CHARGE REVERSAL IN ELECTRICAL DOUBLE-LAYERS - A BALANCE BETWEEN ENERGY AND ENTROPY

Monte Carlo simulations and accurate analytical theories predict a cha rge reversal in an electric double layer under some specific condition s. One typical situation is for very highly charged aggregates in the presence of divalent counterions, where ionic correlations become impo rtant. Here, we reexamine the phenomenon in terms of a balance between energy and entropy contributions. High surface charge density, low te mperature, low dielectric permittivity or multivalent counterions, all these factors increase the influence of the energetic term and favour a charge reversal. Previous studies have focused on the energy term a nd attempts to increase that. An alternative and equally valid way is to reduce the entropy term, which can be accomplished by replacing sin gle atomic or molecular counterions by polyelectrolyte chains. The ent ropy of the charged chains is of course much lower than that of the sm all ions and the dominance of the energy term will again lead to a cha rge reversal, but in this case at a much lower and more realistic valu e of the electrostatic coupling.