Effect of Salt Type and Alkyl Chain Length on the Binodal Curve of an Aqueous Two-Phase System Composed of Imidazolium Ionic Liquids

The binodal data for an aqueous two-phase system (ATPS) containing 1-alkyl-3-methylimidazolium tetrafluoroborate ([C_<i>n</i>mim]­BF₄, <i>n</i> = 2, 3, 4) or 1-alkyl-3-methylimidazolium chloride ([C_<i>n</i>mim]­Cl], <i>n</i> = 2, 4, 6, 8, 10, 12) and inorganic salts (K₂HPO₄, NaH₂PO₄, (NH₄)₂SO₄, and NaCl) or organic salts (sodium citrate, sodium tartrate, and sodium acetate) were widely determined at <i>T</i> = 298.15 K. The various binodal curves could be fitted and summarized as two kinds of empirical nonlinear equations with the lowest standard deviation (SD). The effects of the salt type and cation alkyl chain length of the ionic liquid (IL) on ATPS binodal curves were further discussed. It was displayed that the salting-out ability of different salts was a major factor in these ATPS formations. For [C_<i>n</i>mim]­BF₄-based ATPS, the salting-out ability of various salts decreased in the following order: (NH₄)₂SO₄ > Na₃C₆H₅O₇ > Na₂C₄H₄O₆ > NaH₂PO₄ > NaC₂H₃O₂ > NaCl. For the [C_<i>n</i>mim]Cl aqueous solution, only K₂HPO₄ or (NH₄)₂SO₄ could cause the phase separation and formed ATPS, and the salting-out ability of K₂HPO₄ was stronger than the effect of (NH₄)₂SO₄. The phase separation abilities of the researched [C_<i>n</i>mim]­BF₄ in the corresponding ATPS increased with the increase of cation alkyl chain length, while [C₈mim]Cl had the best phase separation ability of the researched [C_<i>n</i>mim]­Cl-based ATPS