Electrophoretic mobility of model colloids and overcharging: theory and experiment

Several theories claim that ion-ion correlations play an important role in the electric double layer of colloids. One of the most outstanding predictions is overcharging, which would take place at high electrolyte concentrations and surface charge densities. The counterion concentration next to the surface can become so large that the particle charge is overcompensated and the overcharging occurs. Sometimes this would also involve a reversal of the zeta potential, but this phenomenon has been observed rarely through mobility measurements. This study explores the matter further. The electrophoretic mobility is measured for latex particles with moderate and extremely large surface charge densities at high ionic strengths (up to 2 M) in solutions of symmetric electrolytes. The results are analysed within the so-called hypernetted chain/mean-spherical approximation (HNC/MSA) and a Poisson-Boltzmann approach. In this way, the relevance of ion-ion correlations in practice and the occurrence of overcharging are probed experimentally.