The Role of Interfacial Processes in Accelerated Reaction Kinetics in Aqueous Microdroplets

Shu Yang, Meng Li, Justin Wang, Vicki Grassian, Satish Kumar, CARI DUTCHER, University of Minnesota

     Abstract Number: 437
     Working Group: Aerosol Physics

Abstract
Aqueous microdroplets have been shown to exhibit enhanced chemical reactivity compared to bulk solutions, although the mechanisms for these enhancements are not completely understood. Here we combine experimental measurements and kinetic modeling to show the strong coupling of interfacial reactions and gas/droplet partitioning in the condensation reaction of pyruvic acid (PA) to yield zymonic acid (ZA) in acidic aqueous microdroplets. Using a newly developed diffusion-adsorption-reaction-evaporation model, we simulate the intricate kinetics where reactant diffuses and adsorbs to the surface of a microdroplet, where it can remain, react, desorb, or evaporate, while product formed at the surface desorbs and diffuses into the bulk. Tensiometry measurements are utilized to parameterize the adsorption kinetics in the Langmuir adsorption model. The model quantitatively predicts compositional changes during the condensation reaction, and provides insights into how microdroplet reactivity is controlled by coupled interfacial reactions and the gas-phase partitioning of PA and water. Moreover, the model can be generalized to systems where the interplay of reaction, adsorption, diffusion, and evaporation processes varies with droplet size, from nanometer to millimeter scales, or with different initial reactant concentrations, leading to diverse kinetic behaviors. Notably, we observe an intriguing competition between evaporation and reaction rates that determines the optimal droplet size. While smaller droplets exhibit faster reaction rates due to the dominance of surface reactions, they also experience higher PA evaporation rates, leading to more PA being consumed via evaporation rather than reaction. These findings offer insights into the complexity of microdroplet reaction kinetics, with interfacial processes playing a central role, and provide a universal mechanism for understanding reaction kinetics in aqueous nano- and microdroplets.