Lower Critical Solution Temperature (LCST) Phase Separation of Glycol Ethers for Forward Osmotic Control

Abstract

Lower critical solution temperature (LCST) phase transition of glycol ether (GE)/water mixtures induces abrupt change of osmotic pressure driven by mild temperature change. The temperature-controlled osmotic change was applied for the forward osmotic (FO) desalination. Among evaluated three GEs, di(ethylene glycol) n-hexyl ether (DEH) was selected as a potential FO draw solute. A DEH/water mixture with a high osmotic pressure could draw fresh water from a high-salt feed solution such as seawater through a semipermeable membrane at around 10 °C. The water-drawn DEH/water mixture was phase-separated into a water-rich phase and a DEH-rich phase at around 30 °C. The water-rich phase with a much reduced osmotic pressure released water into a low-salt solution, and the DEH-rich phase was recovered into the initial DEH/water mixture. The phase separation behaviour, the residual GEs concentration at the water-rich phase, the osmotic pressure of the DEH/water mixture, and the osmotic flux between DEH/water mixture and salt solutions were carefully analysed for FO desalination. The liquid-liquid phase separation of the GE/water mixture driven by the mild temperature change between 10 °C and 30 °C is very attractive for the development of an ideal draw solute for future practical FO desalination.