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Pure Appl. Chem., 2005, Vol. 77, No. 3, pp. 543-557

Solubilities and thermophysical properties of ionic liquids

Urszula Domańska

Physical Chemistry Division, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland

This report presents the systematic study on the solubilities of 1-alkyl-3-methylimidazolium hexafluorophosphate [e, or bmim][PF6], 1-alkyl-3-methylimidazolium methylsulfate [almim][CH3SO4], 1-hexyloxymethyl-3-methylimidazolium ionic liquids (ILs) [C6H13OCH2mim][BF4], or [C6H13OCH2mim][(CF3SO2)2N] in aliphatic hydrocarbons (heptane, octane), cyclohydrocarbons (cyclopentane, cyclohexane) and aromatic hydrocarbons (benzene, toluene, ethylbenzene, o-xylene, m-xylene, p-xylene), and in alcohols (methanol, ethanol, propan-1-ol, propan-2-ol, butan-1-ol, butan-2-ol, tert-butyl alcohol, and 3-methylbutan-1-ol) as well as of 1-alkyl-3-methyl-imidazolium chloride [C4, C10, or C12mim][Cl] in alcohols. The solubilities have been measured by a dynamic method from 290 K to the melting point of IL or to the boiling point of the solvent. The solubility of [emim][PF6] and [bmim][PF6] in aromatic hydrocarbons decreases with an increase of the molecular weight of the solvent. The difference on the solubility in o-, m-, and p-xylene is not significant. The solubility of [emim][PF6] in alcohols decreases with an increase of the molecular weight of the solvent and is higher in secondary alcohols than in primary alcohols. In every case, with the exception of methanol, the mutual liquid–liquid equilibrium was observed. The shape of the equilibrium curve is similar for [emim][PF6] in every alcohol. The observations of upper critical solution temperatures were limited by the boiling temperature of the solvent. For example, for [emim][PF6], solubilities in methanol and ethanol are higher than that in aromatic hydrocarbons. The miscibility gap for C3 alcohols is higher than that in benzene, but comparable with the solubility in toluene; solubility in 3‑methyl-butan-1-ol is very similar to ethylbenzene and o-, m-, and p-xylene.
The data were correlated by means of the UNIQUAC and modified nonrandom two-liquid (NRTL) equations utilizing parameters derived from the solid–liquid equilibrium (SLE). The root-mean-square deviations of the solubility temperatures for all calculated data depend on the particular system and the equation used.
The solubilities of [C4, C10, or C12mim][Cl] and alkyl-(2-hydroxyethyl)-dimethyl-ammonium ILs in octan-1-ol and water have been measured and used to calculate the octan-1-ol/water partition coefficients as a function of temperature and alkyl substituent. Experimental partition coefficients (log P) are negative for imidazoles, 1,3-dialkylimidazolium chlorides, and investigated ammonium salts. Good knowledge of ILs permits the elimination of volatile organic compounds and makes chemistry cleaner and greener.