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

Abstract 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 Ψ d is deduced from precise measurements of the rate of slow coagulation. These “microsurface potential measurements” allow us to determine a weak repulsion in which Ψ d is of the order of a few k B T . 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 c cr at which charge reversal occurs is found to be always above the critical coagulation concentration c ccc . The analysis of Ψ d and of the ζ potential demonstrates, however, that adsorption of multivalent counterions starts far below c cr . 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.