Effects of Water Dissociation and CO2 Contamination on the Electrophoretic Mobility of a Spherical Particle in Aqueous Salt-Free Concentrated Suspensions

In a very recent paper (Ruiz-Reina, E.; Carrique, F. J. Phys. Chem. B 2008, 112, 11960.) we studied the effect of water dissociation and CO2 contamination on the equilibrium electrical double layer of spherical particles in salt-free concentrated suspensions in aqueous solutions. It was shown that in most cases (dilute to moderately concentrated suspensions), the neglecting of those effects would lead to a very poor description of common salt-free suspensions, especially if the suspensions have been in contact with air. In the present contribution we explore the influence of the latter effects on the de electrophoresis in realistic salt-free suspensions. This kind of system consists of aqueous suspensions without any electrolyte added during the preparation. The ionic species in solution can solely be (i) the "added counterions" stemming from the particles that counterbalance their surface charge, (ii) the H+ and OH- ions from water dissociation, and (iii) the ions produced by the atmospheric CO2 contamination. Our model follows the classical Poisson-Boltzmann approach, a spherical cell model and the appropriate local chemical reactions. We have applied it to the study of the electrophoretic mobility of a spherical particle for different particle volume fractions phi and surface charge densities. The numerical results have shown the quite large influence that water dissociation ions and/or CO2 contamination have on the electrophoretic mobility at low-moderate particle volume fractions. In those situations the role of the added counterions is screened by the other ionic species. These effects yield the mobility to reach plateau values instead of further increasing as volume fraction decreases. It is concluded that it is necessary to take into account the water dissociation influence for phi lower than approximately 10(-2), whereas the atmospheric contamination, if the suspensions have been exposed to the atmosphere, is not negligible if phi < 10(-1). The present work sets the basis for further theoretical models concerning particularly the ac electrokinetics and dielectric response of such systems.