Episode 12: Hydrogel adsorbs more water at elevated temperature

While thermodynamics suggests that water sorption is more favorable at a low temperature, MRS Bulletin podcaster Laura Leay interviews post-doctoral researcher Xinyue Liu from the Massachusetts Institute of Technology (MIT) who reports a hydrogel that can adsorb more water at elevated temperatures. Liu and the research team from MIT and the University of Michigan were searching for a way to harvest water from the air without using a lot of energy. They want to tackle the problem of water scarcity and find a way of generating water sustainably. To do so, they tested many different sorbents. Most sorbents, such as zeolite and silica gel, have a structure that does not change much when it has adsorbed water; however, the polyethylene glycol – or PEG – hydrogel that the team synthesized is different. While it is semi-crystalline at 25°C, it becomes amorphous at 50°C. This structural change means that more adsorption sites are available at the higher temperature. As water is absorbed, it caused the hydrogel to swell, opening up further adsorption sites. The PEG hydrogel monomers are star-shaped, forming a network where the molecular weight can be precisely controlled. The shape of the monomer leads to very homogeneous structures, facilitating crystallization. The PEG hydrogel exhibited a water uptake of 0.050 grams per gram of polymer at 50°C and 50% relative humidity, with half this water uptake at 25°C and the same humidity. This work was published in a recent issue of Advanced Materials.