| Summary: | The applicability of ionic liquids (ILs) as the draw solute in a forward osmosis (FO) system was investigated through a study on the effect of the structural change of the anion on the FO performance. This study evaluated ILs composed of tetrabutylphosphonium cation ([P<sub>4444</sub>]<sup>+</sup>) and benzenesulfonate anion ([BS]<sup>−</sup>), para-position alkyl-substituted benzenesulfonate anions (<i>p</i>-methylbenzenesulfonate ([MBS]<sup>−</sup>) and <i>p</i>-ethylbenzenesulfonate ([EBS<sup>−</sup>]), and methanesulfonate anion ([MS]<sup>−</sup>). The analysis of the thermo-responsive properties suggested that the [P<sub>4444</sub>][MBS] and [P<sub>4444</sub>][EBS] ILs have lower critical solution temperatures (LCSTs), which play a beneficial role in terms of the reusability of the draw solute from the diluted draw solutions after the water permeation process. At 20 wt% of an aqueous solution, the LCSTs of [P<sub>4444</sub>][MBS] and [P<sub>4444</sub>][EBS] were approximately 36 °C and 25 °C, respectively. The water flux and reverse solute flux of the [P<sub>4444</sub>][MBS] aqueous solution with higher osmolality than [P<sub>4444</sub>][EBS] were 7.36 LMH and 5.89 gMH in the active-layer facing the draw solution (AL-DS) mode at osmotic pressure of 25 atm (20 wt% solution), respectively. These results indicate that the [P<sub>4444</sub>]<sup>+</sup>-based ionic structured materials with LCST are practically advantageous for application as draw solutes.
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