Physical and CO2-Absorption Properties of Imidazolium Ionic Liquids with Tetracyanoborate and Bis(trifluoromethanesulfonyl)amide Anions

Ionic liquids with tetracyanoborate (TCB]^?) and bis(trifluoromethanesulfonyl)amide (Tf₂N]^?) anions generally have low viscosities and high CO₂ capacities, and thus they are attractive solvents for CO₂-related applications. Herein, we have investigated physical and CO₂-absorption properties of 1-ethyl-3-methylimidazolium tetracyanoborate ionic liquid (emim]TCB]) to discuss the anion effects of TCB]^? in comparison with the previous results of emim]Tf₂N]. The density, viscosity, electrical conductivity, and isobaric molar heat capacity were measured as a function of temperature at atmospheric pressure. emim]TCB] has both lower density and isobaric molar heat capacity than emim]Tf₂N]. emim]TCB] shows superior transport properties (lower viscosity and higher electrical conductivity) compared to emim]Tf₂N], whereas the Walden plots of molar conductivity against fluidity (reciprocal of viscosity) have smaller values in emim]TCB] than in emim]Tf₂N] at certain fluidities. The high-pressure CO₂ solubilities were also determined in emim]TCB]. The mole fraction scaled solubility of CO₂ in emim]TCB] is slightly larger than that in emim]Tf₂N] at certain pressures and temperatures. The former ionic liquid shows much higher molarity scaled solubility of CO₂ than the latter because of the smaller molar volume. It is suggested that both anions have similar strength of intermolecular interaction with CO₂ and comparable changes in the solvent structure between neat and CO₂ solution, in view of the thermodynamic parameters of dissolution.