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Hydronium Ion Acidity Above and Below the Interface of Aqueous Microdroplets

Colussi, Agustín J. and Enami, Shinichi and Ishizuka, Shinnosuke (2021) Hydronium Ion Acidity Above and Below the Interface of Aqueous Microdroplets. ACS Earth and Space Chemistry, 5 (9). pp. 2341-2346. ISSN 2472-3452. doi:10.1021/acsearthspacechem.1c00067.

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Atmospheric cloud, fog and aerosol microdroplets are more acidic than previously assumed. The fact that interfacial reactions on microdroplets are faster than anticipated has enhanced their role in atmospheric chemistry and raised the question of whether their interfaces are more or less acidic than the bulk phase. It turns out that acidity and its pH dependence sharply change across interfacial layers. Surface-specific experiments show that the protonations of gas-phase molecules at the outermost layer (OTL) of aqueous microdroplets are very different from those dissolved in deeper layers. Trimethylamine (TMA) is protonated, whereas the weak base isoprene (ISO) is not as expected, when dissolved in pH < pK_a(TMA) = 9.8 microdroplets. In dramatic contrast, both gas-phase TMA and ISO are protonated at the OTL of pH < 4 microdroplets. Because ISO is only protonated in concentrated acids H₃O⁺ ions at the OTL of pH < 4 microdroplets are superacidic. Conversely, the OTL of pH > 4 microdroplets lacks the H₃O⁺ ions that protonate TMA in deeper layers. H₃O⁺ ions become more acidic toward the surface (i.e., the free energies of proton transfer, H₃O⁺ + B = H₂O + BH⁺, become more negative) because hydration losses in lower density OTL water destabilize the small H₃O⁺ ion relative to the larger protonated bases BH+. Because the OTL behaves as neutral at pH ∼ 4 (i.e., its pK_w ∼ 8) interfacial water may be more dissociated than in the bulk. In short, the acidity of aqueous microdroplets probed by gas-phase molecules at the OTL is different from the acidity experienced by solutes in deeper layers and should not be confused with pH, which represents the uniform thermodynamic activity rather than the local acidities of H₃O⁺ ions as a function of depth. These concepts should become standard in interfacial atmospheric chemistry.

Item Type:Article
Related URLs:
URLURL TypeDescription
Colussi, Agustín J.0000-0002-3400-4101
Enami, Shinichi0000-0002-2790-7361
Ishizuka, Shinnosuke0000-0002-5915-3576
Additional Information:© 2021 American Chemical Society. Received: March 9, 2021; Revised: August 21, 2021; Accepted: August 24, 2021; Published: August 25, 2021. Shinichi Enami is grateful for receiving a Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific Research (KAKENHI) Grant (19H01154). Agustín J. Colussi received support from the U.S. National Science Foundation (Grant AGS-1744353). The authors declare no competing financial interest.
Funding AgencyGrant Number
Japan Society for the Promotion of Science (JSPS)19H01154
Subject Keywords:Acidity, pH, Reaction mechanisms, Layers, Ions
Issue or Number:9
Record Number:CaltechAUTHORS:20210830-230028625
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Official Citation:Hydronium Ion Acidity Above and Below the Interface of Aqueous Microdroplets. Agustín J. Colussi, Shinichi Enami, and Shinnosuke Ishizuka. ACS Earth and Space Chemistry 2021 5 (9), 2341-2346; DOI: 10.1021/acsearthspacechem.1c00067
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:110624
Deposited By: George Porter
Deposited On:31 Aug 2021 19:37
Last Modified:17 Sep 2021 22:40

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