Ternary Phase Behavior of Ionic Liquid (IL)−Organic−CO₂ Systems

Recently, we have shown that gaseous or supercritical carbon dioxide can be used to induce a liquid−liquid phase split for homogeneous mixtures that consists of methanol−1-butyl-3-methylimidazolium hexafluorophosphate and aqueous/ionic liquid mixtures. [Scurto et al., J. Am. Chem. Soc. 2002, 124, 10276; Chem. Commun. 2003, 572.] Here, the factors that control the vapor−liquid−liquid equilibrium in ionic liquid−organic−carbon dioxide ternary systems at 40 °C are discussed. The lower-critical endpoint pressure (LCEP) locus, i.e., the pressure at which the liquid−liquid phase split occurs, is strongly dependent on the choice of organic, the type of ionic liquid (IL), and the concentration of IL in the organic. On the other hand, the K-point pressure, i.e., the pressure at which one of the liquid phases becomes identical with the CO₂-rich phase, is dependent on the choice of organic only and it was determined to be same as the binary mixture critical point for the given organic−CO₂ system. The compositions of the various phases were measured and they were determined to be strongly dependent on the pressure. The pressure dependence of selectivity and distribution coefficient for the IL between the two liquid phases was measured and observed to improve dramatically with increasing pressure. Specifically, above the LCEP, an increase in pressure resulted in a decrease in the concentration of IL in the organic-rich phase, and above the K-point, the supercritical fluid phase was determined to be free of IL.