A rational approach to CO₂ capture by imidazolium ionic liquids: Tuning CO₂ solubility by cation alkyl branching

Branching at the alkyl side chain of the imidazolium cation in ionic liquids (ILs) was evaluated towards its effect on carbon dioxide (CO₂) solubilization at 10 and 80 bar (1 bar=1×10⁵ Pa). By combining high-pressure NMR spectroscopy measurements with molecular dynamics simulations, a full description of the molecular interactions that take place in the IL-CO₂ mixtures can be obtained. The introduction of a methyl group has a significant effect on CO₂ solubility in comparison with linear or fluorinated analogues. The differences in CO₂ solubility arise from differences in liquid organization caused by structural changes in the cation. ILs with branched cations have similar short-range cation-anion orientations as those in ILs with linear side chains, but present differences in the long-range order. The introduction of CO₂ does not cause perturbations in the former and benefits from the differences in the latter. Branching at the cation results in sponge-like ILs with enhanced capabilities for CO₂ capture.