Prediction of binary VLE for imidazolium based ionic liquid systems using COSMO-RS

A novel method based on unimolecular quantum mechanical calculation has been used to predict the binary vapor-liquid equilibria (VLE) of ionic liquids (ILs). The recently developed conductor-like screening model (COSMO), along with the most common quantum chemical package of GAUSSIAN 03, has been used in this work. These conductor-like screening model calculations combined with exact statistical thermodynamics provide the information necessary for the evaluation of molecular interactions in liquids. An effective parametrization has been done using 10 associated and 22 binary nonassociated systems; these 32 systems are all non-ILs. The effective contact surface area a(eff) and the hydrogen-bonding coefficient c(hb) have been estimated using a sequential scheme. The root-mean-square error obtained for excess Gibb's free energy is similar to 0.1 for a(eff) and c(hb). Values for alpha' (misfit constant), sigma(hb) (cutoff surface charge density for hydrogen bonding), and cavity radii (r(i)) as given in the literature have been used as default. COSMO-RS has then been used to predict the vapor-liquid equilibria for 116 non-IL binary sets out of which 33 are azeotropic systems. COSMO-RS predicts a better pressure a priori with a relative error of similar to 4%, as compared to a 7-8% error for the Wilson/NRTL/UNIQUAC models. Being an a priori model, it does fall short with respect to absolute average deviation in mole fraction for the vapor phase: 0.025 as compared to similar to 0.0075 for the Wilson, NRTL, and UNIQUAC models. Having thus benchmarked extensively, the COSMO-RS model has then been used to predict the VLE for 13 systems based on five imidazolium ILs: (a) 1-methyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [MMIM] [(CF3SO2)(2)N] with (1) benzene and (2) cyclohexane; (b) 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [EMIM][(CF3SO2)(2)N] with (3) acetone, (4) 2-propanol, and (5) water; (c) 1-butyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl) imide [BMIM][(CF3SO2)(2)N] with (6) acetone, (7) 2-propanol, and (8) water; (d) 1-methyl-3-methylimidazolium dimethylphosphate [MMIM][(CH3)(2)PO4] with (9) acetone, (10) tetrahydrofuran, and (11) water; and (e) 1-ethyl-3-methylimidazolium ethoxysulfate [EMIM][C2H5OSO3] with (12) benzene and (13) cyclohexane. The root-mean-square deviation for pressure prediction is 6% as compared to 4%, 1.45%, and 3.13% for the Wilson, NRTL, and UNIQUAC models, respectively. The mole fraction in the vapor phase has also been predicted, confirming the negligible presence of ionic liquids in the vapor phase even at very low pressures.