Jotam Bergfreund

In interfacial experiments and research of fluid mixtures, the hydrophobic or oil phase is often given little attention. This has been the cause for significant inconsistencies in the scientific literature, as experiments were usually performed at arbitrary oils, which impeded the reproducibility and comparability and hampered the pathway to a generic description. In this dissertation, the effect of the hydrophobic phase on the interfacial behavior of surfactants, proteins, and particles was elaborated by studying the adsorption, assembly, deformation, and interfacial rheology at fluid interfaces. Most essentially, we found a scaling law between interfacial tension of the clean oil-water (o/w) interfaces and the equilibrated interfacial pressure after saturated adsorption of the interfacial active substance. Thus, the clean interfacial tension is a promising parameter to describe the oil polarity for interfacial applications. Surfactant adsorption results in a linear increase of  1 with decreasing oil polarity. Further, the area per molecule at the interface increases at more polar interfaces, and the critical micelle concentration increases for ionic surfactants. Proteins, in particular globular and flexible proteins, adsorb stronger to non-polar o/w interfaces. Globular proteins undergo less unfolding at polar o/w interfaces and form weaker interfacial networks. As a result, the network strength of flexible protein layers is lower, and the dependency on the polarity of the oil phase is reduced. Adsorption of particles, e.g., cellulose nanocrystals and pNiPAM microgels, to different fluid interfaces is strongly affected by the properties of the particles. Cellulose nanocrystals are nanosized, anisotropic, and charged needle-like particles. At o/w interfaces of more polar oils, adsorption is drastically reduced, and overall the decrease of interfacial energy is relatively low compared to the other measured substances. pNiPAM microgels are micron-sized, spherical, soft particles. Their adsorption behavior at different fluid interfaces is very similar to globular proteins. In addition, though, the interfacial networks are significantly weaker. Furthermore, experimental guidelines are provided for using oils in interfacial experiments, aiming to harmonize results and protocols in interfacial science. These findings are missing issues in understanding adsorption to fluid interfaces and could significantly improve reproducibility and comparability of past and future research and facilitate industrial applications and developments.