Regular Solution Theory for Low Pressure Carbon Dioxide Solubility in Room Temperature Ionic Liquids: Ionic Liquid Solubility Parameter from Activation Energy of Viscosity Surya S. Moganty † and Ruth E. Baltus* Department of Chemical and Biomolecular Engineering, Clarkson UniVersity, Potsdam, New York 13699-5705 The low pressure solubility of carbon dioxide in eight commercially available room temperature ionic liquids was measured at 10, 25, and 40 °C using a transient thin liquid film technique. In this paper, carbon dioxide solubility is reported as the Henry’s law constant for each system. Experimental results were interpreted using regular solution theory where Eyring’s reaction rate theory was successfully applied to estimate the solubility parameter of each ionic liquid from its activation energy of viscosity. Consistent with the regular solution theory, the carbon dioxide solubility was found to be inversely proportional to the solubility parameter of the ionic liquid, and Henry’s law constants were successfully correlated with the square of the difference between ionic liquid and carbon dioxide solubility parameters. Introduction Room temperature ionic liquids (RTILs) are organic salts consisting of a bulky cation and an inorganic anion with melting points below 100 °C. The large cation size allows for delocal- ization and screening of charges, resulting in a reduction in the lattice energy and thereby the melting or glass transition temperature. RTILs exhibit many interesting properties, which make them suitable for applications such as chemical synthesis, catalysis, electrochemical applications, and gas separations. Knowledge of the solubilities of gases in different RTILs is important for the design and development of ionic liquid-based reaction and separation processes as well as for understanding gas-liquid interactions that govern solubility. Among different gases, carbon dioxide is the most widely studied because its relatively high solubility in many RTILs has focused attention on RTIL-based separation processes for carbon capture from flue gases generated in coal-fired power plants. 1-5 A number of different techniques have been used to measure gas solubilities in RTILs. These include a gravimetric method, 6-10 a quartz crystal microbalance method, 11 and equilibrium pressure and volume techniques. 12,13 In recent years, work in our laboratory has focused on measuring gas solubility and diffu- sivity in RTILs using a technique involving gas uptake into a thin ionic liquid film. 14-16 Efforts have also been directed toward the development of thermodynamic models for predicting gas solubilities in RTILs. Shiflett and Yokozeki 7,9,17,18 developed a Redlich-Kwong cubic equation of state as well as different activity coefficient models to describe NH 3 , CO 2 , and hydrofluorocarbon solubilities in RTILs. Quantitative structure-properties relationship models have also been developed for modeling solubilities in RTILs. 19 However, these approaches require experimental data for each RTIL-gas pair to determine the model parameters. This problem can be avoided with regular solution theory (RST) because model parameters involve only pure component properties. Shi and Maginn 20 compared RST predictions with results from molecular modeling simulations and concluded that RST was the most useful predictive tool for correlating low pressure gas solubilities. Using different approaches to relate measurable properties to RTIL solubility parameters, Noble and co-work- ers 21-24 and Scovazzo and co-workers 25-27 successfully applied RST to interpret and predict the solubilities of a variety of gases in different RTILs. In this paper, we report results from measurements of the low pressure solubility of CO 2 in eight different RTILs at 10, 25, and 40 °C. Results are interpreted using RST, where an alternative approach for estimating the RTIL solubility param- eters is presented. In this approach, RTIL solubility parameters are estimated from activation energies of RTIL viscosity, building upon Eyring’s absolute reaction rate theory of the liquid state, which relates the energy of vaporization to the activation energy of viscosity. 28 This approach is similar yet different than the approach used by Kilaru and Scovazzo 27 for interpreting carbon dioxide and hydrocarbon solubilities in RTILs. The resulting expression allows one to estimate the Henry’s law constant for CO 2 in an ionic liquid from viscosity measurements at several different temperatures. Regular Solution Theory RST assumes that at constant temperature and pressure, the excess entropy of mixing vanishes and that forces of attraction between molecules are primarily short-range dispersion forces. Low columbic interactions are expected for RTILs because the large cation size delocalizes the charge. Hence, it is reasonable to assume that RTIL solutions are dominated by short-range forces. The vapor liquid equilibrium of carbon dioxide dissolved in an RTIL can be expressed in terms of the fugacity of carbon dioxide: Because RTILs are nonvolatile and only CO 2 is introduced into the experimental cell, the gas phase is pure CO 2 and is assumed to be ideal. Therefore, the fugacity of CO 2 can be assumed to be equal to the gas pressure. The CO 2 fugacity in the liquid phase can be written in terms of an activity coefficient: * To whom correspondence should be addressed. E-mail: baltus@ clarkson.edu. † Present address: School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York 14850. f CO 2 G ) f CO 2 IL (1) Ind. Eng. Chem. Res. 2010, 49, 5846–5853 5846 10.1021/ie901837k  2010 American Chemical Society Published on Web 05/17/2010