IMX Colloquium - Does H2O transmute to H2O2 or HO• + H• at the Air–Water Interface of Microscale Droplets / Bubbles?

Recent reports claim that hydrogen peroxide (H2O2) is spontaneously generated – without external energy input or catalyst – at the air–water interface of microdroplets1, bubbles2, and soap films3. Several mechanisms have been proposed to explain this phenomenon, including: (i) ultrahigh instantaneous electric fields at the air–water interface driving HO− à HO• + e− radicals from hydroxide ions; (ii) water’s comproportionation into H2O•+ and H2O•−; and (iii) partial dehydration of interfacial H+ and HO− followed by radical formation. In contrast, our laboratory experiments reveal that H2O2 formation is due to the reduction of dissolved O2 at water– solid interfaces, i.e., with no role for the air–water interface beyond O2 mass transport4-6. Based on these findings, I will present four diagnostic criteria against which any existing or future mechanistic proposal should be assessed. Our approach reconciles the claims of spontaneous H2O2 formation within established physical chemistry and extends naturally to other claims of anomalous microdroplet reactivity. 

References:
(1) Mehrgardi, M. A.; Mofidfar, M.; Zare, R. N. Sprayed Water Microdroplets Are Able to Generate Hydrogen Peroxide Spontaneously. Journal of the American Chemical Society 2022, 144 (17), 7606-7609. DOI: 10.1021/jacs.2c02890.
(2) Nami-Ana, S. F.; Mehrgardi, M. A.; Mofidfar, M.; Zare, R. N. Sustained Regeneration of Hydrogen Peroxide at the Water-Gas Interface of Electrogenerated Microbubbles on an Electrode Surface. Journal of the American Chemical Society 2024, 146 (46), 31945-31949. DOI: 10.1021/jacs.4c11422 From NLM PubMed-not-MEDLINE.
(3) Eldeeb, A. M.; Berbille, A.; Dick, J. E. No Microdroplets? No Problem. Soap Films Amplify the Interface and Produce Hydrogen Peroxide. Journal of the American Chemical Society 2025, 147 (37), 33325-33329. DOI: 10.1021/jacs.5c05887.
(4) Eatoo, M. A.; Mishra, H. Busting the myth of spontaneous formation of H2O2 at the air-water interface: contributions of the liquid-solid interface and dissolved oxygen exposed. Chemical Science 2024, 15 (9), 3093-3103. DOI: 10.1039/d3sc06534k From NLM PubMed-not-MEDLINE.
(5) Eatoo, M. A.; Wehbe, N.; Kharbatia, N.; Guo, X.; Mishra, H. Why do some metal ions spontaneously form nanoparticles in water microdroplets? Disentangling the contributions of the air-water interface and bulk redox chemistry. Chemical Science 2025, 16 (3), 1115-1125. DOI: 10.1039/d4sc03217a From NLM PubMed-not-MEDLINE.
(6) Eatoo, M. A.; Mishra, H. Disentangling the Roles of Dissolved Oxygen, Common Salts, and pH on Spontaneous Hydrogen Peroxide Production in Water: No O2, No H2O2. Journal of the American Chemical Society 2025, 147 (39), 35392-35400. DOI: 10.1021/jacs.5c09028.

Papers:
- Busting the myth of spontaneous formation of H2O2 at the air–water interface: contributions of the liquid–solid interface and dissolved oxygen exposed 
- Disentangling the Roles of Dissolved Oxygen, Common Salts, and pH on Spontaneous Hydrogen Peroxide Production in Water: No O2, No H2O2

Bio: Himanshu Mishra is a scientist-entrepreneur from King Abdullah University of Science and Technology (KAUST), Saudi Arabia. His team has significantly contributed to resolving scientific debates surrounding the spontaneous H2O2 formation in water microdroplets, the electrification at water-hydrophobe interfaces, and liquid–solid adhesion forces. Mishra’s team has also pioneered Carbosoil™ technology to transform desert soils for sustainable greening, food production, and carbon sequestration. His start-up Terraxy is now scaling up Carbosoil™ production to advance sustainability projects in the Middle East and beyond. Mishra’s efforts have been featured by the World Economic Forum, Geneva Inventions, and CNN (LINK).