A theoretical and experimental study of forces between charged mica surfaces in aqueous CaCl2 solutions

Abstract

The interaction between two mica surfaces immersed in CaCl2 solutions has been directly measured. Accurate theoretical calculations, including the anisotropic hypernetted chain (HNC) theory used in this work, predict that at reasonably high surface charge densities the electrical double layer interactions in the presence of divalent counterions should be attractive at short surface separations (in the range 0.6–2 nm). Under most conditions investigated, the experimental results indicate that this indeed is the case. The attraction is a consequence of the correlation between the ions. In addition to the double layer interaction, in most cases the measured force contains an oscillatory contribution. At low CaCl2 concentrations and small surface separations, Ca2+ ions between the surfaces are exchanged for H3O+ ions, which decreases the oscillatory interaction and the ion-correlation attraction. At high concentrations the force is dominated by a strong hydration repulsion, which is related to the adoration of Ca2+ ions onto the mica surface. These complications are not considered in the theoretical treatment—which uses the primitive model for the electrolyte—but the ion-correlation attraction is sufficiently prominent to be detected by surface force measurements. The accuracy of the anisotropic HNC theory for 2:1 electrolytes in the primitive model has been checked by comparing calculated ionic concentration profiles with those obtained by Monte Carlo simulations for identical systems. The agreement is excellent.