Essential Role of Ionic Surfactants in Nitrogen Fixation at the Interface of Microdroplets

Microdroplets produced by ultrasonic atomization have been reported to enable catalyst-free nitrogen fixation under ambient conditions. However, the efficiency is still low, and the fixation mechanism, particularly the role of cavitation and microdroplet interfacial reactions, is not well understood. Here, we employed an ultrasonic atomization system with a distance-adjustable stopper to disentangle the respective roles of cavitation and microdroplet reactions in the nitrogen fixation. We provide solid evidence that in pure water systems, the previously disregarded cavitation process dominates nitrogen fixation, and the contribution of microdroplet reactions is negligible. Remarkably, in the presence of small amounts of ionic surfactants such as sodium dodecyl sulfate (SDS), the atomized microdroplets become substantially reactive for the oxidation of nitrogen to nitrite and nitrate (denoted as NOx-). Mechanism investigations further show that active oxygen species such as hydroxyl radicals, a widely recognized key reactive species for nitrogen fixation, do not account for the observed enhancement; instead, hydrated electrons generated at the charged interfaces are proposed to serve as the underlying reactive species for dinitrogen activation in the surfactant case. These findings reveal the essential role of surfactants in altering microdroplet interfacial reactivity and provide new insights into nitrogen fixation in microdroplets and the atmospheric nitrogen cycle.