Research reveals the chemical underpinnings of how benign water turns into harsh hydrogen peroxide

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A new study puts a fascinating and unexpected chemical genesis on even more solid foundations.

In 2019, researchers and colleagues at Stanford University made the surprising discovery that hydrogen peroxide — a caustic substance used to disinfect surfaces and whiten hair — spontaneously forms into microscopic droplets of benign plain water. Since then, researchers have sought to elucidate how the newly discovered reaction occurs, as well as explore potential applications, such as environmentally friendly cleaning methods.

The latest study revealed that when tiny droplets of water collide with a solid surface, a phenomenon known as contact electrification occurs. The electric charge jumps between the two substances, liquid and solid, producing unstable molecular fragments called reactive oxygen species. Pairs of these species known as hydroxyl radicals, which have the chemical formula OH, can combine to form hydrogen peroxide, H2a2In small quantities, they can be detected.

The new study further shows that this process occurs in humid environments when water touches soil particles as well as fine particles in the atmosphere. These additional results suggest that water can convert to small amounts of reactive oxygen species, such as hydrogen peroxide, anywhere that fine droplets naturally form, including fog, haze, and raindrops, strengthening the relevant 2020 study findings. .

“We have a real understanding now that we didn’t have before about why hydrogen peroxide formation occurs,” said study senior author Richard Zare, Marguerite Blake Wilbur Professor of Natural Sciences and Professor of Chemistry at Stanford College. Humanities and Sciences. “Furthermore, contact electrification that results in hydrogen peroxide appears to be a global phenomenon at solid-water interfaces.”

Zari led this work, in collaboration with researchers from two universities in China, Jiang’an University and Wuhan University, as well as the Chinese Academy of Sciences. The study was published on August 1 in Proceedings of the National Academy of Sciences (PNAS).

About the origins of hydrogen peroxide

For the study, the researchers built a glass device with microscopic channels in which water could be forcibly injected. The channels formed a solid, airtight boundary. The researchers quenched the water with a fluorescent dye that glows in the presence of hydrogen peroxide. One experiment showed that the harsh chemical was present in the vitreous microfluidic channel, but not in a large sample of water that also contained the dye. Additional experiments showed that hydrogen peroxide forms rapidly, within seconds, at the boundary between water and a solid.

To measure whether the extra oxygen atom in hydrogen peroxide (H2a2) resulting from a reaction with glass or inside water (H2O) by itself, the researchers treated the glass lining of some microfluidic channels. These treated channels contained a heavier isotope or version of oxygen, called oxygen-18 or 18Comparison of the post-reaction mixture of water and liquid hydrogen peroxide from treated and untreated channels showed a signal 18O in the former, indicates that the solid is the source of oxygen in hydroxyl radicals and ultimately in hydrogen peroxide.

The new findings could help settle some of the controversy that has swirled in the scientific community since Stanford University researchers first announced their new discovery of hydrogen peroxide in microscopic water droplets three years ago. Other studies have confirmed the main contributions to hydrogen peroxide production via chemical reactions with ozone gas, O3, and a process called cavitation, when steam bubbles arise in areas of low pressure within the accelerating fluid. Zare noted that these two processes also produce hydrogen peroxide in relatively larger amounts.

“All of these processes contribute to the production of hydrogen peroxide, but the current work confirms that this production is also intrinsic to the way micro-droplets are made and interact with solid surfaces through contact electrification,” Zare said.

Turning the tables on seasonal respiratory viruses

Zare explained that determining how and in what situations water can turn into reactive oxygen species, such as hydrogen peroxide, has a range of real-world applications and insights. Of greatest urgency is understanding the formation of hydroxyl radicals and hydrogen peroxide as a neglected contributor to the known seasonality of many viral respiratory diseases, including colds and influenza, and likely COVID-19 once the disease is finally fully endemic.

Viral respiratory infections are transmitted in the air in the form of watery droplets when patients cough, sneeze, sing, or even just talk. These infections tend to rise in the winter and subside in the summer, a trend due in part to people spending more time indoors and in close proximity that are highly contagious during the cold weather season. However, between work, school, and sleeping at night, people actually end up spending roughly the same amount of time indoors during hot weather months as well. Zare said the new study’s findings offer a possible explanation for why winter is associated with more cases of influenza: The main variable at work is humidity, and the amount of water in the air. In the summer, higher relative levels of indoor humidity – associated with higher humidity in the warm air outside – likely facilitate reactive oxygen species in droplets that have enough time to kill viruses. By contrast, in the winter—when indoor air is heated and its humidity drops—the droplets evaporate before the reactive oxygen species can act as a disinfectant.

“Contact electrification provides a chemical basis to explain in part why there is seasonality for viral respiratory diseases,” Zare said. Accordingly, Zare added, future research should investigate any links between indoor humidity levels in buildings and the presence and spread of infection. If the links are further demonstrated, adding humidifiers to HVAC systems could reduce disease transmission.

“A new approach to disinfecting surfaces is just one of the great practical results of this work that includes the basic chemistry of water in the environment,” Zare said. “It just goes to show that we think we know a lot about water, one of the most common substances, but then we humble ourselves.”

Zary is also a member of Stanford BioX, the Heart and Vascular Institute, the Stanford Cancer Institute, Stanford Chem-H, the Stanford Woods Institute for the Environment, and the Wu Tsai Neuroscience Institute.


Chemists discovered that tiny water droplets spontaneously produce hydrogen peroxide


more information:
Bolei Chen et al, Water-steel contact electrification causes hydrogen peroxide to be produced from hydroxyl radical recombination in the sprayed micro-droplets, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2209056119

Presented by Stanford University

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