Distinct Oligomerization of Lactic Acid in Aqueous Microdroplets

Lactic acid (LA), a fundamental building block for poly(lactic acid) (PLA) and a key component of atmospheric organic aerosols, undergoes self-esterification to form oligomers in aqueous environments. While this process has been well studied in bulk solutions, its reaction mechanism in aqueous microdroplets under ambient conditions remains poorly understood. Here, we investigate the reaction of LA in single microdroplets under temperature- and relative humidity (RH)-controlled conditions using <i>in situ</i> confocal micro-Raman spectroscopy. A reaction-evaporation model incorporating surface and bulk reaction pathways, evaporation, and water partitioning quantitatively reproduced the observed reaction kinetics. Both the model and experimental results show that LA undergoes rapid intermolecular esterification, forming oligomers primarily consisting of trimers, tetramers, and pentamers. This oligomerization in microdroplets proceeds 3 orders of magnitude faster than that in bulk solutions, leading to pronounced size-dependent kinetics and a distinct thermodynamic equilibrium. Furthermore, comparison of lactic acid and pyruvic acid (PA) condensation reactions reveals that the kinetic behavior of these reactions in aqueous microdroplets is primarily governed by the interplay between chemical reactivity and evaporation dynamics.