A Molecular Density Functional Theory of aqueous electrolytic solution

TL;DR

This paper extends molecular density functional theory to model inhomogeneous aqueous electrolytic solutions, introducing two models within the HNC approximation, validated against integral equation theory, and capable of studying solvation of arbitrary solutes.

Contribution

It presents a novel generalization of molecular DFT for electrolytic solutions, including a realistic three-component water model with orientational dependence, and implements a 3D methodology validated against existing theories.

Findings

The models accurately predict sodium cation solvation properties.

The methodology can handle solutes of arbitrary shapes.

Near-perfect agreement with integral equation theory for validation.

Abstract

We propose a generalisation of molecular density functional theory to describe inhomogeneous solvent mixture, with the objective of modelling electrolytic solutions. Two electrolytic models are presented, both within the HNC approximation. The first one is a two-components 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 ions densities solely depend on spatial coordinates. Additionally, we develop a realistic three-components 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 (3 spatial coordinates and 3 Euler angles) are validated by comparing the solvation properties of a sodium cation with the predictions of integral equation theory solved in 1D (1 intermolecular distance and 5 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-methylacetamide molecule immersed in both electrolytic solution models.