Distinct Oligomerization of Lactic Acid in Aqueous Microdroplets

Area of Science:

• Environmental Chemistry
• Physical Chemistry
• Materials Science

Background:

• Lactic acid (LA) is crucial for poly(lactic acid) (PLA) and atmospheric aerosols.
• Oligomerization of LA via self-esterification is known in bulk solutions.
• Reaction mechanisms in aqueous microdroplets remain poorly understood.

Purpose of the Study:

• Investigate LA self-esterification in single aqueous microdroplets.
• Elucidate reaction kinetics and mechanisms under controlled temperature and humidity.
• Compare LA and pyruvic acid (PA) condensation dynamics in microdroplets.

Main Methods:

• Utilized in situ confocal micro-Raman spectroscopy for single microdroplet analysis.
• Developed and employed a reaction-evaporation model.
• Controlled temperature and relative humidity (RH) during experiments.

Main Results:

• LA undergoes rapid intermolecular esterification, forming trimers, tetramers, and pentamers.
• Microdroplet oligomerization is 3 orders of magnitude faster than in bulk solutions.
• Reaction kinetics exhibit size-dependence and a distinct thermodynamic equilibrium.

Conclusions:

• Aqueous microdroplet environments significantly accelerate LA oligomerization.
• The interplay between chemical reactivity and evaporation governs reaction kinetics in microdroplets.
• Findings impact understanding of atmospheric organic aerosol formation and PLA precursor synthesis.