Proton Traffic Jam: Effect of Nanoconfinement and Acid Concentration on Proton Hopping Mechanism

• Published in: Angewandte Chemie (International ed.) Vol. 60; no. 48; pp. 25419 - 25427
• Main Authors: Adams, Ellen M., Hao, Hongxia, Leven, Itai, Rüttermann, Maximilian, Wirtz, Hanna, Havenith, Martina, Head‐Gordon, Teresa
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 22.11.2021; Wiley Blackwell (John Wiley & Sons); John Wiley and Sons Inc
• Edition: International ed. in English
• Subjects: Atmospheric chemistry; confined water; Dielectric relaxation; Grotthuss mechanism; Hydrogen; Hydronium ions; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Micelles; proton hopping; Protons; Spectroscopy; THz spectroscopy; Traffic congestion; Traffic jams
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.202108766

The properties of the water network in concentrated HCl acid pools in nanometer‐sized reverse nonionic micelles were probed with TeraHertz absorption, dielectric relaxation spectroscopy, and reactive force field simulations capable of describing proton hopping mechanisms. We identify that only at a critical micelle size of W0=9 do solvated proton complexes form in the water pool, accompanied by a change in mechanism from Grotthuss forward shuttling to one that favors local oscillatory hopping. This is due to a preference for H+ and Cl− ions to adsorb to the micelle interface, together with an acid concentration effect that causes a “traffic jam” in which the short‐circuiting of the hydrogen‐bonding motif of the hydronium ion decreases the forward hopping rate throughout the water interior even as the micelle size increases. These findings have implications for atmospheric chemistry, biochemical and biophysical environments, and energy materials, as transport of protons vital to these processes can be suppressed due to confinement, aggregation, and/or concentration. The mechanism of proton transport within nonionic reverse micelles is dependent on the proton concentration ([H+]) and the size of the nanoconfined environment. For small reverse micelles and/or low [H+], protons diffuse via the Grotthuss mechanism (forward hopping). In larger reverse micelles and/or at high [H+], ions accumulate at the reverse micelles interface, resulting in a proton “traffic jam,” in which an oscillatory hopping mechanism becomes dominant.