Liquid-vapor equilibrium of the systems butylmethylimidazolium nitrate-CO2 and hydroxypropylmethylimidazolium nitrate-CO2 at high pressure: influence of water on the phase behavior.

Ionic liquids (IL) are receiving increasing attention due to their potential as "green" solvents, especially when used in combination with SC-CO 2. In this work liquid-vapor equilibria of binary mixtures of CO 2 with two imidazolium-based ionic liquids (IL) with a nitrate anion have been experimentally determined: butylmethylimidazolium nitrate (BMImNO 3 ) and hydroxypropylmethylimidazolium nitrate (HOPMImNO 3 ), using a Cailletet apparatus that operates according to the synthetic method. CO 2 concentrations from 5 up to 30 mol % were investigated. It was found that CO 2 is substantially less soluble in HOPMImNO 3 than in BMImNO 3 . Since these ILs are very hygroscopic, water easily can be a major contaminant, causing changes in the phase behavior. In case these Ils are to be used in practical applications, for instance, together with CO 2 as a medium in supercritical enzymatic reactions, it is very important to have quantitative information on how the water content will affect the phase behavior. This work presents the first systematic study on the influence of water on the solubility of carbon dioxide in hygroscopic ILs. It was observed that the presence of water reduces the absolute solubility of CO 2 . However, at fixed ratios of CO 2 /IL, the bubble point pressure remains almost unchanged with increasing water content. In order to explain the experimental results, the densities of aqueous mixtures of both ILs were determined experimentally and the excess molar volumes calculated.