UCLA

This dissertation aims to elucidate the behaviors of interfacially active molecules that self-assemble at fluid-fluid interfaces and stabilize them. Examples of such molecules include amphiphilic surfactant molecules that comprise hydrophobic (oil-loving) and hydrophilic (water-loving) moieties and therefore have a natural affinity for oil-water as well as water-air interfaces. Surfactant molecules are ever present in typical household soaps and detergents, and when dissolved in water, self-assemble at the air water interface as well as into multi-molecule aggregates known as micelles. These assemblies, in turn, facilitate foaming and form the basis of the cleaning actions of the soaps and detergents. The first part of this dissertation examines the role of micellar and interfacially self-assembled surfactants on the stability of thin foam films. Specifically, the correspondence between inter-micellar interactions in bulk solutions and in thin films, and the influence of these interactions on the average lifetime of foam films are investigated. I envision that my studies will inspire colloidal scientists to explore foam film studies as a simple but effective method for characterizing nanoscopic colloidal interactions and forces. In the second part of this dissertation, development of interfacially active polymers that stabilize water-water interfaces, designed specifically for stabilization of polyelectrolyte complex coacervates, is demonstrated. Complex coacervates form upon electrostatic complexation of oppositely charged macromolecules and their subsequent condensation into aqueous macromolecules-rich phase. This aqueous two-phase system has been demonstrated to exhibit unique capabilities to achieve dynamic spatial compartmentalization as well as spontaneous sequestering of biological molecules. Despite these exciting prospects, their use has been limited owing to our inability to stabilize coacervate droplets and prevent their macro-phase separation. In my research, I have developed block polymers and comb polymers that have successfully stabilized micron-sized coacervate droplets, leading to first demonstrations of stable coacervate emulsions. I have also demonstrated sequestration of proteins (enzymes) into the stabilized coacervate droplets, paving way for these emulsions to be employed as protein-based bioreactors with selective small molecule transport.