Effect of branch structure of thermoresponsive oligomers on draw solution performance in forward osmosis process

Abstract A series of three types of oligomers—oligo(butylene oxide)m-block-oligo(ethylene oxide)n-block-oligo(butylene oxide)m oligomers (EBs), glycerol-oligo(ethylene oxide)m-block-(butylene oxide)n oligomers (GEBs), and pentaerythritol-oligo(ethylene oxide)m-block-(butylene oxide)n oligomers (PEBs)—having different branch structures were synthesized. EBs, GEBs, and PEBs had linear, three-branch, and four-branch structures, respectively. These oligomers were tested as draw solutes (DSs) for the forward osmosis (FO) process in order to reveal the effect of their branch structures on the FO performance and DS leakage from the FO membrane. The aqueous solutions of EBs, GEBs, and PEBs exhibited a typical lower critical solution temperature-type phase separation. The FO water fluxes in the case of EB, GEB, and PEB were 4.1–4.8 L m−2 h−1 (LMH) at an osmotic pressure of 28 bar, while the reverse solute fluxes (Js) were 0.79–1.83 gMH at DS concentrations corresponding to an osmotic pressure of 28 bar. Js decreased with an increase in the number of branches in the DS molecule, which improved the FO performance index. The results indicate the importance of introducing a branch structure into DS molecules to design useful DSs.