Nanobubbles in Bulk Solution and Electrosprayed Microdroplets Accelerate Chemical Reactions

Bulk nanobubbles (NBs) are metastable gas-filled voids in solution, with diameters of nearly 200 nm that exhibit long lifetimes and large surface-to-volume ratios. Recent studies have shown that chemical reactions are significantly accelerated within sprayed microdroplets during electrospray ionization mass spectrometry (ESI-MS). Although several factors, such as partial solvation of reagents, electric fields, ordered orientation, and pH, have been implicated in the acceleration in microdroplets, the primary governing factor remains unclear and dependent on the system. Similar processes occur on the nanobubble-liquid interface, leading to enhanced reaction rates when they are present in bulk solution. We demonstrate reaction rate acceleration by deliberately adding NBs to the bulk solution and microdroplets by considering two model systems: the Katritzky transamination reaction and the reaction between CO2 and N,N-dibutyl-1,3-propane diamine (DBPA). N2 and CO2 nanobubbles were generated using sonication, pressure cycling (PC), or the Tesla valve (TV) and delivered as bulk or spray solvents. The Katritzky reaction in bulk solution with NB-enriched solvent systems was examined by mass spectrometry under nonaccelerating electrospray conditions. The acceleration factor (AF) for the reaction doubled compared with the control. On the other hand, the microdroplet accelerated the Katritzky reaction, with N2 NB producing a further six-fold acceleration compared to the microdroplet control. For the reaction of the diamine DBPA with N2 nebulization, CO2 reagent delivery by NB resulted in acceleration factors between 2 and 50 compared to the microdroplet reaction control, depending on the solvent fraction, wherein bubbles were generated. With CO2, nebulization where ample reagent is already present, the addition of NBs produced a further two-to-five-fold acceleration factor depending on the solvent fraction in which NBs were generated, indicating a NB catalytic effect. Our findings show that NBs improve reaction kinetics by acting both as improved reagent delivery vehicles and/or as catalysts. Catalytic activity is a consequence of increased liquid-air interfacial surface area and improved reagent transport in bulk solution and within microdroplets.