer. This is even more significant in the case of so-called salt-free systems; ideally, the only charges existing in this case consist of the charge in the layer(s) and the core particle itself, and their corresponding countercharges, with no other ions added. The aims of this paper are as follows: (i) the elaboration of a model for the evaluation of the electrokinetics of multi-grafted polymer particles in the presence of alternating electric fields, in dispersion media where no salts are added; (ii) to carry out an experimental evaluation of the frequency dependence of the dynamic (or AC) electrophoretic mobility and the dielectric permittivity of suspensions of polystyrene latex spherical particles coated with successive layers of cationic, anionic, and neutral polymers; and (iii) finally, to perform a comparison between predictions and experimental results, so that it can be demonstrated that the electrokinetic analysis is a useful tool for the in situ characterization of multilayered particles.Spanish Institution, Ministerio de Ciencia, Innovacion y Universidades GC2018-098770-B-I00Junta de Andalucia BF-FQM-141-UGR1

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

Electrokinetic detection of the salt-free condition in colloids. Application to polystyrene latexes

Financial support from Ministerio de Ciencia, Innovación y Universidades (Spain) ( GC2018-098770-B-I00 ), and Junta de Andalucía (Spain) and European Funds for Regional Development ( BF-FQM-141-UGR18 , PI20-00233 ) is gratefully acknowledged.Financial support from Ministerio de Ciencia, Innovación y Universidades (Spain) (GC2018-098770-B-I00), and Junta de Andalucía (Spain) and European Funds for Regional Development (BF-FQM-141-UGR18, PI20-00233) is gratefully acknowledged.Because of their singular phenomenology, the so-called salt-free colloids constitute a special family of dispersed systems. Their main characteristic is that the dispersion medium ideally contains only the solvent and the ions compensating exactly the surface charge of the particles. These ions (often called released counterions) come into the solution when the surface groups responsible for the particles charge get ionized. An increasing effort is nowadays dedicated to rigorously compare theoretical model predictions for ideal salt-free suspensions, where only the released counterions are supposed to be present in solution, with appropriately devised experiments dealing with colloids as close as possible to the ideal salt-free ones. Of course, if the supporting solution is aqueous, the presence of atmospheric contamination and any other charged species different from the released counterions in the solution must be avoided. Because this is not an easy task, the presence of dissolved atmospheric CO2 and of H+ and OH− from water dissociation cannot be fully discarded in aqueous salt-free solutions (often denominated realistic in such case). Ultimately, at some point, the role of the released counterions will be comparable or even larger in highly charged concentrated colloids than that of added salts. These topics are covered in the present contribution. The model results are compared with experimental data on the dynamic mobility and dielectric dispersion of polystyrene spheres of various charges and sizes. As a rule, it is found that the model correctly predicts the significance of alpha and Maxwell-Wagner-O'Konski relaxations. Positions and amplitudes of such relaxations are well predicted, although it is necessary to assume that the released counterions are potassium or sodium instead of protons, otherwise the frequency spectra of experimental mobility and permittivity differ very significantly from those theoretically calculated. The proposed electrokinetic evaluation is an ideal tool for detecting in situ the possible contamination (or incomplete ion exchange of the latexes). A satisfactory agreement is found when potassium counterions are assumed to be in solution, mostly if one considers that the comparison is carried out without using any adjustable parameters.Ministerio de CienciaMinisterio de Ciencia, Innovación y Universidades GC2018-098770-B-I00European Regional Development Fund BF-FQM-141-UGR18, PI20-00233Junta de Andalucí