Separation factors for [amim]Cl–CO₂ biphasic systems from high pressure density and partition coefficient measurements

In this work, densities and viscosities at atmospheric pressure and densities at high pressures of the ionic liquid, [amim]Cl (1-allyl-3-methylimidazolium chloride) were measured. The densities were evaluated by correlation with the Tait equation and the ePC-SAFT equation of state. Both relationships could describe the experimental density data over the full range of all temperatures (313–373 K) and pressures (0.1–200 MPa) within 0.007% and 0.053%, respectively. Infinite dilution partition coefficients, K_w3^∞, of fundamental benzene derivatives (chlorobenzene, bromobenzene, benzaldehyde and benzyl alcohol) between [amim]Cl and supercritical carbon dioxide were measured at 313–353 K and 6–21 MPa. The ePC-SAFT applied as a predictive model for the experimental partition coefficient gave high deviations (>900%). However, when the ePC-SAFT was applied as a correlative model, average deviation in K_w3^∞ were 20%. A previously proposed semi-empirical LSER-δ model could correlate the K_w3^∞ data to within 8.9%. Separation factors of solutes in [amim]Cl–CO₂ system were calculated with ePC-SAFT model and compared with literature results for [bmim]Cl–CO₂ since both of these ionic liquids have application to biomass processing. For each given ionic liquid, [amim]Cl and [bmim]Cl, the separation factors of non-polar or slightly polar compounds (chlorobenzene, bromobenzene and benzaldehyde) to the reference polar compound (benzyl alcohol) were always greater than 100, with [amim]Cl–CO₂ always having a higher separation factor than [bmim]Cl–CO₂. Thus, [amim]Cl–CO₂ can probably provide better separation between non-polar and polar mixtures than [bmim]Cl–CO₂.