The chemistry of pyruvic acid (PA) under thermal dark
conditions
is limited in bulk solutions, but in microdroplets it is shown to
readily occur. Utilizing in situ micro-Raman spectroscopy as a probe,
we investigated the chemistry of PA within aqueous microdroplets in
a relative humidity- and temperature-controlled environmental cell.
We found that PA undergoes a condensation reaction to yield mostly
zymonic acid. Interestingly, the reaction follows a size-dependent
sigmoidal kinetic profile, i.e., an induction period followed by reaction
and then completion. The induction time is linearly proportional to
the surface area (R
2), and the maximum
apparent reaction rate is proportional to the surface-to-volume ratio
(1/R), showing that both the induction and reaction
occur at the air–water interface. Furthermore, the droplet
size is shown to be dynamic due to changes in droplet composition
and re-equilibration with the relative humidity within the environmental
cell as the reaction proceeds. Overall, the size-dependent sigmoidal
kinetics, shown for the first time in microdroplets, demonstrates
the complexity of the reaction mechanism and the importance of the
air–water interface in the pyruvic acid condensation reaction.