Looking into overcharging in model colloids through electrophoresis: Asymmetric electrolytes

Some theories claim that the Poisson–Boltzmann approach could fail to describe the electric double layer of colloids under certain conditions as a result of neglecting ion size correlations. For instance, if the surface charge density and/or the electrolyte concentration are high enough, the counterion local density in the vicinity of charged surface could become so large that the particle charge would be overcompensated. This phenomenon is theoretically known as overcharging and, sometimes, should involve a ζ-potential reversal. Accordingly, this work looks into overcharging through electrophoresis experiments. The electrophoretic mobility has been measured for latex particles with moderate and large surface charge density in solutions of asymmetric electrolytes $\text{z:1}$ (symmetric electrolytes have been studied in a previous work). In order to find out the relevance of ion size correlations, results are analyzed within the so-called hypernetted-chain/mean-spherical approximation (HNC/MSA) as well as a Poisson–Boltzmann approach. In the case of divalent counterions $\text{(z=2),}$ the HNC/MSA seems to justify why a mobility reversal is hardly observed. For $\text{z=3,}$ our results suggest that ion correlations could play an important role and be mostly (or partially) responsible for mobility reversal in certain cases.