Wetting and drying scenarios of ionic solutions

Abstract

We review recent theoretical results obtained for the wetting and drying behavior of ionic solutions near a charged solid substrate. Three levels of modeling ionic solutions are considered: the primitive model, where the solvent is replaced by a dielectric continuum, the ‘semi-primitive’ model where the solvent is represented by hard spheres with a Yukawa attraction and a dielectric permittivity is introduced, which depends on the local solvent density, and the ‘civilized’ model, where the solvent molecules are dipolar hard spheres with a Yukawa attraction. Calculations on the primitive model are based on a square gradient functional for the solvent, combined with a Poisson–Boltzmann description of the ions; the discrete solvent models are treated within a multi-component density functional theory, combining Rosenfeld's fundamental measure description of excluded volume effects with the mean-field approximation for the Yukawa and electrostatic interactions. Qualitative agreement is found between the predictions of the three models, while those of the two discrete solvent models agree quantitatively. The relative size of anions and cations is shown to have a crucial influence on interfacial properties, as observed experimentally. A novel drying scenario is predicted near a charged wall. New results are reported for the variation of the surface tension γ with ion concentration c. Contrary to the observed behavior of aqueous ionic solutions, the liquid/vapor surface tension is found to systematically decrease with c, presumably because our solvent models are insufficient to describe water, due to the neglect of strong hydrogen bonding. The solid/liquid surface tension is, in contrast, shown to be a non-monotonic function of c and to possess a minimum.