Ozone Fine-Bubble Oxidation of Water-Soluble Alcohols via Electron Transfer across the Gas–Liquid Interface, Quantified by Kinetic Studies

A novel fine-bubble reactor was developed to enable stable and quantitative kinetic studies of reactions at the gas-liquid interface, a process previously challenging to elucidate. Using this advanced analytical platform, we investigated ozone fine-bubble oxidation of a series of water-soluble alcohols, revealing fundamentally new insights into interfacial reactivity. Under single-substrate conditions, ozone fine bubbles exhibited comparable reactivity toward primary 1-butanol and secondary 2-butanol. However, under competitive conditions with a 1:1 mixture, ozone fine bubbles preferentially oxidized secondary 2-butanol over primary 1-butanol. This marked difference between single-substrate and competitive reactivity cannot be explained by single-substrate kinetics. For the reaction with methanol, a kinetic isotope effect of <i>k</i>_H/<i>k</i>_D = 1.5 was observed under competitive conditions, but no isotope effect was found in the single-substrate reaction. The most plausible explanation is that the oxidation sequence is initiated by an electron transfer from surface-bound OH^- to O₃, which generates reactive HO^• radicals at the gas-liquid interface. The subsequent reaction of these HO^• radicals with alcohols is not rate-determining, thus giving rise to the observed difference between single-substrate and competitive conditions. Overall, this study reveals a two-step mechanism governing ozone fine-bubble oxidation at the gas-liquid interface: (1) a rate-determining substrate uptake step, as evidenced by hydrophobicity-dependent reactivity for a series of alcohols and by observed bubble size reduction upon substrate uptake; (2) the electron-transfer initiated process described above. Our study established a powerful analytical framework for probing gas-liquid interfacial reactions.