Understanding the High Solubility of CO2 in an Ionic Liquid with the Tetracyanoborate Anion

The ionic liquid 1-ethyl-3-methylimidazolium tetracyanoborate, [mini] [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 CO2 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.