Dynamic Processes of Surfactants at Fluid Interfaces

This thesis addresses knowledge gaps in several complex dynamic processes of surfactants at fluid interfaces. Topics covered in this work include (1) interfacial dynamics of complex structured polymeric surfactants, (2) interfacial properties of binary surfactant systems where two surfactants adsorb from different sides of the interface, (3) the coupling of capillary waves and surface tension-driven (Marangoni) spreading on deep and low-viscosity subphases, and (4) Marangoni spreading in binary surfactant mixtures. While those topics are different interfacial dynamic processes, the central and repeating themes in this thesis are to determine surfactants’ thermodynamic and dynamic effects, by experiments or modeling or both, in those processes of interest, and to report discoveries of new phenomena, correlations, or synergistic effects. By establishing the correlation between molecular structure, interfacial rheology and emulsifying efficiency, the interfacial stiffening behaviors and the polymer structures leading to such behaviors, e.g., the anchoring groups that fix polymers at the interface, were identified as key features contributing to high emulsifying efficiency. By probing the interfacial properties of systems where two surfactants access the interface from different bulk phases, binary surfactant attraction strength at the interface was identified as a key parameter dictating the occurrence of synergistic effects in interfacial tension reduction and dilatational modulus. By performing computational modeling on Marangoni spreading with the full Navier-Stokes equations, new features, e.g., the Marangoni shoulder, pertaining to surfactant spreading on deep and low-viscosity subphases were discovered. By correlating the synergistic composition ranges of surface tension synergism and the novel Marangoni spreading synergism for various binary surfactant systems, the importance of binary surfactant interaction as well as adsorption kinetics was revealed. Overall, this thesis provides critical insights into those specific topics and advances the general field of interfacial phenomena.