Room Temperature Ionic liquids (RTILs) which contain organic cations and anions, possess unique properties; such as, non-volatility, non-flammability, superior ionic and thermal conductivity. These properties make them potential solvent replacements for various Volatile Organic Compounds (VOCs) as separation agents. However, application of RTILs in separations is hindered by the lack of scientific data on physical and transport properties of these liquids. In this study, we build on the previous success of using a Regular Solution Theory (RST) approach to model gas solubilities. We will, instead, use surface free energies (from surface tension studies) to calculate RTIL solubility parameters. In addition, we will present ab initio and quantum chemistry calculations of solubility parameters for non-existing RTILs and extension of RST to these liquids. Finally we will use the results to test the viability of commercially available Phosphonium- and newly synthesized Ammonium-based RTILs for the separation of CO2 and other industrial gases.