Liquid-Liquid Equilibrium between Aprotic Heterocyclic Anion (AHA) Ionic Liquids and Water, and the Effect of Water on CO2 Capture by AHA Ionic Liquids

The effect of the cation and anion on the liquid-liquid equilibrium of aprotic heterocyclic anion (AHA) ionic liquids (ILs) and water was studied by volumetric Karl Fischer titration. It was found that the cation size and the anion nature impact the hydrophobicity of the ionic liquid.

In the context of post combustion CO₂ capture, small tetraalkylphosphonium cations and different AHA anions were probed for CO₂ capture. The AHA ILs studied showed high affinity for CO₂ binding; however, some of them showed low chemical stability and high viscosity even when paired with smaller alkyl chain cations.

As well, a proton NMR spectroscopy method was developed, in order to obtain a full understanding of the reaction mechanism, kinetics and equilibrium values of the IL-CO₂-H₂O reaction. The results indicate that the AHA anions undergo reprotonation, although the reaction appears to be reversible. The rate, equilibrium values, and CO₂ solubility vary significantly with the nature of the anion.