(222ad) Prediction of Phase Behaviors of Ionic Liquids Over a Wide Range of Conditions

Here we compare several predictive approaches for the properties and phase behavior of mixtures containing ionic liquids (ILs). These models combine the Pitzer-Debye-Hückel (PDH) model for the long-range electrostatic interactions and the COSMO-SAC model for the short-range interactions between contacting molecules. While the COSMO-SAC model alone has been shown to be accurate for vapor-liquid equilibrium (VLE) and infinite dilution  activity coefficient (IDAC) of solvents in IL, it is inaccurate for dilute IL solution properties such as osmotic coefficient (OC) and mean ionic activity coefficient (MIAC), where long-range interactions are important. The PDH model alone is accurate for dilute IL solutions; however, it is not applicable at high IL concentrations. Here we propose a novel method for attenuating the long-range effect from the PDH model at high IL concentrations. The resultant model is thus accurate over the whole concentration ranges of the IL solutions. We have examined this method for three versions of the COSMO-SAC model, denoted as COSMO-SAC(2007)+PDH(α), COSMO-SAC(2010)+PDH(α), and COSMO-SAC(ion,α), for mixtures involving 73 ILs formed from the combination of 44 cations and 31 anions. The value of absolute relative deviation (ARD) for IDAC (3552 points) is 107.89% (COSMO-SAC(2007)+PDH(α)), 165.37% (COSMO-SAC(2010)+PDH(α)) and 116.94% (COSMO-SAC(ion,α)), respectively. The errors for VLE (1626 points) are 23.98%, 28.88% and 22.02%. The prediction results for osmotic coefficient (1348 points) of solutions showed 40.86, 48.10% and 24.09% and the predicted MIAC (484 points) of ionic liquid showed a similar result with about 48%. The proposed COSMO-SAC models provide prediction of properties of IL fluids with consistent accuracy over the full range of concentrations.