Do Acid–Base Equilibria of Phenols in Pneumatic Sprays Follow the Bulk Expectations?

Abstract

Microdroplets have been reported to exhibit accelerated chemical reactivity relative to bulk solutions, yet the fundamental origins of this behavior remain poorly understood. Extremely unlikely protonation equilibria have been reported for some reactions in microdroplets. This necessitates systematic investigation of protonation equilibria in microdroplets using a series of molecules with well-defined acid–base chemistry. To address this problem, we investigate deprotonation of a family of substituted phenols spanning a wide range of pKa values (∼1–9.2 in water and ∼14–25 in acetonitrile). We generate microdroplets of solutions of phenols over a range of concentrations and measure the corresponding phenolate ion yield using mass spectrometry. In aqueous microdroplets, the observed ion yields are quantitatively consistent with the expected bulk equilibrium behavior. Therefore, phenol deprotonation in water is not affected by any previously reported unusual microdroplet acid–base chemistry. In contrast, phenols in acetonitrile microdroplets exhibit ion yields unexpectedly similar to those observed in water despite the dramatically different pKa scales in the two solvents. This suggests that deprotonation in nonaqueous microdroplets generated by nebulization may not follow the bulk equilibrium expectations and may require alternative explanations.