Phase Behavior of Imidazolium-Based Ionic Liquid + Phosphonium-Based Ionic Liquid Mixtures with a Common Anion: Effects of the Alkyl-Chain Length of Cation

The phase behavior of imidazolium-based ionic liquid (IL) + phosphonium-based IL binary mixtures with a common anion has been investigated in the temperature range 253–453 K. Bis­(trifluoromethanesulfonyl)­amide ([NTf₂]⁻) and tetrafluoroborate ([BF₄]⁻) were selected as common anions. The imidazolium-based IL + phosphonium-based IL mixtures with a common anion showed an upper critical solution temperature (UCST). The phase separation temperatures of the mixtures decreased with increasing alkyl-chain length of the imidazolium cation. In contrast, the phase separation temperatures of the mixtures increased as the alkyl-chain length of the phosphonium cation increased. The phase separation temperatures of the imidazolium-based IL + phosphonium-based IL mixtures with [BF₄]⁻ as a common anion were remarkably higher than those with [NTf₂]<sup>−