Impact of oil-water-surfactant interactions on stability and non-equilibrium processes in emulsion droplets
Restricted (Penn State Only)
- Author:
- Birrer, Samuel
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 20, 2025
- Committee Members:
- Kenneth Knappenberger, Program Head/Chair
Mark Maroncelli, Major Field Member
Christine Keating, Major Field Member
Lauren Zarzar, Chair & Dissertation Advisor
Kristen Fichthorn, Outside Unit & Field Member - Keywords:
- emulsions
droplets
surfactants
ionic liquids
interfaces
solubilization
stability
phase inversion
coalescence
colloids
active matter
fluid physics - Abstract:
- Emulsions are commonplace materials in a variety of fields and applications, but they are out of equilibrium, and understanding how they proceed towards equilibrium is necessary for emulsion formulation and control over time-dependent physical properties. Emulsions are mixtures of immiscible liquids where one liquid is dispersed as droplets in the other, and they are usually stabilized by a surfactant or other stabilizer against various degradation processes leading towards equilibrium. Understanding the pathways through which emulsions equilibrate is essential for the targeted formulation of emulsions for applications including coatings, advanced food materials, chemical separation, emulsion polymerization, and enhanced oil recovery. This dissertation explores interactions between surfactants and different types of emulsions and investigates the impact of these interactions on droplet properties and behavior, focusing on the stability of emulsions to coalescence and other degradation processes, and dynamic behaviors of droplets in response to solubilization and partitioning by surfactants. In this work, we investigate the properties and stability of ionic liquid-in-water emulsions made with commercially available surfactants. We show that many of these emulsions are unstable and rapidly coarsen, but that some of the selected ionic liquid-surfactant combinations display longer-term stability. We also examine the solubilization behavior of ionic liquid-in-water emulsions and in some cases observed a related phenomenon, Marangoni-driven self-propulsion, which as far as we know has not been reported for this class of emulsion before. We propose explanations for trends in droplet stability as well as solubilization and self-propulsion. In addition to characterizing well-known properties and behaviors of ionic liquid-in-water emulsions, we note an unfamiliar process by which droplets of certain ionic liquids exchange with water on a molecular or nanoscopic level to develop complex internal structure, becoming double (water-in-ionic liquid-in-water) or triple (ionic liquid-in-water-in-ionic liquid-in-water) emulsions. Double and triple emulsion formation occurred spontaneously from single emulsions, with no special method of emulsification or continued energy input of any kind. This process is interesting both for the low-energy, low-effort generation of complex droplets and for the unusual quality of increasing rather than decreasing the interfacial area over time. After observing how dramatically transport across emulsion interfaces could change the behavior of ionic liquid-in-water emulsions, we explored how bromoalkane droplets that solubilize and partition nonionic surfactants, can follow an unexpected pathway wherein a spherical droplet goes through an interfacial instability and dissociates, giving the appearance of an “explosion”. We demonstrated that this process depends on oil chain length, concentration and ethylene oxide number of the surfactant, initial droplet diameter, and presence of neighboring droplets. We proposed a mechanism based on local phase inversion explaining both the appearance of the dissociation behavior as well as its dependence on specific chemical conditions like oil and surfactant chemical structure and surfactant concentration. Through these studies of emulsion properties, stability, and dynamic behavior, we gained insight into the design of new emulsions for future applications and an improved understanding of how complex interplay among the components of non-equilibrium droplets and emulsions can produce dynamic active behavior that is unpredictable from investigation of equilibrium phase data.