Controlled Generation and Characterization of Multi-component Fluid/Fluid Interfaces

The stabilization of multiphase fluid formulations via addition of multiple surface active materials is widespread across various industries and applications. The properties of the air/water and oil/water interfaces that are affected by these materials, including surface tension and elasticity, have been shown to correspond to measurable changes in desirable bulk properties, such as foam and emulsion stability. However, equilibrium interfacial tension values are rarely enough to predict or design stability. Understanding the dynamic interfacial behavior of these materials is critical for improving formulation design. This work aims to characterize the interfacial properties of multi-component systems at air/water and oil/water interfaces. A microtensiometer platform is used to measure interfacial tension and interfacial mechanics to quantify multi-species adsorption under a variety of conditions, including adsorption time, concentration, and ionic strength. This technique is used to characterize several model systems, encompassing nonionic and anionic surfactants, rhamnolipid biosurfactants, hydrophobin proteins and surfactant-particle complexes. Dynamic interfacial tension measurements of surfactant systems reveal the irreversible adsorption behavior of a polymeric surfactant and rhamnolipids, as well as the effects of added small molecule surfactants on their persistence at the interface. For larger adsorbed species, measuring the interfacial mechanics is important to capture adsorption behavior more completely. Through precise control of bulk solution properties, rigid incompressible protein and nanoparticle films are created that can effectively stabilize interfaces. The unique interfacial mechanics of the films are only observed when the interfaces are generated utilizing a controlled adsorption procedure. The results on these model systems highlight the complex nature of the interactions of multiple species on an interface. Systematic modification of the bulk solution allows for the generation of mixed interfaces where interfacial properties that are traditionally confounded by bulk species interaction or dispersion instability can be probed. These interfacial measurements contribute to the overall understanding of the importance of processing as well as composition on desirable formulation properties.