Electrokinetic and dielectric response of a concentrated salt-free colloid: Different approaches to counterion finite size effects.

Abstract

In the present work, a general model is developed for the electrokinetics and di- electric response of a concentrated salt-free colloid that takes into account the finite size of the counterions released by the particles to the solution. The effects associ- ated with the counterion finite size have been addressed using a hard-sphere model approach elaborated by Carnahan and Starling (N. F. Carnahan and K. E. Starling, Equation of state for nonattracting rigid spheres, J. Chem. Phys. 51, 635 (1969)). A more simple description of the finite size of the counterions based on that by Bikerman has also been considered for comparison. The studies carried out in this work include predictions on the effect of the finite counterion size on the equilibrium properties of the colloid and its electrokinetic and dielectric response when it is sub- jected to constant or alternating electric fields. The results show how important the counterion finite size effects are for most of the electrokinetic and dielectric proper- ties of highly charged and concentrated colloids, mainly for the static and dynamic electrophoretic mobilities. Furthermore, new insights are provided on the counte- rion condensation effect when counterions are allowed to have finite size. Focus is placed on the changes undergone by their concentration in the condensation layer, for low-salt and highly charged colloids.