Cation-Anion and Cation-Cation Interactions in Mixtures of Hydroxy-functionalized and Aprotic Ionic Liquids

Abstract

A combination of molecular dynamics simulations, quantum chemical calculations, and vibrational spectroscopy is employed to study ionic interactions in 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C_2OHmim]NTf₂, 1-methyl-3-propylimidazolium bis(trifluoromethanesulfonyl)imide [C₃mim]NTf₂, and their double salt mixtures. Ionic liquids containing hydroxy groups are unique in their ability to form unconventional hydrogen bonds between neighboring cations, despite the strong repulsive forces these ions exert upon one another. Our computational and spectroscopic data reveal a rich array of ionic interactions in pure [C_2OHmim]NTf₂. The hydrogen bonding network inherent to [C_2OHmim]NTf₂ is partially disrupted when [C₃mim]NTf₂ is added to the ionic liquid to produce [C_2OHmim]_0.5[C₃mim]_0.5NTf₂. This leads to increased amounts of uncoordinated hydroxy groups and minor changes in the distribution functions for side chain orientations. Furthermore, OH groups from [C_2OHmim]⁺ are shown to hydrogen bond with the acidic hydrogen atoms attached to the [C₃mim]⁺ or [C_2OHmim]⁺ imidazolium rings. Cation-cation hydrogen bonding is not restricted solely to situations where both cations contain hydroxy groups (i.e., homo-aggregation of cations through OH‧‧‧OH interaction motifs). Rather, hetero-aggregation of chemically distinct cations may also occur through CH‧‧‧OH hydrogen bonds, where the CH hydrogen atom is directly connected to the imidazolium ring. Taken together, our data shows the cationic interactions that occur in the ionic liquid mixtures are controlled by competitive interactions originating from the hydrophobic propyl groups and hydrophilic hydroxyethyl groups.