Theoretical Study of Fe³⁺ and Ni²⁺ Ion Interactions in Ethaline as the Deep Eutectic Solvent and Water Solutions Using Molecular Dynamics, Quantum Theory of Atoms in Molecules, and Non-Covalent Interactions

Deep eutectic solvents (DES) have several advantages compared to water and traditional solvents, making them an alternative, especially in applications that require better solvation, greater thermal stability, and a lower environmental impact. This study aimed to analyze the behavior of Fe³⁺ and Ni²⁺ ions in two quantities of water (300 and 5580 molecules) and in a eutectic solvent based on choline chloride and ethylene glycol (1ChCl:2EG). The computational methods used involved molecular dynamics, quantum theory of atoms in molecules (QTAIM), and noncovalent interactions simulations. Analysis of the radial distribution function multiplied by the number density [<i>g</i>(<i>r</i>)ρ] and the cumulative number (CN) indicated that the interactions between the metal ions and the water molecules were strongest for the systems with the most water. QTAIM determined the bond critical point, the electron density [ρ(<i>r</i>)], the Laplacian of the electronic density [∇²ρ(<i>r</i>)], and the electron localization function (η, ELF), allowing the interactions to be analyzed. Polarizability of the metal ions in both water and ethaline was compared; the increasing order of polarizability was Fe³⁺ < Ni²⁺ < Fe²⁺, with Fe³⁺ being the least polarizable due to its high charge and smaller ionic radius. In the mixed systems with Fe²⁺ or Fe³⁺ added to Ni²⁺, the metal species with the same charge competed similarly in water and DES. The intermolecular forces in DES are weaker due to the solvent's lower polarity and dielectric constant than water. In the systems with the highest water content (5580 molecules), Fe³⁺ ions were surrounded by the largest water molecules, followed by Fe²⁺ and Ni²⁺. These results may help to better understand the solvation and behavior of these ions in different media, which has implications for use in electrodeposition, batteries, and corrosion inhibition.