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Contributed by Richard N. Zare, October 12, 2020 (sent for review September 25, 2020; reviewed by Raoul Kopelman and Veronica Vaida)
Significance
Water molecules in bulk liquid are stable and inert under ambient conditions. In sharp contrast, we show that the condensation of water vapor in air to form microdroplets on cold surfaces causes the formation of hydrogen peroxide (H2O2) which we suggest is promoted by the large, intrinsic electric field at the air–water interface. This finding provides an alternative pathway for the production of atmospheric H2O2, for example, in raindrops as well as fogs and mists. This easy means of naturally producing H2O2 suggests many interesting possibilities, from the use of condensed steam for disinfection to how water microdroplets might have promoted formation of the building blocks of life in the prebiotic era.
Abstract
It was previously shown [J. K. Lee et al., Proc. Natl. Acad. Sci. U.S.A., 116, 19294–19298 (2019)] that hydrogen peroxide (H2O2) is spontaneously produced in micrometer-sized water droplets (microdroplets), which are generated by atomizing bulk water using nebulization without the application of an external electric field. Here we report that H2O2 is spontaneously produced in water microdroplets formed by dropwise condensation of water vapor on low-temperature substrates. Because peroxide formation is induced by a strong electric field formed at the water–air interface of microdroplets, no catalysts or external electrical bias, as well as precursor chemicals, are necessary. Time-course observations of the H2O2 production in condensate microdroplets showed that H2O2 was generated from microdroplets with sizes typically less than ∼10 µm. The spontaneous production of H2O2 was commonly observed on various different substrates, including silicon, plastic, glass, and metal. Studies with substrates with different surface conditions showed that the nucleation and the growth processes of condensate water microdroplets govern H2O2 generation. We also found that the H2O2 production yield strongly depends on environmental conditions, including relative humidity and substrate temperature. These results show that the production of H2O2 occurs in water microdroplets formed by not only atomizing bulk water but also condensing water vapor, suggesting that spontaneous water oxidation to form H2O2 from water microdroplets is a general phenomenon. These findings provide innovative opportunities for green chemistry at heterogeneous interfaces, self-cleaning of surfaces, and safe and effective disinfection. They also may have important implications for prebiotic chemistry.
Footnotes
↵1J.K.L. and H.S.H. contributed equally to this work.
- ↵2To whom correspondence may be addressed. Email: fprinz{at}stanford.edu or zare{at}stanford.edu.
Author contributions: J.K.L., H.S.H., D.P.M., R.M.W., F.B.P., and R.N.Z. designed research; J.K.L., H.S.H., and S.C. performed research; J.K.L., H.S.H., and R.N.Z. analyzed data; and J.K.L., H.S.H., D.P.M., and R.N.Z. wrote the paper.
Reviewers: R.K., University of Michigan; and V.V., University of Colorado Boulder.
The authors declare no competing interest.
This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2020158117/-/DCSupplemental.
Data Availability.
All study data are included in the article and SI Appendix.
Published under the PNAS license.
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Condensing water vapor to droplets generates hydrogen peroxide
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