Rotational dynamics of carbon dioxide in ionic liquids

Abstract The rotational dynamics of carbon dioxide (CO 2 ) in four ionic liquids (ILs) has been investigated by 17 O NMR T 1 measurement. The ILs have the bis(trifluoromethanesulfonyl)imide anion ([NTf 2 ] − ) in common while they have diversity in the cationic component. The cations studied are 1-ethyl-3-methylimidazolium ([C 2 C 1 im] + ), 1-butyl-3-methylimidazolium ([C 4 C 1 im] + ), tributylmethylphosphonium ([P 4441 ] + ) and tetraoctylphosphonium ([P 8888 ] + ) cations. The observed rotational relaxation times ( τ 2R ) 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 CO 2 in [C 2 C 1 im][NTf 2 ] and [C 4 C 1 im][NTf 2 ]. The modified theory, however, still overestimates the τ in [P 4441 ][NTf 2 ] and [P 8888 ][NTf 2 ]. These results demonstrate that the hydrodynamic model does not work well for the extreme end of the solvent/solute size ratio.