Most absorbent materials lose their ability to retain water as temperatures rise. Even materials designed to absorb moisture, like silica gel packs, lose their sponge-like properties as the environment heats up.
However, one material, polyethylene glycol (PEG), unusually withstands the drying effects of heat. MIT engineers discovered that PEG, a hydrogel used in cosmetics, industrial coatings, and pharmaceutical capsules, can absorb moisture from the atmosphere even at increasing temperatures. PEG doubles its water absorption as temperatures rise from 25 to 50 degrees Celsius. Its resilience comes from a heat-triggered transformation: the hydrogel's microstructure changes from a crystal to a less-organized "amorphous" phase, enhancing its water-capturing ability.
Researchers developed a model based on PEG's unique properties to create other heat-resistant materials that absorb water. These materials could be used in devices for harvesting humidity from the air and obtaining drinking water, especially in arid desert areas. The materials could also be used in heat pumps and air conditioners for more efficient temperature and humidity regulation.
Lenan Zhang, an MIT researcher, says this material could be a key component of passive climate control systems. The team discovered PEG's unusual properties while evaluating a range of similar hydrogels for their water-harvesting abilities. Most materials lose their ability to capture moisture from the air as temperatures rise. But PEG has an inverse relationship, becoming denser and absorbing more water as temperatures increase from 25 to 50 degrees Celsius.
Researchers found that PEG's unusual performance is due to phase transformation. The hydrogel's crystalline microstructure breaks down and changes to an amorphous phase, which provides more opportunities for the material's polymers to capture rapidly moving water molecules.
The team developed a theory to predict how hydrogels absorb water, which can explain PEG's unusual behavior. Discovering PEG's unique properties was mostly accidental. The material's melting temperature happens to be in the range where water is a liquid, allowing them to capture PEG's phase transformation and resulting super-absorption behavior. Other hydrogels have melting temperatures outside this range, but researchers suspect these materials could have similar phase transformations once they reach their melting temperatures.
Other polymers could, in theory, exhibit the same behavior if their melting points can be designed within a selected temperature range, explains Shaoting Lin, a team member. They now intend to use the developed theory as a model to design materials specifically for capturing water at higher temperatures. The goal is to create a material that can absorb a large amount of water at low humidity and high temperatures. This could be used for atmospheric water harvesting, bringing drinking water to hot and arid environments.
The research was partly supported by the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy. This discovery shows promise in developing innovative technologies and materials to address energy and water challenges, with the potential to improve lives in drought-stricken and water-scarce regions.
Zhang and his colleagues detail their work in a study published in Advanced Materials. MIT co-authors include Xinyue Liu, Bachir El Fil, Carlos Diaz-Marin, Yang Zhong, Xiangyu Li, and Evelyn Wang, along with Shaoting Lin from Michigan State University.
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