Ion correlation forces between uncharged dielectric walls

Abstract

The interaction pressure between two uncharged planar walls immersed in various electrolyte solutions containing mono- and/or divalent ions is investigated. The solution is treated as a primitive model electrolyte, and the wall surfaces constitute dielectric discontinuities. Ionic image charge and ion-wall dispersion interactions are included. The interaction parameters are appropriate for hydrocarbon (polystyrene)/water interfaces, and the electrolyte concentrations considered lie between 0.250M and 1.00M. The anisotropic hypernetted chain method is used to self-consistently calculate the ion density profiles and the ion-ion correlation functions in the inhomogeneous electrolyte. Thereby, the effects of image charge interactions and dispersion interactions on the pressure and the electrolyte structure are included in a fully consistent manner. The explicit consideration of correlations between the ions in the presence of image charges ensures that the screening of the zero-frequency van der Waals interaction is taken into account. Of special interest are the effects of asymmetries between anions and cations with respect to valency and/or dispersion interaction with the walls. Such asymmetries create an electric double layer in the electrolyte outside each electroneutral surface. This causes the wall-wall interaction for large surface separations to be similar to the interaction between charged surfaces. For intermediate separations, around 1-2 nm, a substantial repulsive peak appears in the ionic pressure. In some cases the repulsion is larger than the van der Waals attraction between the walls, which implies that there is a repulsive barrier in the total pressure despite that the surfaces are uncharged. The strongest repulsion is found for 2:1 electrolytes where the monovalent anions interact strongly with the walls via dispersion forces. In general, ion-wall dispersion forces acting on ions of lower valency have a much greater effect than equally strong dispersion forces acting on ions of higher valency. This is mainly due to the more strongly repulsive image charge forces on ions of higher valency that counteract the attractive dispersion forces. Effects of confinement on the ion-ion correlations also contribute to this difference. For all electrolytes the interaction pressure from the ions is attractive for small surface separations. The main cause is a depletion of ions between the walls from the self-image repulsion and confinement effects. For totally symmetric electrolytes the attractive pressure extends to large separations in most cases.