Thermoresponsive Ionic Liquid with Different Cation–Anion Pairs as Draw Solutes in Forward Osmosis

We synthesized various phosphonium- and ammonium-based ionic liquids (ILs), using benzenesulfonate (BS) and 4-methylbenzenesulfonate (MBS) to establish the criteria for designing an ideal draw solute in a forward osmosis (FO) system. Additionally, the effects of monocationic, dicationic, and anionic species on FO performance were studied. Monocationic compounds ([P(4444)][BS], [P(4444)][MBS], [N(4444)][BS], and [N(4444)][MBS]) were obtained in one step via anion exchange. Dicationic compounds ([(P(4444))(2)][BS], [(P(4444))(2)][MBS], [(N(4444))(2)][BS], and [(N(4444))(2)][MBS]) were prepared in two steps via a Menshutkin SN(2) reaction and anion exchange. We also investigated the suitability of ILs as draw solutes for FO systems. The aqueous [P(4444)][BS], [N(4444)][BS], [N(4444)][MBS], and [(N(4444))(2)][BS] solutions did not exhibit thermoresponsive behavior. However, 20 wt% [P(4444)][MBS], [(P(4444))(2)][BS], [(P(4444))(2)][MBS], and [(N(4444))(2)][MBS] had critical temperatures of approximately 43, 33, 22, and 60 °C, respectively, enabling their recovery using temperature. An increase in IL hydrophobicity and bulkiness reduces its miscibility with water, demonstrating that it can be used to tune its thermoresponsive properties. Moreover, the FO performance of 20 wt% aqueous [(P(4444))(2)][MBS] solution was tested for water flux and found to be approximately 10.58 LMH with the active layer facing the draw solution mode and 9.40 LMH with the active layer facing the feed solution.