Molecular Dynamics Insight into the Role of Water Molecules in Ionic Liquid Mixtures of 1-Butyl-3-methylimidazolium Iodide and Ethylammonium Nitrate

Imidazolium-based ionic liquids are well known for their versatility as solvents for various applications such as dye-sensitized solar cells, fuel cells, and lithium-ion batteries; however, their complex interactions continue to be investigated to further improve upon their design. Ionic liquids (ILs) are commonly mixed with co-solvents such as water, organic solvents, or other ionic liquids to tailor their physiochemical properties. To better predict these properties and fundamentally understand the molecular interactions within the electrolyte mixtures, molecular dynamics (MD) simulations are often employed. In this study, MD simulations are performed on ternary solutions containing ionic liquids of 1-butyl-3-methylimidazolium iodide ([BMIM][I]) and ethylammonium nitrate ([EA][NO₃]) with increasing concentration of water. As previously reported, these ternary solutions displayed a wide temperature window of thermal stability and electrochemical conductivity. Utilizing MD simulations, the complex intermolecular interactions are identified, and the role of water as a co-solvent is disclosed to correlate with changes in their bulk properties. The MD results, including simulation box snapshots, radial distribution functions, and self-diffusion coefficients, reveal the formation of heterogeneous regimes with increasing water concentration, hydrogen bonding between iodide-water, iodide-[EA]⁺, and a change in IL ordering when in mixtures containing water. The simulations also display the formation of water aggregates and networks at high water concentrations, which can contribute to the thermal behavior of the respective mixtures. As the design of IL-based electrolytes grows in demand with increasing complexity, this work demonstrates the capability of MD simulations containing multiple constituents and their necessity in material development through identification of microscopic structure-property relationships.