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.