Dual-functional ionic liquids exhibiting upper critical solution temperature behavior and antibacterial activity as draw solutes for forward osmosis

• Draw solutes with facile recovery and antibacterial functionality were developed. • UCST-type ionic liquids (ILs) were synthesized and evaluated as draw solutes. • [N 1118 ][BF 4 ] exhibited highest water flux, conductivity, and antibacterial property. • Antibacterial property may reduce biofouling and improve forward osmosis (FO). • Thermoresponsive phase behavior enabled facile recovery in the FO process. Eight ionic liquids (ILs) were systematically synthesized by pairing alkylammonium-based cations—dodecyltrimethylammonium ([N 11112 ] + ) and octyltrimethylammonium ([N 1118 ] + )—with fluorinated anions, including trifluoromethanesulfonate ([OTf] ‒ ), tetrafluoroborate ([BF 4 ] ‒ ), bis(trifluoromethane)sulfonimide ([NTf 2 ] ‒ ), and hexafluorophosphate ([PF 6 ] ‒ ). The resulting ILs—[N 11112 ][OTf], [N 11112 ][BF 4 ], [N 11112 ][NTf 2 ], [N 11112 ][PF 6 ], [N 1118 ][OTf], [N 1118 ][BF 4 ], [N 1118 ][NTf 2 ], and [N 1118 ][PF 6 ]—were investigated for their phase behavior in aqueous solutions. Among them, [N 11112 ][OTf], [N 11112 ][BF 4 ], and [N 1118 ][BF 4 ] displayed upper critical solution temperature (UCST) behavior with phase transition temperatures of 35, 45, and 39°C, respectively, at 20 wt%. Their conductivities were 9.22, 19.01, and 32.13 mS/cm. Forward osmosis (FO) performance was tested in both active layer-facing feed solution (AL-FS) and active layer-facing draw solution (AL-DS) modes using deionized (DI) water and brackish water (2,000 ppm, NaCl). With DI water, water fluxes reached 18.75, 27.87, and 34.59 LMH in AL-FS mode and increased to 20.34, 36.09, and 37.53 LMH in AL-DS mode. With brackish water, fluxes were 12.39, 22.80, and 31.56 LMH in AL-FS mode, and 15.39, 29.94, and 33.15 LMH in AL-DS mode. Antibacterial testing showed >99% inhibition of Escherichia coli and Staphylococcus aureus by [N 1118 ][BF 4 ]. These results highlight the potential of UCST-type ILs as multifunctional draw solutes that combine thermoresponsive separation, high osmotic pressure, and antibacterial activity—key features for advancing high-performance FO systems.