Recent reports on the formation of hydrogen peroxide (H$_2$O$_2$) in water microdroplets produced via capillary condensation or pneumatic spraying have garnered significant attention. How covalent bonds in water could break under such conditions challenges our textbook understanding of physical chemistry and the water substance. While there is no definitive answer, it has been speculated that ultrahigh electric fields at the air-water interface are responsible for this chemical transformation. This thesis documents the findings of our exploration of this mystery via a comprehensive experimental investigation of H$_2$O$_2$ formation in (i) water microdroplets condensed on hydrophobic and hydrophilic substrates formed via hot water in the 50–70 ℃ range or ultrasonic humidifier under controlled air composition, and (ii) water microdroplets sprayed over a range of liquid flow-rates, the (shearing) air flow rates, and the air composition. Our glovebox experiments, with controlled gas composition, revealed that no H$_2$O$_2$(aq) was produced in water microdroplets condensed via heating water (detection limit ≥ 0.25 μM), regardless of the droplet size or the substrate wettability. In contrast, water droplets condensed via ultrasonic humidification contained significantly higher (~1 μM) H$_2$O$_2$ concentration. We pinpointed that ultrasonic humidifiers induced cavitation of tiny bubbles in water, which is known to form H$_2$O$_2$(aq) and other reactive species. Next, in the case of sprayed water microdroplets, also, we did not detect H$_2$O$_2$(aq) unless O$_3$(g) was present in the ambient atmosphere. In contrast, water microdroplets (sprayed or condensed) exposed to O$_3$(g) concentration in the range 2–5000 ppb formed 2–100 μM H$_2$O$_2$(aq); increasing the gas–liquid surface area, mixing, and contact duration enhanced the H$_2$O$_2$(aq) concentration. Therefore, we submit that the original reports suffered from experimental artifacts due to the high regional O$_3$(g) giving rise to H$_2$O$_2$(aq), and the air–water interface does not spontaneously produce H$_2$O$_2$(aq). The water surface merely facilitates the O$_3$(g) mass transfer, which then undergoes chemical transformations in the water to form H$_2$O$_2$(aq). Taken together, these findings offer an alternative explanation to the mysterious production of H$_2$O$_2$ in water microdroplets; These findings should also advance our understanding of the implications of this chemistry in natural and applied contexts.
| Identifer | oai:union.ndltd.org:kaust.edu.sa/oai:repository.kaust.edu.sa:10754/675648 |
| Date | 03 1900 |
| Creators | Musskopf, Nayara H. |
| Contributors | Mishra, Himanshu, Biological and Environmental Science and Engineering (BESE) Division, Nunes, Suzana Pereira, Thoroddsen, Sigurdur T |
| Source Sets | King Abdullah University of Science and Technology |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | 2023-03-01, At the time of archiving, the student author of this thesis opted to temporarily restrict access to it. The full text of this thesis will become available to the public after the expiration of the embargo on 2023-03-01. |
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