Abstract
Reaction kinetics can be significantly accelerated in microconfinement, where interfacial processes play a critical role. We developed a kinetic model describing diffusion, adsorption, evaporation, partitioning, and surface reactions in a microdroplet. Tensiometry measurements are utilized to parametrize the adsorption kinetics using a Langmuir adsorption model. The model quantitatively reproduces previous experimental measurements of the concentration and droplet size evolution during the condensation reaction of pyruvic acid (PA) to zymonic acid (ZA) in microdroplets. We further generalize the model to systems where the interplay between reaction and transport processes varies with droplet size from nanometer to millimeter scales, leading to diverse kinetic behaviors unique to the droplet environment. Notably, we observe an intriguing competition between evaporation and reaction 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 the reaction. These findings offer insights into the complexity of microdroplet reaction kinetics and elucidate general mechanisms for understanding processes that control the reaction kinetics in droplets over a wide range of length scales.