Development of thermoresponsive star oligomers with a glycerol backbone as the draw solute in forward osmosis process

Recently, forward osmosis (FO) is attracting research attention once again. In a FO process, it is important to develop a draw solution (DS) with a high osmotic pressure and low solute leakage. In this study, thermoresponsive star-shaped oligomers with a glycerol backbone were developed as new draw solutes for FO. A series of glycerol-oligo(ethylene oxide)-block-oligo(butylene oxide) (GEB) oligomers were systematically designed and synthesized. The average degrees of polymerization of ethylene oxide (EO; m) and butylene oxide (BO; n) units of GE_mB_n were varied to control the hydrophilic/hydrophobic balance of the molecule. Aqueous solutions of GEBs were evaluated in terms of their osmotic pressures, phase diagrams, and viscosities. Most of them showed a lower critical solution (LCST)-type phase separation at temperatures below 60 °C. The osmotic pressure of 68 wt% GE₇B₃ (concentration of the dense phase after phase separation at 60 °C) was 74 bar, about 2.6 times higher than that of seawater. Moreover, the leakage of GE₇B₃ was much lower than that of conventional draw solutes. The osmotic pressure of the dilute phase of a GE₇B₃ solution at 60 °C was less than 2 bar, implying reduced energy consumption during post-processing by low-pressure reverse osmosis to collect pure water.