Examination of the atypical electrophoretic mobility behavior of charged colloids in the low salt region using the O'Brian-White theory

Polystyrene microspheres having roughly the same size but different negative surface charge densities were prepared by emulsion polymerization. The amount of sulfate groups on the surface of the particles was controlled by variation of the amount and the decomposition rate of the initiator used, potassium, persulfate. After the cleaning process involving dialysis and extensive ultrafiltration the surface-charge density of the samples was determined and their electrokinetic behavior was studied. A simple model based on the Gouy-Chapman theory and the O'Brian-White approach allows the calculation of the dependence of the electrophoretic mobility on salt concentration. Comparison of the theoretical and experimental curves showed that they were in good agreement in a number of qualitative features. Moreover, the model revealed that a monotonously increasing zeta potential with falling electrolyte concentration results in a mobility maximum, and that this so-called atypical behavior is in accordance with the standard electrokinetic theory. No ion adsorption mechanism or the existence of a charged hairy layer, current standard explanations for this anomality, had to be invoked.