Molecular dynamics simulation of thermodynamic properties for CO2/ionic liquid systems

Supercritical carbon dioxide and ionic liquids(ILs) are two kinds of green solvents. Supercritical carbon dioxide can be dissolved in ionic liquids, but ionic liquids cannot be dissolved in supercritical carbon dioxide. CO2/IL binary systems, therefore, have many advantages for supercritical carbon dioxide and ILs such as decreasing the viscosity of ILs and easy phase separations. It is a new kind of coupled green solvent. Its physical chemical properties are very important for the design of reaction and separation processes. The CO2/IL binary systems (CO2/[bmim][PF6] and CO2/[bmim][NO3]) were selected as model compounds and the thermodynamic properties of these systems were simulated by molecular dynamics simulation method with available molecular force field parameters and ensembles. Results show that the ILs expanded only 15% at a CO2 molar fraction of 0.5. The diffusion coefficients of CO2/ILs are much smaller than those of CO2/methanol and CO2/ethanol systems. With the content of CO2 increasing, the diffusion coefficients of the ILs increased while their viscosities decreased significantly. These results indicate that CO2 can effectively overcome the shortcomings of ILs that have poor diffusion coefficients and high viscosities. We conclude that CO2 can improve the transport properties of ionic liquid solvent systems and enhanced the reaction and separation processes in these systems. © Editorial office of Acta Physico-Chimica Sinica.