Experimental study of electrostatically stabilized colloidal particles: Colloidal stability and charge reversal

We consider the interaction of colloidal spheres in the presence of mono-, di-, and trivalent ions. The colloids are stabilized by electrostatic repulsion due to surface charges. The repulsive part of the interaction potential Ψd is deduced from precise measurements of the rate of slow coagulation. These “microsurface potential measurements” allow us to determine a weak repulsion in which Ψd is of the order of a few kBT. These data are compared to ζ potential measured under similar conditions. At higher concentrations both di- and trivalent counterions accumulate at the very proximity of the particle surface leading to charge reversal. The salt concentration ccr at which charge reversal occurs is found to be always above the critical coagulation concentration cccc. The analysis of Ψd and of the ζ potential demonstrates, however, that adsorption of multivalent counterions starts far below ccr. Hence, colloid stability in the presence of di- and trivalent ions cannot be described in terms of a DLVO ansatz assuming a surface charge that is constant with regard to the ionic strength.

ζ Potential and diffuse potential Ψd of anionic, surface charged colloids as a function of the concentration of lanthanum ions. The results show charge reversal (cr) beyond the critical coagulation concentration (ccc) together with a comparison to the classical Poisson–Boltzmann theory (dashed line).Figure optionsDownload high-quality image (47 K)Download as PowerPoint slideResearch highlights
► We measure the weak repulsion of charged colloidal spheres near charge reversal.
► We combine for the first time microsurface potential measurements with measurements of the zeta potential under identical conditions.
► We determine and discuss the critical coagulation concentration and the concentration of charge reversal of a model system.
► We discuss the implications of our findings for the DLVO theory.
► We demonstrate that adsorption of multivalent ions is the major cause for destabilization of colloidal particles in the presence of multivalent ions.