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.