Iodide-Based Ionic Liquid and Water Mixtures: A Combined Experimental and Molecular Dynamics Study

Iodide-containing ionic liquids (Ils) have been explored for use in electrochemical applications utilizing the triiodide/triiodide redox couple including dye-sensitized solar cells and MET sensors. Advantages of these ILs include low vapor pressure and a wide temperature range as a liquid, but high viscosity can reduce their usefulness. Mixing the IL with a low viscosity solvent such as water can alleviate this difficulty, but the properties of iodide-IL/water mixtures have never been studied. We systematically studied, for the first time, mixtures of 1-butyl-3-methylimadazolium iodide ([BMIM][I]) and water through both experimental studies and molecular dynamics simulations. The addition of water was found to lower the melting point of [BMIM][I] up to a composition of 50% water by mole, after which it increased rapidly. Trends in density, viscosity, viscosity deviation, and ionic conductivity were also observed.Molecular dynamics simulations representing the same mixture compositions were performed in parallel to the experimental work. Enthalpy, entropy, and Gibb’s energy of mixing were calculated from the simulations. Molecular ordering between the ions in the ionic liquid and the water molecules was observed and related to trends in the macroscopic properties. Hydrogen bonding was found to have a significant effect on ordering and properties.