Estimation of Partial Ionic Dissociation and Fluid Phase Equilibria in Systems Containing Ionic Liquids Luke D. Simoni, Alexander Augugliaro, Joan F. Brennecke, Mark A. Stadtherr Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana Introduction The prediction of phase equilibria in systems containing ionic liquids (ILs) is crucial to evaluating an IL’s utility as an alternative solvent for a myriad of applications. Indeed, the use of an IL as an extraction solvent, reactive medium, or novel electrolyte requires accurate knowledge of the IL’s partitioning and miscibility with the system components. As there are a large number of ILs possible, experimental verification of all phase behaviors is an intractable endeavor. Therefore, simple predictive estimation methods are desired. Part of any accurate, predictive method will require realistic representation of the molecular state in solution. In this work, we show that a simple method of ionic partial dissociation estimation improves LLE predictions of IL-containing systems over previous contributions 1 Using a novel version of the electrolyte-NRTL (eNRTLpd), we show that satisfactory LLE predictions are possible for Type 1 systems. Ongoing work includes predictions for other Types of LLE systems. Furthermore, we show, in line with a previous contribution, 2 that VLE vapor pressure predictions for an aqueous IL-containing system is more accurately estimated using our eNRTLpd model fitted to experimental excess enthalpy (h E ) and activity coefficients of water at infinite dilution (   ). 3 Ionic Liquids Considered The following ions comprise the ILs studied in this presentation. cation [Rmim]1-Alkyl-3- Methylimidazolium anion [TfO]Trifluoromethanesulfonate [EtSO 4 ]Ethylsulfate [Tf 2 N]Bis(trifluoromethylsulfonyl) imide Dissociation Degree Estimates As examples we demonstrate the above calculations for the IL systems IL (1)/water (2). Uncertainty in associated viscosity measurements renders estimates more reliable as neat IL is approached. Therefore a roughly average median value of ξ is taken for each system (independent of composition) Vapor-Liquid Equilibrium Predictions The following are VLE predictions for [emim][TfO] (1)/water (2) using experimental h E and estimated infinite dilution activity coefficients (   ) for water. Examination of both plots reveals that eNRTLpd provides intermediate predictions compared to NRTL and eNRTL using the same data for fitting. Clearly, eNRTLpd is more accurate for pressure prediction of this system. It should be noted that   values are crucial for model parameter estimation when the system’s excess enthalpy data is endothermic (e.g. [emim][TfO]/water). Degree of Dissociation Dramatically easier and less experimentally intensive than previous contributions, 4 we rely only on conductivity and viscosity measurements to determine the degree of dissociation of the ionic liquid in the solution. Assuming that the ions diffuse like spheres where the dominating resistance is viscous friction, we can relate the viscosity and conductivity of a solution to the degree of ionic dissociation with the Stokes-Einstein and Nernst-Einstein equations. Note that for ILs Also, we estimate the Stokes Radius and for pure ILs of ion i by the Bondi Method where σ i = 3(volume) i /(area) i Conclusions Estimates of ionic degree of dissociation from viscosity and conductivity measurements are easier and more reliable than those obtained through PG-NMR. Predictions that consider partial dissociation provide an improvement over previous conventional g E models that are direct predecessors to the eNRTLpd model for binary VLE and ternary LLE. Acknowledgments Department of Energy References 1.L. D. Simoni, Y. Lin, J. F. Brennecke, M. A. Stadtherr. Modeling Liquid-Liquid Equilibrium of Ionic Liquid Systems with NRTL, Electrolyte-NRTL and UNIQUAC. Ind. Eng. Chem. Res. 47, 2008, L.D. Simoni, L.E. Ficke, C.A. Lambert, M.A. Stadtherr, and J.F. Brennecke. Thermodynamics of Ionic Liquid + Water Systems; manuscript in preparation, 2008b. 3.L.E. Ficke, H. Rodríguez, and J.F. Brennecke. Heat Capacities and Excess Enthalpies of 1-Ethyl-3-methylimidazolium Based Ionic Liquids and Water. J. Chem. Eng. Data, 2008, in press. 4.H. Tokuda, S. Tsuzuki, Md. Abu Bin Hasan Susan, K. Hayamizu, M. Watanabe. How Ionic are Room Temperature Ionic Liquids? An Indicator of Physicochemical Properties. J. Phys. Chem. B, 110, 2006, Liquid-Liquid Equilibrium Predictions The following are ternary LLE predictions for [emim][EtSO 4 ] (1)/EtOH (2)/ETBE (3) at 298 K using published binary LLE and VLE data for model fitting. We notice a remarkable improvement in the eNRTLpd predictions over NRTL and eNRTL. Liquid-Liquid Equilibrium Predictions The following are ternary LLE predictions for [hmim][Tf 2 N] (1)/EtOH (2)/water (3) at 295 K using published binary LLE and VLE data for model fitting. We notice a slight improvement in the eNRTLpd binodal curve accuracy. Verification of Neat [Tf 2 N]-based IL ξ Values CATION ξ ξ(previous) 4 [mmim] [emim] [bmim] [hmim] [omim] Note: ξ(prevoius) 4 = ( Λ imp /Λ NMR )/ν