Understanding Interfacial Transport, Reactions, and Phase Behavior in Emulsion Systems

Download or read book Understanding Interfacial Transport, Reactions, and Phase Behavior in Emulsion Systems written by Rebecca Balaj and published by . This book was released on 2022 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Complex fluids are of increasing interest because of their ability to provide a unique environment for the exploration of interfacial phenomena. Emulsions, which are mixtures of immiscible liquids, allow for separation of reagents whilst simultaneously providing a liquid-liquid interface wherein reactants can be easily transported, and reactions are able to readily occur. Thus, utilizing complex emulsions as dynamic materials is potentially important to understand localized reaction pathways and kinetics in a controllable and predictable manner. However, a significant limitation of fluid droplets for long-term applications is their eventual breakdown over time. Elucidating approaches by which to enhance droplet stability while simultaneously maintaining their reconfigurability and responsive character is therefore essential. This dissertation, thus, exists in three main sections: first, it aims to investigate a surfactant-free gelation pathway to form oil-core hydrogel capsules to stabilize complex droplets; secondly, it examines the non-equilibrium partitioning of amphiphilic surfactants into biphasic emulsions of miscible oils to dynamically and controllably induce phase separation; and, finally, it attempts to explore the reconfigurable complex droplet as a responsive soft material which can be used to easily manipulate chemical reactions at the oil-water interface. One approach to enhanced stabilization of droplets is by encapsulation, where a shell, often a polymer, encases the liquid droplet to prevent coalescence and limit exchange of droplet contents with the continuous phase. A common approach to droplet encapsulation is interfacial polymerization, where monomers dissolved in immiscible phases react to form capsules localized at the interface. Hydrogels, which are composed of crosslinked, charged polymers swollen in water, are highly permeable to molecules in the aqueous phase and would permit the diffusion of surfactants or other analytes to the droplet surface to trigger a response in droplet configuration. In the first major project of this work, we utilized a Pickering complex droplet intermediate to localize hydrogel formation at the droplet-continuous phase interface. Enhanced stability of droplets with both ionically and covalently crosslinked capsules was observed, with a complete spectrum of reconfigurability possible upon addition of droplet-internal fluorosurfactant, Capstone FS-30. The next major work examines the partitioning of surfactant molecules across the droplet oil-water interface. In general, the partitioning of chemicals across interfaces and their subsequent concentration into droplets and coacervates is important in many fields of study, including organic reaction chemistry and in mimicking individual properties of natural systems, such as living cells. When considering partitioning occurring within emulsions, the dynamics and relative concentration of all components--oil, water, and surfactant--are critical to understanding the behavior of non-equilibrium systems. We became interested in the dynamics and degree of surfactant partitioning while examining the behavior of two-component oil droplets in aqueous surfactant. We produced microscale droplets containing two oils, which in bulk at room temperature are miscible in all proportions. Upon preparing this emulsion, we expected that the droplets would remain in a single phase during their lifetime when undergoing micellar solubilization; however, we noticed phase separation occurring within the droplets. To confirm the partitioning of surfactant into these droplets, we developed a protocol for the LCMS analysis of single microscale droplets. Furthermore, we have shown that, upon addition of ionic surfactant, we can controllably and dynamically induce phase separation through various stimuli. In the last major project of this work, we examine reaction rates at oil-water interfaces and within surfactant micelles. Reactions occurring at the interface of immiscible fluids are important for a variety of different fields, largely due to the separation of reagents into distinct phases only to be reacted specifically and controllably at the interface. This allows unique opportunity to tune reaction kinetics, location, and pathways. Studies have previously been conducted observing reaction kinetics at the oil-water interfaces of emulsions, therefore exploring reactions at interfaces in complex droplets should then allow for an even more specific morphology-dependent, tunable reaction environment.