Physical and CO_2-Absorption Properties of Imidazolium Ionic Liquids with Tetracyanoborate and Bis(trifluoromethanesulfonyl)amide Anions

摘要

Ionic liquids with tetracyanoborate ([TCB]~-) and bis(trifluoromethanesulfonyl) amide ([Tf_2N]~-) anions generally have low viscosities and high CO_2 capacities, and thus they are attractive solvents for CO_2-related applications. Herein, we have investigated physical and CO_2-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][Tf_2N]. 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][Tf_2N]. [emim][TCB] shows superior transport properties (lower viscosity and higher electrical conductivity) compared to [emim][Tf_2N], whereas the Walden plots of molar conductivity against fluidity (reciprocal of viscosity) have smaller values in [emim][TCB] than in [emim][Tf_2N] at certain fluidities. The high-pressure CO_2 solubilities were also determined in [emim][TCB]. The mole fraction scaled solubility of CO_2 in [emim][TCB] is slightly larger than that in [emim][Tf_2N] at certain pressures and temperatures. The former ionic liquid shows much higher molarity scaled solubility of CO_2 than the latter because of the smaller molar volume. It is suggested that both anions have similar strength of intermolecular interaction with CO_2 and comparable changes in the solvent structure between neat and CO_2 solution, in view of the thermodynamic parameters of dissolution.