Understanding the high solubility of CO₂ in an ionic liquid with the tetracyanoborate anion

The ionic liquid 1-ethyl-3-methylimidazolium tetracyanoborate, [emim][B(CN)₄], shows greater CO₂ solubility than several popular ionic liquids (ILs) of different anions including [emim] bis(trifluoromethylsulfonyl)imide [emim][Tf₂N]. Herein, both classical molecular dynamics simulation and quantum mechanical calculations were used to understand the high solubility of CO₂ in the [emim][B(CN)₄] IL. We found that the solubility is dictated by the cation-anion interaction, while the CO₂-anion interaction plays a secondary role. The atom-atom radial distribution functions (RDFs) between cation and anion show weaker interaction in [emim][B(CN)₄] than in [emim][Tf₂N]. A good correlation is observed between gas-phase cation-anion interaction energy with CO₂ solubility at 1 bar and 298 K, suggesting that weaker cation-anion interaction leads to higher CO ₂ solubility. MD simulation of CO₂ in the ILs showed that CO₂ is closer to the anion than to the cation and that it interacts more strongly with [B(CN)₄] than with [Tf₂N]. Moreover, a higher volume expansion is observed in [emim][B(CN)₄] than in [emim][Tf₂N] at different mole fractions of CO₂. These results indicate that [B(CN)₄] as a small and highly symmetric anion is unique in giving a high CO₂ solubility by interacting weakly with the cation and thus allowing easy creation of cavity for close contact with CO₂.