Rotational dynamics of carbon dioxide in ionic liquids

Abstract

The rotational dynamics of carbon dioxide (CO2) in four ionic liquids (ILs) has been investigated by 17O NMR T1 measurement. The ILs have the bis(trifluoromethanesulfonyl)imide anion ([NTf2]−) in common while they have diversity in the cationic component. The cations studied are 1-ethyl-3-methylimidazolium ([C2C1im]+), 1-butyl-3-methylimidazolium ([C4C1im]+), tributylmethylphosphonium ([P4441]+) and tetraoctylphosphonium ([P8888]+) cations. The observed rotational relaxation times (τ2R) are smaller than the prediction by the Stokes-Einstein-Debye (SED) theory (subslip behavior). When correction is made to the SED theory with respect to the solvent/solute molecular size ratio according to Gierer and Wirtz (Z. Naturforschg 1953, 8a, 832), a reasonable agreement between theory and experiment is reached for the τ of CO2 in [C2C1im][NTf2] and [C4C1im][NTf2]. The modified theory, however, still overestimates the τ in [P4441][NTf2] and [P8888][NTf2]. These results demonstrate that the hydrodynamic model does not work well for the extreme end of the solvent/solute size ratio.