On a theoretical model of microdroplet redox chemistry

Microdroplet chemistry has emerged as a fascinating field. In particular, a plethora of unexpected oxidation and reduction reactions are observed at air–water interface with wide implications to chemical science and beyond. However, the explanation of microdroplet redox chemistry is challenging and controversial. Mechanistically, thermodynamic reversal over the bulk reaction, surprisingly fast kinetics (often less than milliseconds) and generality of a broad scope of substrates defy textbook understanding. Herein, we lay out a theoretical model of concerted interfacial electron transfer of OH- (partially solvated) + A^n → OH· (partially solvated) + A^{n-1}, rather than the conventional stepwise ionization-capture picture. Four different electron acceptors A^n (including a transition metal, an organic molecule, oxygen, ozone) are chosen to cover a wide range of experimental scenarios and chemical diversity. When A^n in the system is reduced upon receiving an electron, the accompanied product of OH· further serves as the oxidizing agent, capturing the intrinsic oxidative capacity of microdroplet. We then introduce a general energetic treatment to account for the partial solvation of OH- at heterogeneous water interface and develop a Marcus-theory-based reaction rate formalism that integrates the energetic distribution. We quantitatively analyze both the thermodynamic and the kinetic consequences of this model. For all four cases analyzed, the otherwise unfavorable thermodynamic penalty (ΔG° > 0) for the bulk reaction is reversed at water interface (ΔG° < 0) after a certain dehydration level. Remarkably, for all these cases, interfacial electron transfer becomes near-barrierless (ΔG° ≈ -λ, ΔG‡ ≤ 3 kcal/mol) for a substantial sub-population of partially solvated OH-. Thus, the water interface can prepare and gate a substantial donor population whose electrons are nearly ready to tunnel into a broad range of acceptors across the interface, which explains the surprisingly fast reaction rate observed in experiments. This model further predicts absolute rate constants that are in agreement with kinetic measurement on microdroplets. Finally, a phase diagram is constructed to depict the vast thermodynamic and kinetic (barrierless reaction) zones that are newly opened by the water interface, thus accounting for reaction generality. Despite its simplicity, this model provides a robust prediction for the rich redox chemistry at water interface.