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

Abstract

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 [C4mim]x[P66614](1-x)[NTf2] ([C4mim]+ = 1-butyl-3-methylimidazolium, [P66614]+ = trihexyl(tetradecyl)phosphonium, [NTf2]- = bis(trifluoromethylsulfonyl)imide) by increasing the [P66614]+ concentration. Additionally, density and viscosity of the DSILs were also modified with the [P66614]+ concentration. The solubility of solvents totally miscible with [P66614][NTf2], such as benzene and toluene, could only be tuned in the DSILs when [P66614]+/[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 1H NMR, 19F NMR, and FT-IR spectroscopy, a preferential interaction was observed between the [NTf2]- anion with [C4mim]+ compared to that with [P66614]+. Also, interactions between [C4mim]+ and [P66614]+ through alkyl chain aggregations were demonstrated.