Pseudolattice theory of the surface tension of ionic liquid-water mixtures.

A theoretical model for ionic liquids (ILs) based on a pseudoreticular structural model for the bulk mixture is reported. The original Bahe-Varela pseudolattice theory of concentrated ionic solutions is modified and the short-range interactions modeled by a Lennard-Jones potential. In this framework, the surface tension of the pure IL is calculated and the correct dependence of this magnitude on the density of the liquid, as provided by the parachor, is recovered. The anions in the mixture are assumed to form a continuum structureless neutralizing background, and that the organic cations and water molecules are placed in the nodes of the pseudolattice. The surface pressure of IL-water mixtures is calculated using a localized model for the adsorption of particles in the surface of the mixture and a mean-field Bragg-Williams approximation for the chemical potential of the adsorbed particles in the pseudolattice. The theoretical predictions are tested with experimental data of several ionic liquid aqueous mixtures.