Ionic liquid-alkane association in dilute solutions

Enthalpies and entropies of transfer were measured by gas chromatography for dilute solutions of a homologous series of eight even n-alkanes (from octane to docosane) into six different ionic liquids (ILs) (namely, 1-butyl-3-methylimidazolium chloride, bromide, iodide, triflate and hexafluorophosphate; plus N-butylmethylpyridinium bis {(trifluoromethyl) sulfonyl}-imide) over the 80-150 degrees C temperature range, all ILs being in the liquid state. Over a narrow concentration range, the entropic change may be consistent with a solvophobic association model of n-alkanes in ILs. A very simple model is proposed to account for the thermodynamic data. This approach can be used to approximate interionic distances and possible dielectric constants for ILs. Although the model may have some use in dilute alkane-IL solutions, more sophisticated models, particularly for the enthalpic contributions, are desirable.