Solute partitioning between the ionic liquid 1-n-butyl-3-methylimidazolium tetrafluoroborate and supercritical CO2 from capillary-column chromatography.

Open-tubular capillary-column supercritical fluid chromatography (SFC) with the room-temperature ionic liquid (RTIL) 1-n-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) as the stationary liquid and supercritical carbon dioxide (scCO2) as the mobile phase was employed to measure solute retention factors within 313-353 K and 8.1-23.2 MPa. The selection of solutes included 18 compounds of diverse volatilities and chemical functionalities. The retention factors were converted to infinite-dilution solute partition coefficients in the biphasic [bmim][BF4]-scCO2 system. At a constant temperature, an increase in scCO2 density produced distinct shifts in relative retention (=separation factor), thus providing some pressure-tunable selectivity. At a particular temperature and density of CO2, solute partition coefficients can be correlated in terms of linear solvation energy relationships. This important finding indicates a future possibility to estimate the partitioning data in RTIL-scCO2 systems using limited experimental information and the molecular descriptors available for a large variety of prospective solutes. Analysis of the relative retention data by regular solution theory resulted in approximate values of the solubility parameter of CO2-expanded [bmim][BF4].