Primitive models and electrophoresis: an experimental study

Several theories (developed in the context of the so-called primitive models) argue that the classical Poisson–Boltzmann (PB) approach could fail to describe the electrical double layer in the case of high surface charge densities and/or ionic strengths. Under such conditions, the concentration of counterions in the vicinity of the surface would be larger than expected. What is more, the particle charge could be overcompensated (which is known as overcharging). This would lead to a considerable reduction in the magnitude of the ζ-potential (as compared with PB predictions) and, even, a reversal in its sign. Although these predictions are quite remarkable, these new approaches have been applied just scarcely in practice. Obviously, electrophoresis offers an excellent possibility of testing these theories. Accordingly, we have measured the electrophoretic mobility for latex particles with moderate and large surface charge densities at high ionic strengths. A commercial setup specifically designed for measuring extremely low mobilities has been used. The results have been interpreted with the help of the so-called hyper-netted-chain/mean-spherical approximation (HNC/MSA), a theory that includes ion size effects. In this way, overcharging has been experimentally investigated. Although the HNC/MSA is not a completely successful approach, our analysis suggests that the role of ion size correlations is important, particularly in electrolytes with a counterion valency of 2 or greater.