Capturing the Effect of Anion Type on the Intermolecular Interactions between Water and Imidazolium-Based Ionic Liquids: A Comparative DFT Study.

Abstract

The studies on ionic liquids (ILs) and their interaction with different solvents have always been an interesting topic for experimental and computational chemists. Recently, however, deep insights on the molecular structures of the IL-water binary mixtures have been mainly performed through classical simulations. Here, a comprehensive quantum mechanical study is presented on seven 1-butyl-3-methylimidazolium-based ILs in the absence and presence of water. As the most important intermolecular interaction between ionic moieties of ILs and water molecules, hydrogen bonding is studied through different bonding analyses. The effect of different anions, [NO₃]^-, [HSO₄]^-, [SCN]^-, [DCA]^-, [BF₄]^-, [PF₆]^-, and [NTf₂]^-, on the behavior of ILs interacting with a sample of water molecules is investigated. Comparing the implicit and explicit approaches to consider water solvent indicated that the structure of ILs in the solvent depends on the selected solvent model. By considering explicit water molecules, we analyzed the intermolecular interactions between ILs and the water sample. The energy decomposition analysis indicated that the stability of the IL···water systems is mainly due to the electrostatic component of the total interaction energy. The interaction region indicator (IRI) analysis discovered that chemical bond and van der Waals (vdW) interactions are important in the IL···water systems. Indeed, investigation of each ion/ion pair surrounded by ten nearest neighbor water molecules discovered that the vdW interactions are responsible for the cation···anion and the cation···water interactions, while chemical bonding is important in the anion···water and the water···water interactions. Therefore, the anion···water interaction requires further analysis. The quantum theory of atoms in molecules verified the ionic nature of the H-bond in the anion···water interaction. The IRI analysis showed that the interaction between water molecules and cyano-based anions, [SCN]^- and [DCA]^-, is only due to chemical bonding, while in the oxygenated anions, [NO₃]^- and [HSO₄]^-, the vdW forces are also important. For the other anions, [BF₄]^-, [PF₆]^-, and [NTf₂]^-, the vdW forces have the main contribution in the anion···water interaction. Natural bond orbital analysis indicated that these intermolecular interactions originate from n_anion → σ_O-H^* electron transfer. Finally, the law of matching water affinity (LMWA) using energy-based parameters was used to predict the hydrophilicity of ILs as follows: [BMIM][NO₃] > [BMIM][SCN] > [BMIM][DCA] > [BMIM][HSO₄] > [BMIM][BF₄] > [BMIM][NTf₂] > [BMIM][PF₆]. Results obtained in the current work give insights into the electronic nature of intermolecular interactions between ILs and water molecules, which is necessary due to importance of water in modifying properties of ILs in various applications.