(384d) Determination of Infinite Dilution Activity Coefficients in Ionic Liquids by Capillary Gas Chromatography

Infinite dilution activity coefficients (γ^∞) provide information regarding the solute-solvent interactions in the absence of solute-solute interactions and are used, in particular, for the selection of solvents for extraction and extractive distillation. Several methods developed for the measurement of γ^∞ include ebulliometry method, static method, gas-stripping method (or dilutor method), liquid-liquid chromatography, gas-liquid chromatography (GLC) and headspace GLC, of which GLC is a rapid and exact method for experimental determination of γ^∞. Ionic liquids (ILs) are considered to be a promising substitute for traditional solvents. The properties of extremely low vapor pressure even at high temperatures, high viscosity, high thermal stability and high polarity make ILs suitable as stationary phases for gas chromatography. Infinite dilution activity coefficients of organic solvents in a great deal of ILs have been determined by packed column gas chromatography (PGC) over the last decade.

Capillary gas chromatography (CGC) has several advantages over PGC for γ^∞ measurement such as high column efficiency and lower surface-to-volume ratio. Nonetheless, CGC has rarely been applied to the determination of γ^∞. To our knowledge there have been no reports concerning determination of infinite dilution activity coefficients in ILs by CGC. This is probably due to the large errors raised by the low carrier gas flow rate and the small amount of stationary phase.

In this work, we rationally revised the calculation equation of γ^∞ for CGC, which, as a consequence, eliminated the corresponding errors mentioned above. The respective γ^∞ values of organic compounds including alkanes, alkenes, alkynes, cycloalkanes, cycloalkenes, aromatic hydrocarbons, alcohols, aldehydes, ethers, ketones, esters, chloromethanes and nitroalkanes in two ILs 1-butyl-3-methylimidazolium hexafluorophosphate and 1,3-bis(cyanopropyl) imidazolium bis(trifluoromethylsulfonyl)-amide at 313.15-333.15K were successfully determined by CGC using the new equation. The second IL was newly synthesized by our group. Our γ^∞ values of the first IL agreed well with literatures, the results further proved the feasibility of the revised equation for CGC to investigate infinite dilution activity coefficients. Using the γ^∞ values determined by CGC with ionic liquid stationary phases, selectivities of selected compounds could be easily calculated.