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.

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We show that the screening of the electrostatic potential in electrolytes can in exact theory be expressed in terms of a generalized screened Colomb potential, analogous to the Yukawa potential from the Debye–Hückel approximation, provided the source charge of the potential is renormalized. The renormalized charge distribution is identical to that of a “dressed particle” in dressed ion theory, DIT, of Kjellander and Mitchell. Using DIT we analyze the leading terms of the decay of density profiles and electrostatic potential outside a charged planar wall in contact with 1:2 electrolytes. The formalism leads in a natural manner to the definition of a primary and a secondary effective charge of an object immersed in an electrolyte. These charges are associated with the leading and second leading decay modes of the potential, which have different decay lengths. It is found that both leading terms in the decay are important; together, they give in many cases a very good representation of the density profiles and the potential for distances larger than about a couple of ionic diameters from the wall. When varying the actual (bare) surface charge density σ of the wall, two points of zero effective surface charge density are found: one at low (but nonzero) and one at high value of σ. The former occurs when the counterions are monovalent and the latter when the counterions are divalent and the electrolyte concentration is sufficiently high. Both are associated with effective charge reversals where the surface appears to attain a charge of opposite sign compared to its bare charge. The double layer interaction between two equally charged particles is attractive at large separations at a point of zero effective surface charge.

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Grand Canonical Monte Carlo calculations are used to examine the phase behavior of model binary liquid mixtures confined between parallel plates. The stable and metastable states are identified by direct evaluation of the grand potential. It is shown that demixing transitions can lead to long-range attractive forces acting between the plates, verifying earlier predictions based on approximate integral equation theories. The nature of these transitions and the physical origin of the accompanying attractive forces are discussed.