A molecular density functional theory of aqueous electrolytic solution

We propose a generalization of molecular density functional theory to describe inhomogeneous solvent mixtures, to model electrolytic solutions. Two electrolytic models are presented, both within the HNC approximation. The first one is a two-component mixture representing a primitive-like model of sodium chloride, where the solvent is described as a dielectric continuum. This popular model has the advantage of simplicity, as the ion densities solely depend on spatial coordinates. In addition, we develop a realistic three-component electrolyte model, in which water solvent is described by a third density field that depends on both spatial and orientational coordinates. The proposed methodology and its tridimensional implementation (three spatial coordinates and three Euler angles) are validated by comparing the solvation properties of a sodium cation with the predictions of integral equation theory solved in 1D (one intermolecular distance and five Euler angles), showing near-perfect agreement. This methodology enables the study of solvation properties of solutes of arbitrary shapes in electrolytic solutions, as demonstrated with the prototypical N-methyl acetamide molecule immersed in both electrolytic solution models.