Distribution of Organic Solutes in Biphasic 1-n-Butyl-3-methylimidazolium Methyl Sulfate−Supercritical CO₂ System

Interphase distribution of organic nonelectrolytes can be important in applications of biphasic solvent systems composed of a room-temperature ionic liquid and supercritical carbon dioxide (scCO₂). Here, open tubular capillary-column supercritical fluid chromatography (SFC) with 1-n-butyl-3-methylimidazolium methyl sulfate ([bmim][MeSO₄]) as the stationary liquid and scCO₂ as the carrier fluid was employed to measure retention factors of organic solutes within 313−353 K and 8.5−23.2 MPa. Solute selection included 18 compounds of diverse volatilities and chemical functionalities. The retention factors were converted to infinite-dilution solute partition coefficients in the biphasic [bmim][MeSO₄]−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. Analysis of the relative retention data by regular solution theory resulted in approximate values of the solubility parameter of CO₂-expanded [bmim][MeSO₄].