Unraveling Reaction Acceleration in Microdroplets: Exploring Unique Chemistry at the Gas/Solution Interface
Chemical reactions in micron-sized droplets under ambient conditions are often orders of magnitude faster than the equivalent bulk reactions due to the large interfacial effects. The investigation of the underlying mechanisms driving the unique surface chemistry of droplets, as well as their applications and implications in synthesis, has garnered considerable interest. This dissertation delves into three key subtopics: (1) Exploring partial solvation as a mechanism for accelerating reactions in microdroplets, (2) Investigating the spontaneous oxidation and reduction of heteroatom double bonds induced by water radical cations and anions generated from water, and (3) Examining the role of oxazolone intermediates in prebiotic peptide synthesis and the emergence of homochirality in living systems.
Chemical reactions can be accelerated in microdroplets but with previously unclear mechanisms. Here we report a systematic study of organic reactions of common types in microvolumes and compare their rates with those in bulk solution. The observed interfacial area effect, molecularity effect and solvent effect provided experimental evidence for partial solvation at gas/liquid interface as one of the major contributors to the observed more than 104-fold acceleration in microdroplets.
Recent spectroscopic results as well as computations demonstrate the existence of a strong electric field at aqueous droplet surfaces, which can result in microdroplet-specific reactions, especially their intrinsic redox properties. Spontaneous oxidation or reduction without external oxidants or reductants has been reported. One explanation for the existence of active species is dissociation of the radical cation/anion pair (H2O+∙/ H2O-∙), recently argued to occur in pure bulk water, to provide the free radical cation and radical anion. In this work, we reported spontaneous oxidation of heteroatom double bonds (e.g. sulfone to sulfonic acid, ketone to carboxylic acid) in non-aqueous microdroplets containing traces of water (<1%). Meanwhile, the simultaneous oxidation and reduction of several phosphonates was discovered, supporting the radical pair as the source of reactive species in water microdroplets.
One implication of microdroplet chemistry lies in its connection to prebiotic synthesis. Peptide formation from amino acids is thermodynamically unfavorable but a recent study provided evidence that the reaction occurs at the air/solution interfaces of aqueous microdroplets. Here we show that (i) the suggested amino acid complex in microdroplets undergoes dehydration to form oxazolone; (ii) addition of water to the oxazolone forms the dipeptide; and (iii) reaction of oxazolone with other amino acids forms tripeptides. Furthermore, the chirality of the reacting amino acids is preserved in the oxazolone, and strong chiral selectivity is observed when converting the oxazolone to tripeptide. This last fact ensures that optically impure amino acids will undergo chain extension to generate homochiral peptides. Peptide formation in bulk by wet-dry cycling shares a common pathway with the microdroplet reaction, both involving the oxazolone intermediate.
Accelerated Reactions in Droplets and Thin Films: Surface Effects and Analytical Implications
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Multi-University Research Initiative (MURI) of the Air Force Office of Scientific Research (FA9550-21-1-0170) via Stanford University (sub-award 62741613-204669)
- Doctor of Philosophy
- West Lafayette