Controlling emulsion and foam stability with stimuli-responsive peptide surfactants

Emulsions and foams are thermodynamically unstable dispersions that will eventually succumb to coalescence, leading to phase separation. However the kinetic stability of emulsions and foams can vary from transiently stable systems with lifetimes of seconds to indefinitely stable systems with lifetimes of many years. Understanding and controlling emulsion and foam stability is fundamental to their widespread application in consumer products and industrial processes. Designed stimuliresponsive peptide surfactants that allow the stability of emulsions and foams to be controlled by changes in solution conditions have recently been developed at the University of Queensland. The research objective of this thesis was to establish the mechanism by which these switchable biosurfactants control emulsion and foam stability and hence contribute design rules for future generations of peptide surfactants. In particular, research focused on the control of emulsion coalescence kinetics and the fundamental insights that these peptide-based emulsions provide into the coalescence phenomena. It was proposed that these switchable peptide surfactants allow the mechanical strength of the viscoelastic surfactant layer to be decoupled from other contributions to emulsion stability. It was found that the established Derjaguin– Landau–Vervey–Overbeek (DLVO) theory, which is frequently used as the basis for predicting emulsion stability, was not able to describe the stability switching observed in the peptide-based emulsions. Different designs of peptide surfactant were used to demonstrate that the kinetics of emulsion coalescence could be shifted by changing the interfacial elasticity, clearly illustrating the critical role of the surfactant layer’s mechanical properties in the coalescence mechanism. Where the peptide-surfactant-based emulsions enabled triggering a rapid transition to coalescence from a flocculation stable system it was shown that both the electrostatic repulsion (flocculation barrier) and the interfacial elasticity (coalescence barrier) were switched. This work made use of a number of experimental techniques to study the coalescence mechanism, including the observation of droplet interactions in microfluidic channels. The switchable peptide surfactants were shown to enable triggered coalescence in droplet based microfluidics, something that had hereto with proved an intractable challenge for surfactant containing oil-in-water systems. Having established the importance of the mechanical properties of the adsorbed peptide layer in enabling control over coalescence kinetics, it was of interest to study the effect of adding other surfactant species. Mixed surfactant systems are likely to be encountered in industrial applications or commercial products. The peptide surfactant AM1 was mixed with the common anionic surfactant sodium dodecyl sulfate (SDS) and synergistic behaviour was identified, including enhanced interfacial adsorption and reversible association of structures in the bulk solution. Furthermore the interfacial layers formed by AM1-SDS retained the switchable mechanical behaviour despite considerable increases in the absolute mechanical strength