First Principle Approach to Solvation by Methylimidazolium-Based Ionic Liquids

Abstract

Understanding the nature of the inter- and intramolecular interactions of solutes and ionic liquid (IL) ion pairs from an electronic point of view is necessary for explaining the mechanisms behind the selectivity of ILs toward a certain solute. Due to the complexity of the underlying physicochemical interactions, and aiming at a reliable representation of the solute-IL interactions, the model system chosen in this work is formed by one single ion pair and the solute of interest, in the gas phase. Ab initio molecular dynamics (MD) techniques are used for ensuring a complete scan of the potential energy surface. A representative number of structures extracted from this trajectory are optimized using more sophisticated DFT methods. Posterior bond analysis (with natural bonding orbitals (NBO), and Morokuma-like energy partition) provide a detailed picture of the solute-IL bond nature for a set of various solutes, anions, and cations, to find a relationship between the gas phase electronic characteristics and the experimentally observed behavior. The approximation to the ILs solvation properties employing this very basic model shows that, on one side, the specific interaction of the solute with methylimidazolium-based IL is a reliable indication of the overall affinity between the bulk IL and the solute, and can be considered a predictive tool for the bulk behavior. Furthermore, the systematic study carried out has permitted the rational comprehension of such properties and thus permits us to extend it to other systems.