Modeling the vapor–liquid equilibria and water activity coefficients of alternative refrigerant-absorbent ionic liquid–water pairs for absorption systems

The aim of this study is to describe the phase equilibrium behavior of aqueous solutions of selected ionic liquids (ILs) as new potential pairs of absorption cooling systems. The ILs under study have a shared cation, 1-butyl-3-methylimidiazolium, [C₄mim]⁺, and the following anions: methanesulfonate, [CH₃SO₃]⁻, trifluoromethanesulfonate, [CF₃SO₃]⁻, acetate, [CH₃CO₂]⁻, trifluoroacetate, [CF₃CO₂]⁻, tosylate, [TOS]⁻, bromide, [Br]⁻, and thiocyanate, [SCN]⁻. Each binary system was individually studied within the soft-SAFT framework. A mesoscopic simple molecular model was proposed for each IL, being characterized by a specific set of molecular parameters. In combination with one or, in some cases, two additional binary parameters, it was possible to simultaneously describe the vapor–liquid equilibrium (VLE) of the aqueous binary systems at three different pressures (0.10 MPa, 0.07 MPa and 0.05 MPa) and the correspondent isothermal water activity coefficients at 298.15 K, correctly incorporating the different deviations from ideality and water interactions characteristics for each IL. Infinite dilution coefficients were also predicted in reasonable agreement with the experimental data. The simple model used with soft-SAFT to represent ILs provides an overall good description of most of the systems. Results for particular systems showed some space for improvement, nevertheless at expense of increasing the model complexity by changing the originally proposed molecular model.