{"id":90731,"date":"2023-04-22T02:43:00","date_gmt":"2023-04-22T00:43:00","guid":{"rendered":"https:\/\/www.sonnenseite.com\/?p=90731"},"modified":"2023-04-20T08:44:07","modified_gmt":"2023-04-20T06:44:07","slug":"even-as-temperatures-rise-this-hydrogel-material-keeps-absorbing-moisture","status":"publish","type":"post","link":"https:\/\/www.sonnenseite.com\/en\/science\/even-as-temperatures-rise-this-hydrogel-material-keeps-absorbing-moisture\/","title":{"rendered":"Even as temperatures rise, this hydrogel material keeps absorbing moisture"},"content":{"rendered":"\n

MIT engineers identified an unusually absorbent material that could be used for passive cooling or water harvesting in warm climates.<\/p>\n\n\n\n

The vast majority of absorbent materials will lose their ability to retain water as temperatures rise. This is why our skin starts to sweat and why plants dry out in the heat. Even materials that are designed to soak up moisture, such as the silica gel packs in consumer packaging, will lose their sponge-like properties as their environment heats up.<\/p>\n\n\n\n

But one material appears to uniquely resist heat\u2019s drying effects. MIT engineers have now found that polyethylene glycol (PEG) \u2014 a hydrogel commonly used in cosmetic creams, industrial coatings, and pharmaceutical capsules \u2014 can absorb moisture from the atmosphere even as temperatures climb.<\/p>\n\n\n\n

The material doubles its water absorption as temperatures climb from 25 to 50 degrees Celsius (77 to 122 degrees Fahrenheit), the team reports.<\/p>\n\n\n\n

PEG\u2019s resilience stems from a heat-triggering transformation. As its surroundings heat up, the hydrogel\u2019s microstructure morphs from a crystal to a less organized \u201camorphous\u201d phase, which enhances the material\u2019s ability to capture water.<\/p>\n\n\n\n

Based on PEG\u2019s unique properties, the team developed a model that can be used to engineer other heat-resistant, water-absorbing materials. The group envisions such materials could one day be made into devices that harvest moisture from the air for drinking water, particularly in arid desert regions. The materials could also be incorporated into heat pumps and air conditioners to more efficiently regulate temperature and humidity.<\/p>\n\n\n\n

\u201cA huge amount of energy consumption in buildings is used for thermal regulation,\u201d says Lenan Zhang, a research scientist in MIT\u2019s Department of Mechanical Engineering. \u201cThis material could be a key component of passive climate-control systems.\u201d<\/p>\n\n\n\n

Zhang and his colleagues detail their work in a study<\/a> appearing today in Advanced Materials<\/em>. MIT co-authors include Xinyue Liu, Bachir El Fil, Carlos Diaz-Marin, Yang Zhong, Xiangyu Li, and Evelyn Wang, along with Shaoting Lin of Michigan State University.<\/p>\n\n\n\n

Against intuition<\/h5>\n\n\n\n

Evelyn Wang\u2019s group in MIT\u2019s Device Research Lab aims to address energy and water challenges through the design of new materials and devices that sustainably manage water and heat. The team discovered PEG\u2019s unusual properties as they were assessing a slew of similar hydrogels for their water-harvesting abilities.<\/p>\n\n\n\n

\u201cWe were looking for a high-performance material that could capture water for different applications,\u201d Zhang says. \u201cHydrogels are a perfect candidate, because they are mostly made of water and a polymer network. They can simultaneously expand as they absorb water, making them ideal for regulating humidity and water vapor.\u201d<\/p>\n\n\n\n

The team analyzed a variety of hydrogels, including PEG, by placing each material on a scale that was set within a climate-controlled chamber. A material became heavier as it absorbed more moisture. By recording a material\u2019s changing weight, the researchers could track its ability to absorb moisture as they tuned the chamber\u2019s temperature and humidity.<\/p>\n\n\n\n

What they observed was typical of most materials: as the temperature increased, the hyrogels\u2019 ability to capture moisture from the air decreased. The reason for this temperature-dependence is well-understood: With heat comes motion, and at higher temperatures, water molecules move faster and are therefore more difficult to contain in most materials.<\/p>\n\n\n\n

\u201cOur intuition tells us that at higher temperatures, materials tend to lose their ability to capture water,\u201d says co-author Xinyue Liu. \u201cSo, we were very surprised by PEG because it has this inverse relationship.\u201d<\/p>\n\n\n\n

In fact, they found that PEG grew heavier and continued to absorb water as the researchers raised the chamber\u2019s temperature from 25 to 50 degrees Celsius.<\/p>\n\n\n\n

\u201cAt first, we thought we had measured some errors, and thought this could not be possible,\u201d Liu says. \u201cAfter we double-checked everything was correct in the experiment, we realized this was really happening, and this is the only known material that shows increasing water absorbing ability with higher temperature.\u201d<\/p>\n\n\n\n

A lucky catch<\/h5>\n\n\n\n

The group zeroed in on PEG to try and identify the reason for its unusual, heat-resilient performance. They found that the material has a natural melting point at around 50 degrees Celsius, meaning that the hydrogel\u2019s normally crystal-like microstructure completely breaks down and transforms into an amorphous phase. Zhang says that this melted, amorphous phase provides more opportunity for polymers in the material to grab hold of any fast-moving water molecules.<\/p>\n\n\n\n

\u201cIn the crystal phase, there might be only a few sites on a polymer available to attract water and bind,\u201d Zhang says. \u201cBut in the amorphous phase, you might have many more sites available. So, the overall performance can increase with increased temperature.\u201d<\/p>\n\n\n\n

The team then developed a theory to predict how hydrogels absorb water, and showed that the theory could also explain PEG\u2019s unusual behavior if the researchers added a \u201cmissing term\u201d to the theory. That missing term was the effect of phase transformation. They found that when they included this effect, the theory could predict PEG\u2019s behavior, along with that of other temperature-limiting hydrogels.<\/p>\n\n\n\n

The discovery of PEG\u2019s unique properties was in large part by chance. The material\u2019s melting temperature just happens to be within the range where water is a liquid, enabling them to catch PEG\u2019s phase transformation and its resulting super-soaking behavior. The other hydrogels happen to have melting temperatures that fall outside this range. But the researchers suspect that these materials are also capable of similar phase transformations once they hit their melting temperatures.<\/p>\n\n\n\n

\u201cOther polymers could in theory exhibit this same behavior, if we can engineer their melting points within a selected temperature range,\u201d says team member Shaoting Lin.<\/p>\n\n\n\n

Now that the group has worked out a theory, they plan to use it as a blueprint to design materials specifically for capturing water at higher temperatures.<\/p>\n\n\n\n

\u201cWe want to customize our design to make sure a material can absorb a relatively high amount of water, at low humidity and high temperatures,\u201d Liu says. \u201cThen it could be used for atmospheric water harvesting, to bring people potable water in hot, arid environments.\u201d<\/p>\n\n\n\n

This research was supported, in part, by U.S. Department of Energy\u2019s Office of Energy Efficiency and Renewable Energy.<\/p>\n\n\n\n