Modeling the melting temperature depression of ionic liquids caused by supercritical carbon dioxide

The melting temperature depression (MTD) that happens when an ionic liquid (IL) is pressurized by a soluble gas is modeled using phase equilibrium relations. MTD of hydrocarbons, polymers and lipids has been reported by several authors and the subject has been widely discussed in the literature. However, the phenomenon in ionic liquids (ILs) has received recent attention especially because some ILs that are potentially attractive for organic synthesis could not be used due to their relatively high melting temperatures. When solid is formed from the gas-pressurized liquid, three phase equilibrium in the binary system IL + gas is produced. In this paper the equation of state method is used to model the gas–liquid equilibrium and activity coefficients for representing the solid–gas equilibrium. Thus a consistent set of equations is formulated and solved by an optimization method using available experimental data of pressure, and temperature of IL + gas systems. The Peng–Robinson equation with the Wong–Sandler mixing rules showed to be appropriate for correlating the gas–liquid equilibrium data while the van Laar model for the activity coefficient was appropriate for correlating the solid–gas equilibrium data.