Double Salt Ionic Liquids Containing the Trihexyl(tetradecyl) phosphonium Cation: The Ability to Tune the Solubility of Aromatics, Ethers, and Lipophilic Compounds

The chemical and physical properties of an ionic liquid (IL) can be tuned with the addition of a second IL, forming a Double Salt Ionic Liquid (DSIL). Here, the solubilities of lipophilic solutes with long alkyl chains (e.g., heptane, methyl palmitate) were tuned in the DSIL [C(4)mim](x)[P-66614]((1-x))[NTf2] ([C(4)mim](+) = 1-butyl-3-methylimidazolium, [P-66614](+) = trihexyl(tetradecyl) phosphonium, [NTf2](-) = bis(trifluoromethylsulfonyl) imide) by increasing the [P-66614](+) concentration. Additionally, density and viscosity of the DSILs were also modified with the [P-66614](+) concentration. The solubility of solvents totally miscible with [P-66614][NTf2], such as benzene and toluene, could only be tuned in the DSILs when [P-66614](+)/[NTf2](-) <= 0.33, since at higher ratios these solutes were totally miscible with the DSILs. The interactions between the ions account for the different solvent behaviors in the DSILs. Using H-1 NMR, F-19 NMR, and FT-IR spectroscopy, a preferential interaction was observed between the [NTf2](-) anion with [C(4)mim](+) compared to that with [P-66614](+). Also, interactions between [C(4)mim](+) and [P-66614](+) through alkyl chain aggregations were demonstrated.