Electric Double Layer of Spherical Particles in Salt-Free Concentrated Suspensions: Water Dissociation and CO₂ Influence

Abstract

We present a model for the theoretical description of the electric double layer of realistic salt-free colloidal suspensions. This kind of systems consist of aqueous suspensions deionized maximally without any electrolyte added during the preparation, in which the only ions present can be (i) the added counterions that counterbalance the surface charge, (ii) the H(+) and OH(-) ions from water dissociation, and (iii) the ions produced by the atmospheric CO2 contamination. Our theory is elaborated in the framework of the classical Poisson-Boltzmann theory, the spherical cell model approach, and the appropriate local equilibrium reactions, and it also includes an efficient mathematical treatment for dealing with the resulting integro-differential equations. We have applied it to the study of the surface electric potential in a wide range of volume fraction and surface charge density values in a variety of cases. The numerical results show that it is necessary to consider the water dissociation influence for volume fractions lower than approximately 10(-2), whereas the atmospheric contamination, if the suspensions are open to the atmosphere, is important in the region of phi<10(-1). The present work sets the basis for theoretical models concerning the equilibrium phase diagram, electrokinetics, and rheology of such systems.

Electric double layer for spherical particles in salt-free concentrated suspensions including ion size effects

“…If the suspension is concentrated [5], the particle-particle electrohydrodynamic interactions introduce severe limitations when dealing with the response to external fields, not only for the mathematical difficulties of dealing with many-body interactions, but also for the additional numerical problems raised when it comes to solve the equations, no matter the approximation chosen. The authors have addressed this problem by using a cell model approach to deal with electrokinetic and rheological properties in concentrated suspensions [10][11][12][13][14][15][16][17][18].…”

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

“…The further dissociation of the bicarbonate anion HCO 3 − to give H + and CO 3 = has been disregarded due to its minor quantitative role in the phenomena we are concerned with [22]. A salt is also added to the solution, KCl in this study, introducing in the problem two new ionic species K + (z 5 =+1, D 5 =1.9×10 −9 m 2 s −1 ) and Cl − (z 6 = −1, D 6 = 2.0×10 −9 m 2 s −1 ).…”

“…A further model improvement was done by considering, what can be denominated realistic conditions in the chemistry of ionic species in aqueous solution, assuming in all cases equilibrium in all chemical reactions involved [22]. A more realistic model model considers the role of ions coming from water dissociation and from the chemistry of possible carbon dioxide contamination of the solution under a non-equilibrium chemical approach for chemical reactions.…”

Electroviscous effect of concentrated suspensions in salt-free media: Water dissociation and CO2 influence

“…The problems of finding the equilibrium functions n 0 i ðrÞ and W 0 ðrÞ in the case of water dissociation were treated in an earlier work [11]. The resulting equation is…”

“…We will call these kinds of systems ''realistic salt-free suspensions", in the sense that they are deionized as far as possible and there is no other salt added during the preparation, although there are actually various ionic species in them. The problem of the determination of the electric potential and ionic distributions in this kind of suspension has been very recently treated by Ruiz-Reina and Carrique [11].…”