Viscosity of 1-Alky1-1-methylpyrrolidinium Bis(trifluoromethylsulfonyl)imide Ionic Liquids Saturated with Compressed CO2

The viscosities of mixtures of pyrrolidinium-based ionic liquids saturated with compressed CO2 were measured using a high-pressure viscometer at three different temperatures (298.15, 318.15, and 338.15 K). The high-pressure viscosities of 1-propyl-1-methylpyrrolidinium ([C(3)C(1)Pyr]), 1-butyl-1-methylpyrrolidinium ([C(4)C(1)Pyr]), and 1-hexyl-1-methylpyrrolidinium ([C(6)C(1)Pyr]) cations with a common anion, bis(trifluoromethylsulfonyl)imide ([NTf2]), were measured up to a maximum CO2 pressure of 25 MPa. These viscosities ranged from 3.3 to 98.3 mPas. The viscosities of the ionic liquid/CO2 mixtures dramatically decrease with increasing CO2 pressure up to approximately 7 MPa, beyond which the viscosity decrease becomes more marginal. This is due to the vapor-liquid equilibrium (CO2 solubility) of the IL/CO2 system. The viscosity decrease at lower temperatures is more significant because of the higher CO2 solubility. The effect of the alkyl chain length of the cation on the viscosity of ionic liquid/CO2 mixtures was also measured. Though the pure [CnC1Pyr][NTf2] viscosities increase significantly with increasing alkyl chain length of the cation, the viscosities of the ionic liquid/CO2 mixtures become quite similar at higher CO2 pressures (composition).