pH-Dependent Kinetics of Imidazole Production in Aqueous Glyoxal/Ammonium Sulfate Microdroplets

The kinetics of imidazole formation in optically levitated glyoxal/ammonium sulfate aqueous microdroplets has been investigated using single-particle Raman spectroscopy. The microdroplet diameters (∼25 μm) were measured using brightfield microscopy. Individual microdroplets were produced from bulk solutions with pH in the range 4.0–9.3 and confined for up to 6 h in the optical levitation apparatus, which was maintained at a constant relative humidity (RH) of ∼70%. Imidazole bands at 920, 1230, and 1470 cm^–1 were observed to grow in as the confined microdroplets aged; the effective formation rate constants increased with initial bulk solution pH, reflecting the increased concentration of nucleophilic NH₃ as the acid–base equilibrium is shifted. However, the imidazole formation rates do not increase to the extent that would be expected based on previous bulk kinetics measurements. Preliminary experiments observing microdroplets dispensed sequentially from the same bulk solution suggest that imidazole formation is accelerated in the microdroplets by a factor of ∼7. The apparent rate acceleration can be explained by preconcentration of the reactants as the initially dispensed microdroplets lose water and equilibrate with the apparatus RH.