Multiphase Chemical Kinetics in Aqueous Microdroplets

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

Distinguishing Surface and Bulk Reactivity: Concentration-Dependent Kinetics of Iodide Oxidation by Ozone in Microdroplets.

Iodine oxidation reactions play an important role in environmental, biological, and industrial contexts. The multiphase reaction between aqueous iodide and ozone is of particular interest due to its prevalence in the marine atmosphere and unique reactivity at the air-water interface. Here, we explore the concentration dependence of the I- + O3 reaction in levitated microdroplets under both acidic and basic conditions. To interpret the experimental kinetics, molecular simulations are used to benchmark a kinetic model, which enables insight into the reactivity of the interface, the nanometer-scale subsurface region, and the bulk interior of the droplet. For all experiments, a kinetic description of gas- and liquid-phase diffusion is critical to interpreting the results. We find that the surface dominates the iodide oxidation kinetics under concentrated and acidic conditions, with the reactive uptake coefficient approaching an upper limit of 10-2 at pH 3. In contrast, reactions in the subsurface dominate under more dilute and alkaline conditions, with inhibition of the surface reaction at pH 12 and an uptake coefficient that is 10× smaller. The origin of a changing surface mechanism with pH is explored and compared to previous ozone-dependent measurements

A Detailed Reaction Mechanism for Thiosulfate Oxidation by Ozone in Aqueous Environments

The ozone oxidation, or ozonation, of thiosulfate is an important reaction for wastewater processing, where it is used for remediation of mining effluents, and for studying aerosol chemistry, where its fast reaction rate makes it an excellent model reaction. Although thiosulfate ozonation has been studied since the 1950s, challenges remain in developing a realistic reaction mechanism that can satisfactorily account for all observed products with a sequence of elementary reaction steps. Here, we present novel measurements using trapped microdroplets to study the pH-dependent thiosulfate ozonation kinetics. We detect known products and intermediates, including SO32-, SO42-, S3O62-, and S4O62-, establishing agreement with the literature. However, we identify S2O42- as a new reaction intermediate and find that the currently accepted mechanism does not directly explain observed pH effects. Thus, we develop a new mechanism, which incorporates S2O42- as an intermediate and uses elementary steps to explain the pH dependence of thiosulfate ozonation. The proposed mechanism is tested using a kinetic model benchmarked to the experiments presented here, then compared to literature data. We demonstrate good agreement between the proposed thiosulfate ozonation mechanism and experiments, suggesting that the insights in this paper can be leveraged in wastewater treatment and in understanding potential climate impacts

The role of the droplet interface in controlling the multiphase oxidation of thiosulfate by ozone.

Predicting reaction kinetics in aqueous microdroplets, including aerosols and cloud droplets, is challenging due to the probability that the underlying reaction mechanism can occur both at the surface and in the interior of the droplet. Additionally, few studies directly measure the surface activities of doubly charged anions, despite their prevalence in the atmosphere. Here, deep-UV second harmonic generation spectroscopy is used to probe surface affinities of the doubly charged anions thiosulfate, sulfate, and sulfite, key species in the thiosulfate ozonation reaction mechanism. Thiosulfate has an appreciable surface affinity with a measured Gibbs free energy of adsorption of -7.3 ± 2.5 kJ mol-1 in neutral solution, while sulfate and sulfite exhibit negligible surface propensity. The Gibbs free energy is combined with data from liquid flat jet ambient pressure X-ray photoelectron spectroscopy to constrain the concentration of thiosulfate at the surface in our model. Stochastic kinetic simulations leveraging these novel measurements show that the primary reaction between thiosulfate and ozone occurs at the interface and in the bulk, with the contribution of the interface decreasing from ∼65% at pH 5 to ∼45% at pH 13. Additionally, sulfate, the major product of thiosulfate ozonation and an important species in atmospheric processes, can be produced by two different pathways at pH 5, one with a contribution from the interface of >70% and the other occurring predominantly in the bulk (>98%). The observations in this work have implications for mining wastewater remediation, atmospheric chemistry, and understanding other complex reaction mechanisms in multiphase environments. Future interfacial or microdroplet/aerosol chemistry studies should carefully consider the role of both surface and bulk chemistry

Competitive Adsorption and Reaction at the Air-Water Interface studied by Iodide Ozonolysis in Microdroplets

