Electrophoresis of large polyelectrolyte-coated colloidal particles

We perform electrophoretic mobility mu measurements of spherical colloidal particles coated with a charged polyelectrolyte shell versus 1:1 electrolyte concentration c. Instead of the expected Smoluchowski scaling law mu approximately c(- 1 / 2) for large kappaa, with kappa the inverse of the Debye length, we find that mu scales as mu approximately c(- 1 / 3). We account for this result using a general theory for the electrophoresis of soft particles [H. Ohshima, Adv. Colloid Interface Sci. 62, 189 (1995)] combined with the salt concentration dependence of the shell thickness, as described by Pincus [P. Pincus, Macromolecules 24, 2912 (1991)].