(230d) Effect of CO2 Pressure on the Viscosity of 1?Alkyl-1-Methylpyrrolidinium Bis(tri?uoromethylsulfonyl)Imide Ionic Liquids

The effect of CO₂ pressure on the viscosity of pyrrolidinium-based ionic liquids (ILs) were measured using a high-pressure viscometer at various temperatures (298.15, 318.15, and 338.15 K). The high-pressure viscosities of 1-propyl-1-methylpyrrolidinium ([C₃C₁Pyr]), 1-butyl-1-methylpyrrolidinium ([C₄C₁Pyr]), and 1-hexyl-1-methylpyrrolidinium ([C₆C₁Pyr]) cations with a common anion, bis(trifluoromethylsulfonyl)imide ([NTf₂]), were measured up to 25 MPa of CO₂ pressure. The viscosities of the IL/CO₂ mixtures dramatically decrease with increasing CO₂ pressure up to approximately 7 MPa, beyond which the reduction becomes marginal. This is due to the vapor−liquid equilibrium (CO₂ solubility in IL) of the IL/CO₂ system. The viscosity reduction is more significant at lower temperatures because of the higher CO₂ solubility under those conditions. The effects of cation alkyl chain length on the viscosities of IL/CO₂ mixtures were also measured. Though the pure [C_nC₁Pyr][NTf₂] viscosities increase significantly with increasing alkyl chain length of the cation, the viscosities of the IL/CO₂ mixtures are similar at higher CO₂ pressures (composition).