Spherical polyelectrolyte brushes' constant zeta potential with varying ionic strength: an electrophoretic study using a hairy layer approach

The zeta-potential of spherical brushes of poly[2-(methacryloyloxy) ethyl] trimethylammonium chloride (PMETAC) and poly(potassium sulfopropyl methacrylate) (PSPM) was measured as a function of ionic strength. The zeta-potential of PMETAC brushes varies from +30 mV at 10 mM NaCl to +20 mV at 200 mM, and the zeta-potential of PSPM changes from -30 mV to -25 mV, at their respective ionic strengths. This unusual weak dependence of the zeta-potential on ionic strength is quantitatively explained on the basis of the responsiveness of brushes toward changes in the ionic strength as well as taking into account the specific hydrodynamics of the hairy brush solution interface. The electric potential distribution is described in the framework of the Debye-Huckel approximation. An analytical equation describing the dependence of the zeta-potential of polyelectrolyte brushes on ionic strength is provided.