# Distinct Oligomerization of Lactic Acid in Aqueous Microdroplets

## TL;DR

Lactic acid forms oligomers much faster in tiny water droplets than in bulk solutions, with the process influenced by evaporation and reaction dynamics.

## Contribution

The study reveals size-dependent kinetics and a distinct thermodynamic equilibrium of lactic acid oligomerization in microdroplets.

## Key findings

- Lactic acid oligomerization in microdroplets is 3 orders of magnitude faster than in bulk solutions.
- Oligomers primarily consist of trimers, tetramers, and pentamers in microdroplets.
- Reaction kinetics are governed by the interplay between chemical reactivity and evaporation dynamics.

## Abstract

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 in situ 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.