Hydrolysis versus IonCorrelation Models in ElectrokineticCharge Inversion: Establishing Application Ranges.

In this article, we investigate experimentally a widerange ofsituations where charge inversion (i.e., overcompensation of the surfacecharge of a colloidal particle by the countercharge) can occur. Tothat end, the electrophoretic mobility of sodium montmorillonite,silica, and polystyrene latex as functions of pH and concentrationof different salts is presented, and conditions are established wherecharge inversion occurs. The reason for this study is to provide experimentalevidence for distinguishing between two existing models for the explanationof charge inversion. One of these is the specific adsorption of ionslocated in the Stern layer in combination with a Gouy–Chapmandiffuse part of the double layer. The other ion-correlation theoriesexplain the phenomenon in terms of purely physical arguments basedon Coulombic pair interactions between ions and surface charges andon excluded volume effects. In distinguishing between these two interpretations,the influence of the pH plays a central role because of its effecton the hydrolysis of multivalent cations. In our experiments, it isfound that although 1–2 and 2–2 electrolytes provokea decrease in the absolute values of the electrophoretic mobilitieswhen their concentration in solution is increased, they never leadto charge inversion, whatever the surface charge or the pH. However,in the case of salts of trivalent cations, electrokinetic charge reversalis often observed above a certain critical electrolyte concentration.In addition, the extent of overcharging increases when the concentrationis raised above the critical value. This trend occurs for any systemin which the surface charge is pH-independent, as in polystyrene latexand montmorillonite. Most of the results presented here are compatiblewith the specific adsorption of hydrolyzed metal ions as the maindriving force for charge inversion. At low pH, when the hydrolysisof trivalent cations is likely to be absent, overcharging can be attributedto ion correlation effects. [ABSTRACT FROM AUTHOR]