UC Berkeley

Multiphase chemistry occurs when reactivity involves two or more distinct phases. Chemical transformations of this kind are ubiquitous in all domains of science, with particular relevance to environmental and biological chemistry. Aerosols, including cloud droplets, sea-spray, smoke, and dust are prime examples of gas-liquid and gas-solid systems that undergo heterogeneous chemical changes while persisting in the atmosphere. A fundamental understanding of how multiphase reactions proceed is critical, then, to the study of our environment and the causal networks between anthropogenic activity, global ecosystems, and atmospheric composition. Moreover, a mechanistic perspective of reactivity in gas-liquid or gas-solid systems provides a useful framework for the study of multiphase interactions more generally, even informing on similar mechanics encountered in liquid-liquid and solid-liquid systems. In this work, experiment and theory are brought together to develop a kinetic framework of multiphase reactivity in aqueous microdroplets with particular focus on the role of the air-water interface. The experiments presented investigate the ozone-oxidation of aqueous sodium iodide contained in levitated microdroplets. This system is not only relevant to oxidation reactions in the environment, but also a compelling platform for studying mass-transfer across the air-water interface due to its unique reactive properties. As explored in Chapters 2 and 3, both I- and O3 possess a unique affinity for the air-water interface relative to their bulk phases, which directly affects the chemical kinetics at the microdroplet surface. This effect is studied by measuring microdroplet-oxidation kinetics while varying the solution pH and the concentration of both reactants. Experiments in Chapter 4 perturb this surface chemistry by the addition of surfactant to the microdroplet solution—effectively suppressing the surface reaction and producing a kinetic signature consistent with a diffusion limited reaction rate in the bulk phase. Insights from the specific systems in Chapters 2-4 provide the basis for a general framework of mass-transport and chemical reactivity in microdroplets which is developed in Chapter 5. This work aims to provide a route to analyzing an array of multiphase experiments from a critical lens by disentangling the underlying physical and chemical phenomena.