Charge inversion in electric double layers and effects of different sizes for counterions and coions

The molecular mechanisms behind the phenomena of charge inversion in the diffuse electric double layer (i.e., inversion of sign in the charge distribution profile) and attractive double-layer interactions between equally charged surfaces have been investigated with statistical mechanical methods. In aqueous systems with monovalent electrolytes these phenomena can, for example, be induced by having larger coions than counterions, while for divalent electrolytes they occur primarily for electrostatic reasons. We have identified several different mechanisms for the charge inversion. In one of the mechanisms, which only occurs at low surface charge density, the different distances of closest approach to the surface for the ions are crucial, while in some other mechanisms active in the monovalent systems the different size in the ion–ion interaction is the important factor. For divalent electrolytes the electrostatic part of the ion–ion correlations is the dominating effect and ion sizes are not as important, while in the monovalent case core–core collisions between the ions play a very important role.