Determination of activity coefficients at infinite dilution of organic solutes in the ionic liquid, tributylmethylphosphonium methylsulphate by gas–liquid chromatography

Activity coefficients at infinite dilution, γi∞, have been determined for organic solutes in the phosphonium-based ionic liquid, tributylmethylphosphonium methylsulphate at T = 308.15 K, 318.15 K and 328.15 K. The experimental activity coefficient data presented are the first to be reported for this ionic liquid, through the use of the technique of steady-state gas–liquid chromatography. The organic solutes that were investigated were normal alkanes (pentane, hexane, heptane and octane), alkenes (1-hexene, 1-heptene and 1-octene), alkynes (1-hexyne, 1-heptyne and 1-octyne), cycloalkanes (cyclopentane, cyclohexane and cycloheptane) and benzene. The partial molar excess enthalpies at infinite dilution, HiE,∞, were also determined for the solutes from the temperature dependency of the γi∞ values.The γi∞ data obtained for tributylmethylphosphonium methylsulphate in this investigation have been compared to those obtained for other ionic liquids available in literature; most notably that of trihexyl(tetradecyl)-phosphonium tris(pentafluoroethyl) trifluorophosphate. This comparison allows for an assessment of the potential for the use of different ionic liquids as solvents in industrial solvent-enhanced separation processes such as extractive distillation.