The ozonolysis of iodide in seawater and sea-salt aerosol is a primary sink for ozone in the marine boundary layer and a major source of atmospheric iodine. While the chemical composition of the air/water interface has been shown to influence the overall chemistry in this system, it remains unclear to what extent the reaction occurs at the interface and how non-reactive solutes (i.e., surfactants and other salts) might alter the multiphase reaction mechanism of O3 with I-. Using a quadrupole electrodynamic trap (QET) and single-droplet paper-spray mass spectrometry, we examine the competition between solute adsorption and reaction at the air-water interface by measuring the ozonolysis kinetics of I- in aqueous microdroplets in the presence of surface-active chlorate ions (ClO3-). Iodide consumption kinetics depend upon both pH and the gas phase ozone concentration [O3], with a transition from zero to first-order kinetics (in [I-]) with increasing [O3]. To explain these observations, a kinetic model is constructed that accounts for reaction and mass-transport of both I- and O3 and the competitive adsorption of iodide and chlorate at the microdroplet surface. Under our experimental conditions the reaction occurs at the air-water interface, where significant depletion of both ozone and iodide produces the observed shift from zero to first order kinetics with increasing [O3]. Analytical expressions for surface concentrations are derived to accurately predict the reactive uptake coefficients obtained from experiments (γ = 2×10-4). These predictions are extended over a range of [O3] and [I-] to assess the impact of competitive adsorption on the multiphase reaction mechanism under more dilute reaction conditions

Chemical Kinetics in Microdroplets

Micron-sized compartments play significant roles in driving heterogeneous transformations within atmospheric and biochemical systems as well as providing vehicles for drug delivery and novel reaction environments for the synthesis of industrial chemicals. Many reports now indicate that reaction kinetics are accelerated under micro-confinement; for example in sprays, thin films, droplets, aerosols, and emulsions. These observations are dramatic, posing a challenge to our understanding of chemical reaction mechanisms with potentially significant practical consequences for predicting the complex chemistry in natural systems. Here we introduce the idea of “kinetic confinement,” which is intended to provide a conceptual backdrop for understanding when and why microdroplet reaction kinetics differ from their macroscale analogs

Competitive Adsorption and Reaction at the Air-Water Interface studied by Iodide Ozonolysis in Microdroplets

The ozonolysis of iodide in seawater and sea-salt aerosol is a primary sink for ozone in the marine boundary layer and a major source of atmospheric iodine. While the chemical composition of the air/water interface has been shown to influence the overall chemistry in this system, it remains unclear to what extent the reaction occurs at the interface and how non-reactive solutes (i.e., surfactants and other salts) might alter the multiphase reaction mechanism of O3 with I-. Using a quadrupole electrodynamic trap (QET) and single-droplet paper-spray mass spectrometry, we examine the competition between solute adsorption and reaction at the air-water interface by measuring the ozonolysis kinetics of I- in aqueous microdroplets in the presence of surface-active chlorate ions (ClO3-). Iodide consumption kinetics depend upon both pH and the gas phase ozone concentration [O3], with a transition from zero to first-order kinetics (in [I-]) with increasing [O3]. To explain these observations, a kinetic model is constructed that accounts for reaction and mass-transport of both I- and O3 and the competitive adsorption of iodide and chlorate at the microdroplet surface. Under our experimental conditions the reaction occurs at the air-water interface, where significant depletion of both ozone and iodide produces the observed shift from zero to first order kinetics with increasing [O3]. Analytical expressions for surface concentrations are derived to accurately predict the reactive uptake coefficients obtained from experiments (γ = 2×10-4). These predictions are extended over a range of [O3] and [I-] to assess the impact of competitive adsorption on the multiphase reaction mechanism under more dilute reaction conditions.</jats:p

A Kinetic Model for Predicting Trace Gas Uptake and Reaction

A model is developed to describe trace gas uptake and reaction with applications to aerosols and microdroplets. Gas uptake by the liquid is formulated as a coupled equilibria that links gas, surface and bulk regions of the droplet or solution. Previously, this framework was used in explicit stochastic reaction-diffusion simulations to predict the reactive uptake kinetics of ozone with droplets containing aqueous aconitic acid, maleic acid and sodium nitrite. Using prior data and simulation results, a new equation for the uptake coefficient is derived, which accounts for both surface and bulk reactions. Lambert W functions are used to obtain closed form solutions to the integrated rate laws for the multiphase kinetics; similar to previous expressions that describe Michaelis–Menten enzyme kinetics. Together these equations couple interface and bulk processes over a wide range of conditions and don’t require many of the limiting assumptions needed to apply resistor model formulations to explain trace gas uptake and reaction