In the past decade, room temperature ionic liquids (ILs) have received increased attention as more environmentally benign alternatives in a variety of applications ranging from reaction media, separation solvents, heat transfer fluids and electrolytes in fuel cells (Brennecke and Maginn, 2001). Recent research efforts - experimental and computational - have provided a wealth of data and understanding about the nature of ILs. As widespread industrial use of ILs approaches, there is a need for relatively simple thermodynamic models, such as equations of state (EOS), that are easy to use in the context of process design, optimization and control. Such models must be capable of correlating and predicting the properties of ILs and mixtures containing ILs with reasonable accuracy. Of particular interest here is the thermodynamics of IL-gas mixtures, as occur in applications such as gas separations and absorption-refrigeration systems (Blanchard and Brennecke, 2001; Yokozeki, 2005). Since IL-CO₂ mixtures have been the most widely studied, we will focus on this system as an example. Molecular dynamics and Monte Carlo simulations have been shown to accurately predict pure IL properties and IL-CO₂ properties from ab initio calculations, but they are computationally very expensive. Therefore, there is interest in simpler models that might be more useful for process engineering computations. In order to model IL-CO₂ systems, recent research efforts have focused on using EOS models to correlate vapor-liquid equilibrium (VLE) behavior (Kroon et al., 2006; Wang et al., 2006). Classical cubic EOS, such as Redlich-Kwong, have been shown to correlate and predict the VLE of IL-CO₂ systems (Shifflet and Yokozeki, 2005; 2007). Given the necessary number of interaction parameters, such models can properly correlate IL-CO₂ VLE data and have been shown to be predictive of VLE behavior outside of the data range used to fit the interaction parameters (Shifflet and Yokozeki, 2007). Unfortunately, prediction of VLE behavior alone is not sufficient in some applications, such as in an absorption-refrigeration cycle for which mixture enthalpies and densities are needed. The suitability of cubic EOS models for prediction of properties other than VLE has not been widely studied to date. In this work, we investigate the applicability of cubic EOS for modeling the properties of ILs and mixtures containing ILs. We use the Peng-Robinson EOS, and parameterize the EOS with the goal of being able to model the pure IL behavior given available data. We then fit interaction parameters to IL-CO₂ VLE data. The parameterized equation is then used to predict mixture density and heat of absorption. The results from this approach are then compared to the predictions provided by previous cubic EOS models for IL-CO₂ systems. This allows us to make comparisons of different approaches for developing cubic EOS models for mixtures containing ILs, and to address the question of how good such models are in this context. References Blanchard, L. A. and J. F. Brennecke, “Recovery of organic products from ionic liquids using supercritical carbon dioxide.” Ind. Eng. Chem. Res., 40, 287 (2001). Brennecke J. F. and E. J. Maginn, “Ionic liquids: innovative fluids for chemical processing.” AIChE J., 47, 2384 (2001). Shiflett, M. B. and A. Yokozeki, “Solubilities and diffusivities of carbon dioxide in ionic liquids: [bmim][PF6] and [bmim][BF4].” Ind. Eng. Chem. Res., 44, 4453 (2005). Kroon, M. C., Karatsani, E. K., Economou, I. G., Witkamp G. J. and C. J. Peters, “Modeling the carbon dioxide solubility in imidazolium-based ionic liquids with the tPC-SAFT equation of state.” J. Phys. Chem. B, 110, 9262 (2006). Shiflett, M. B. and A. Yokozeki, “Solubility of CO2 in room temperature ionic liquid [hmim][Tf2N].” J. Phys. Chem. B, 111, 2070 (2007). Wang, T. F., Peng, C. J., Liu, H.L and Y. Hu, “Description of the pVT behavior of ionic liquids and the solubility of gases in ionic liquids using an equation of state.” Fluid Phase Equilib., 250, 150 (2006). A. Yokozeki, “Theoretical performances of various refrigerant-absorbent pairs ia a vapor-absorption refrigeration cycle by the use of equations of state.” Applied Energy, 80, 383 (2005).