Date of Award

2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Chemical and Biomolecular Engineering

First Advisor

Daeyeon Lee

Abstract

Emulsions are dispersions of droplets of one fluid within a second, immiscible fluid and have a wide range of applications from foodstuffs to pharmaceuticals to personal care products and agrochemicals. Emulsions are intrinsically unstable because of large interfacial area associated with the system. To obtain stable emulsions, the interfaces between immiscible fluids must be stabilized by emulsifying agents such as surfactants and colloidal particles. Surfactants refer to surface-active agents which prefer to segregate to interfaces between two immiscible fluids. Their surface activity originates from their amphiphilic structure. Colloidal particles can stabilize emulsions due to their tendency to attach strongly to the interface. It has been demonstrated that particles with amphiphilic structure (also known as Janus particles) can be synthesized. An important potential application of Janus particles comes from the fact that they could make unique solid surfactants, however, several questions needs to be answered: (1) It has been recognized for more than 100 years that surfactant molecules and homogeneous particles can attach to interfaces and stabilize emulsions. How would Janus particles be different from and, more importantly, advantageous over molecular surfactants and homogeneous particles in emulsion stabilization? (2) How does the structure or geometry of Janus particles influence their properties as solid surfactants? To answer these questions, thermodynamics of emulsion stabilization using amphiphilic Janus particles is investigated showing that they can indeed generate thermodynamically stable emulsion whereas emulsions stabilized by surfactant molecules and homogeneous particles are only kinetically stable. In addition, a new synthesis method is developed enabling the bulk synthesis of highly uniform pH-responsive Janus particles that are able to completely reverse their surfactant properties in response to changes in the solution pH. These Janus particles can stabilize different types of simple emulsions (oil-in-water and water-in-oil) at different pH and, more importantly, induce phase inversion of emulsions in response to changes in solution pH. Furthermore, one-step formation of stable multiple emulsions is demonstrated using these amphiphilic Janus. Multiple emulsions stabilized by these stimuli-responsive Janus particles can be induced to release the encapsulant by simply increasing the pH of the continuous phase. In conclusion, Janus particles represent promising systems as solid surfactants for making stable and smart emulsions.

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