Scientists find that water can turn into hydrogen peroxide when it condenses on cold surfaces

A Stanford research team that recently discovered an unexpected new chemical behavior of water when tiny droplets are formed from water vapor extended the results to natural, everyday water condensation.

By Adam Hadhazy

In its bulk liquid form, whether in a bathtub or an ocean, water is a relatively benign substance with little chemical activity. But on the scale of tiny droplets, water can become surprisingly reactive, the Stanford researchers found.

In micro-drops of water, just millionths of a meter wide, a portion of H.₂The O molecules present can convert to a close chemical cousin, hydrogen peroxide, H.₂OR₂, a harsh chemical commonly used as a disinfectant and hair bleaching agent.

Stanford scientists first reported this unexpected behavior in forcefully sprayed micro-drops of water last year. Now, in a new study, the research team has demonstrated the same transformation as Jekyll and Hyde when micro-drops simply condense from the air on cold surfaces. The new results suggest that the transformation of hydrogen peroxide in water is a general phenomenon, occurring in mists, mists, raindrops and wherever micro-drops naturally form.

The surprising discovery could lead to greener methods of disinfecting surfaces or promoting chemical reactions. “We have shown that the process of hydrogen peroxide formation in water droplets is a widespread and surprising phenomenon that is happening right under our noses,” said senior study author Richard Zare, Professor Marguerite Blake Wilbur in Natural Sciences and a professor of chemistry at the Stanford School of Humanities and Sciences.

The researchers also speculate that this recently recognized chemical ability of water may have played a key role in restarting the chemistry for life on Earth billions of years ago, as well as producing our planet’s first atmospheric oxygen before life emerged. “This spontaneous production of hydrogen peroxide may be a missing part of the story of how the building blocks of life first formed,” Zare said.

The lead co-authors of the new study, published in Proceedings of the National Academy of Sciences, are Stanford staff scientists Jae Kyoo Lee and Hyun Soo Han.

Together with Zare and other Stanford colleagues, Lee and Han made the initial discovery of hydrogen peroxide production in micro-drops of water last year. Some outside researchers who reviewed the study’s findings were skeptical, Zare said, that such a potentially common phenomenon could have gone unnoticed for so long. A debate also ensued over how hydrogen peroxide would ever actually form.

“The argument was that people have been studying aerosols of water for years, and of course water is omnipresent and has been intensively studied since the dawn of modern science, so if this hydrogen peroxide formation in micro-drops were real, surely someone would have already seen it, “Zare said. ‘This led us to want to explore the phenomenon further, to see under what other circumstances it might occur, as well as to learn more about the fundamental chemistry going on.’

The micro-drops have done another way

Zare and colleagues decided to investigate condensation, a scenario in which micro-drops form easily naturally, without the help of an external force such as a nebulizer tool. Condensation occurs when water vapor (gas) in the air turns into a liquid upon contact with a colder surface; for example, when the bathroom mirror fogs up after a shower.

The Stanford team condensed water into multiple refrigerated materials, including silicon, glass, plastic, and metal. The researchers then cleaned a test strip that changes color in the presence of hydrogen peroxide on the condensed water. Sure enough, the stripe turned blue. The low but detectable amounts of hydrogen peroxide (on the order of parts per million) that formed varied based on factors such as surface temperature and relative humidity in the test chamber. The researchers also noted that the hydrogen peroxide formed in the micro-drops diluted as the size of the water droplets increased, which could explain why this chemical transformation has been neglected for so long.

The new experiments also support the researchers’ initial hypothesis about how hydrogen peroxide was forming. They showed that a strong electric field generated at the interface between water and air, right at the periphery of the micro-drop, appears to activate the water molecules, forming various so-called reactive oxygen species. These species are unstable molecular fragments that can quickly react with other molecules to produce hydrogen peroxide.

A process always with us and well before us

Such chemistry at the micro-drop level could have enhanced the chemical transition from non-life to life on Earth over four eons ago, Zare said. The origin of life has a kind of chicken or egg dilemma, in which the catalytic molecules that speed up chemical reactions and which seem necessary to restart the chemistry of life, require life itself to create the catalytic molecules in the first place. place. But the natural creation of hydrogen peroxide could have instead promoted reactions that led to the molecular building blocks that eventually assembled into complex, self-replicating entities.

Zare speculates that this ancient and widespread chemical reaction may have even provided a source of oxygen for the first years of life (as hydrogen peroxide breaks down into water and oxygen molecules) before the appearance of organisms that could produce oxygen through photosynthesis. .

Zare’s team is currently investigating how the production of hydrogen peroxide via micro-drops could be harnessed for cleaning and disinfection purposes. One intriguing possibility, suggests Zare, is the use of micro-drops and their associate H.₂OR₂ to eliminate SARS-CoV-2 (the virus that causes COVID-19) from surfaces.

“With this new study and our ongoing work, we are explaining how and why water droplets are so markedly different from bulk water in terms of their chemical reactivity,” said Zare. “It’s amazing that water still has a few tricks up its sleeve chemically.”

Other Stanford authors include Robert Waymouth, the Robert Eckles Swain Professor in Chemistry; Fritz Prinz, Leonardo professor and professor of mechanical engineering and materials science and engineering; and the PhD students Settasit Chaikasetsin in mechanical engineering and Daniel P. Marron in chemistry.

Zare is also a member of the Stanford Bio-X, the Cardiovascular Institute, the Stanford Cancer Institute, the Stanford ChEM-H, the Stanford Woods Institute for the Environment and the Wu Tsai Neurosciences Institute.

The research was funded in part by a grant from the US Air Force Office of Scientific Research and the Volkswagen Group of America.