Wetting of a solid substrate by a 'civilized' model of ionic solutions

摘要

We use classical density functional theory (DFT) and an explicit solvent description to investigate the wetting and drying behavior of ionic solutions in contact with a charged solid substrate. The solvent is modeled by dipolar hard spheres, while the monovalent ions are oppositely charged hard spheres; cohesion is ensured by a Yukawa attraction between all three species. The free energy functional describing the inhomogeneous solution includes the best available fundamental measure description of excluded volume correlations in a ternary mixture of hard spheres, whereas all electrostatic and cohesive interactions are treated within the mean-field approximation. We find both first and second order wetting transitions which are rather little affected by ions at low and moderate concentrations, compared to the wetting behavior of the pure solvent. A novel drying scenario is predicted, where complete drying is prevented by the electrostatic attraction between a positively charged substrate and the anions, while near a negatively charged substrate, a first order transition with a predrying line is observed. The various scenarios are surprisingly similar to our previous predictions based on a "semiprimitive" model where the solvent particles carry no dipole, but the ion-ion interactions are reduced by a local dielectric permittivity.