A structural investigation of ionic liquid mixtures

Abstract

The structures of mixtures of ionic liquids (ILs) featuring a common 1-butyl-3-methylimidazolium ([C₄C₁im]⁺) cation but different anions have been investigated both experimentally and computationally. ¹H and ¹³C NMR of the ILs and their mixtures has been performed both on the undiluted liquids and those diluted by CD₂Cl₂. These experiments have been complemented by quantum chemical density functional theory calculations and molecular dynamics simulations. These techniques have identified the formation of preferential interactions between H² of the imidazolium cation and the most strongly hydrogen bond (H-bond) accepting anion. In addition, a preference for the more weakly H-bond accepting anion to interact above the imidazolium ring through anion–π⁺ interactions has been identified. The modelling of these data has identified that the magnitude of these preferences are small, of the order of only a few kJ mol⁻¹, for all IL mixtures. No clustering of the anions around a specific cation could be observed, indicating that these interactions arise from the reorientation of the cation within a randomly assigned network of anions. π⁺–π⁺ stacking of the imidazolium cations was also studied and found to be promoted by ILs with a strong H-bond accepting anion. Stacking interactions are easily disrupted by the introduction of small proportions (<50 mol%) of a weakly coordinating anion due to their propensity to form anion–π⁺ interactions. These results suggest that the formation of IL mixtures with different anions leads to subtle structural changes of much lower energy than the Coulombic ordering of ions, accounting for why most IL mixtures exhibit ideal, or nearly ideal, behaviour.