Electrophoretic mobility of latex spheres in mixture solutions containing mono and divalent counter ions

The electrophoretic mobilities of polystyrene sulfate latex spheres carrying constant surface charge densities in aqueous solutions were measured in the presence of monovalent K+ and divalent Mg2+ counter ions. The measurements were carried out as functions of the concentrations of Mg2+ and K+ ions (CMg and CK) and the ratio of the concentration of divalent counter ions to that of monovalent ions, X = CMg/CK. The data were analyzed on the basis of standard electrokinetic and electric double layer models. The calculated values of the mobility are in reasonably good agreement with the experimental data assuming that the distance of the slipping plane from the surface, xs ranges from 0 to 0.25 nm and the values of xs increase with increasing X.