Spontaneous Interfacial Redox Transformation of 6PPD-Quinone on Water Microdroplets

Spontaneous chemical transformations, including simultaneous reduction and oxidation at air-water interfaces on microdroplets, provides an important pathway for atmospheric chemistry processes. As an emerging tire-derived contaminant, N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q) has drawn intense scrutiny owing to its ubiquitous formation in the atmospheric environment and acute toxicity effects. Herein, we demonstrate that 6PPD-Q undergoes spontaneous, ultrafast transformation at the air-water interface of microdroplets under room temperature and atmospheric conditions. This reaction exhibits a remarkably short half-life of <2 min, representing a 1176-fold acceleration relative to its degradation rate in bulk water. Integrated microdroplet experiments and molecular simulations suggest that the reductive-oxidative species synergy effect (ROSE) is triggered by the strong electrification on the surface of the microdroplets. The simultaneous air-water interfacial redox reaction of 6PPD-Q occurs, mediated by ROSE, with the generation of emerging derivatives, of which hydroquinone derivatives are computationally predicted to have higher acute and chronic toxicity, as well as human health risks, than their parent 6PPD-Q by up to 1.1-2.6-fold. This study reveals a previously overlooked route for toxic hydroquinone derivatives generation, which enhances insights into the atmospheric chemistry of 6PPD-Q and raises critical concerns regarding their elevated toxicity.