In its bulk liquid form, whether in a bathtub or an ocean, water is a relatively benign substance with little chemical activity. But down at the scale of tiny droplets, water can turn surprisingly reactive, Stanford researchers have discovered.
In microdroplets of water, just millionths of a meter wide, a portion of the H2O molecules present can convert into a close chemical cousin, hydrogen peroxide, H2O2, a harsh chemical commonly used as a disinfectant and hair bleaching agent.
Stanford scientists first reported this unexpected behavior in forcibly sprayed microdroplets of water last year. Now in a new study, the research team has shown the same Jekyll-and-Hyde transformation happens when microdroplets simply condense from the air onto cold surfaces. The new results suggest that water’s hydrogen peroxide transformation is a general phenomenon, occurring in fogs, mists, raindrops and wherever else microdroplets form naturally.
The surprising discovery could lead to greener methods for disinfecting surfaces or promoting chemical reactions. “We’ve shown that the process of forming hydrogen peroxide in water droplets is a widespread and surprising phenomenon that’s been happening right under our noses,” said study senior author Richard Zare, the Marguerite Blake Wilbur Professor in Natural Science and a professor of chemistry in the Stanford School of Humanities and Sciences.
The researchers also speculate that this newly recognized chemical ability of water could have played a key role in jumpstarting the chemistry for life on Earth billions of years ago, as well as produced 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 were formed on early,” Zare said.
The co-lead 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.
Along with Zare and other Stanford colleagues, Lee and Han made the initial discovery of hydrogen peroxide production in water microdroplets last year. Some outside researchers who went over the study’s results were skeptical, Zare said, that such a potentially common phenomenon could have gone undiscovered for so long. Debate also ensued over just how the hydrogen peroxide would ever actually form.
“The argument was that people have been studying water aerosols for years, and of course water is ubiquitous and has been intensively studied since the dawn of modern science, so if this hydrogen peroxide formation in microdroplets were real, surely someone would have seen it already,” said Zare. “That led us to want to explore the phenomenon further, to see in what other circumstances it might occur, as well as learn more about the fundamental chemistry going on.”