Accelerated Keto–Enol Tautomerization Kinetics of Malonic Acid in Aqueous Droplets

Abstract

Chemical reactions in atmospheric organic aerosol (OA) add large uncertainties to accurate predictions of the effect of aerosol on health, visibility, and climate. The acceleration of reaction rates of organic compounds in droplets compared to bulk solution has been reported; however, the mechanism and the principle of acceleration largely remains unknown. Malonic acid (MA) is a dicarboxylic acid that exhibits keto–enol tautomerization and is ubiquitous in organic aerosols found in the nature. An environment controlled electrodynamic balance (EDB) coupled with Mie scattering imaging (MSI) and Raman spectroscopy was used to levitate single charged MA droplets and investigate the effect of relative humidity (RH: 90%, 70%, 50%, and 30%) and size (28–91 μm diameter) on the reaction kinetics of keto–enol tautomerization of MA. Raman spectroscopy of hydrogen–deuterium isotopic exchange in MA droplets enabled quantitative analysis of MA tautomerization kinetics. The result showed slower reaction rates of MA droplets at lower RH as well as in larger-sized droplets. Application of a step-by-step isotopic exchange model to the MA droplets at 90% RH condition was used to determine the enolization rate of MA, yielding a value 10-fold higher than determined in bulk solution. Keto and enol forms of MA have distinctive physicochemical properties, such as reactivity and hydrogen bond structure. The result from our work suggests that keto–enol tautomerization can play an important role for aging of MA-containing OA in nature, as the enol form of MA can undergo accelerated chemical reactions due to the presence of the reactive carbon–carbon double bond.