Spectroscopically probing microscopic solvent properties of room-temperature ionic liquids with the addition of carbon dioxide

Room-temperature ionic liquids (RTILs) provide an alternative for elimination of solvent emissions to the atmosphere for many reactions, but the subsequent separation of the products by conventional methods can be a challenge. However, the use of supercritical carbon dioxide (scCO(2)) as an extractant offers potential for a novel class of environmentally benign media for chemical reaction and downstream separation. We have investigated the solvent properties of mixtures of 1-butyl-3-methyl imidazolium hexafluorophosphate ([bmim]-[PF6]) and CO2 as functions of temperature (35-50 degreesC) and CO2 pressure (0-230 bar). We report the Kamlet-Taft dipolarity/polarizability parameter, volume expansion, and microviscosity. The results are consistent with a picture of local enhancement of RTIL composition around it chromophore, maintaining solvent strength even at fairly high loadings of CO2, whereas the microviscosity in the vicinity of the solute is dramatically reduced, leading to enhanced mass transport and facilitated separation.