CO2 absorption properties, densities, viscosities, and electrical conductivities of ethylimidazolium and 1-ethyl-3-methylimidazolium ionic liquids

We have focused on a protic ionic liquid, ethylimidazolium bis(trifluoromethanesulfonyl)amide ([eimH][Tf2N]), and the analogous aprotic ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([emim][Tf2N]). Densities, viscosities, and electrical conductivities of the two ionic liquids have been measured over the temperature range T = (273.15–363.15) K at atmospheric pressure. The densities of [eimH][Tf2N] and [emim][Tf2N] decrease with increasing temperature. The protic ionic liquid has the higher density (the smaller molar volume) than the aprotic one. The transport properties of the two ionic liquids show ordinary temperature dependencies. [eimH][Tf2N] has the higher viscosities (smaller electrical conductivities) than [emim][Tf2N]. Empirical Walden products indicate that [eimH][Tf2N] shows smaller conductivities than [emim][Tf2N] at certain viscosities. To investigate CO2 absorption properties of the ionic liquids, binary phase equilibria in CO2 + ionic liquid systems have been investigated under high pressures up to 6 MPa at T = (298.15, 313.15, 333.15) K. The solubilities of CO2 in [eimH][Tf2N] and [emim][Tf2N] show typical pressure dependencies in both mole fraction and molarity scales as physical absorbents. The mole fraction of CO2 in [eimH][Tf2N] is slightly smaller than that in [emim][Tf2N] under the conditions investigated, while the molarity of CO2 in [eimH][Tf2N] is comparable to that in [emim][Tf2N]. The molar volumes in ionic liquid phase almost linearly decrease with the mole fraction of CO2 in the present systems. The partial molar volume of CO2 in [eimH][Tf2N] at infinite dilution is almost same as that in [emim][Tf2N].