Microgels at Oil-Water Interfaces : Deformation, Assembly and Compression

Emulsions are composed of two immiscible liquid phases where one phase is dispersed in the other. The large interfacial area induces thermodynamic instability and droplets tend to coalesce. Stabilizers like surfactants decrease the surface tension and thus prevent phase separation. In Pickering emulsions, hard nano- or microparticles form a protective particle layer around the droplets, leading to high stability of the respective emulsions. Soft microgel particles can act as a new class of stimuli-sensitive stabilizers due to their high affinity to liquid interfaces. Microgels are hydrophilic, crosslinked, polymeric particles that are swollen by a good solvent. Microgels containing poly-(N-isopropyl acrylamide) as main monomer are sensitive to temperature changes and the microgel collapses above a temperature of 32°C. Additional sensitivity towards pH can be induced by using weak acidic or basic comonomers. The sensitivity of the microgels can be transferred to the respective microgel-stabilized emulsions and selective phase separation can be achieved by changing external parameters.The mechanism behind phase separation is not known thoroughly. It is thus inevitable to investigate the microgel-covered interface between oil and water. In this thesis, the microgel-covered interface is investigated in different manners with regard to the properties of microgels at interfaces and the influence of charges. On the one hand, the droplet interface is investigated directly with microscopy and scattering methods; on the other hand, flat oil-water interfaces are created as model systems to investigate the structure at the interface in more detail. The influence of charges is examined and differences between Pickering emulsions and microgel-stabilized emulsions are illustrated. Emulsion droplets are investigated with cryogenic scanning electron microscopy (cryo-SEM) that reveals the arrangement of microgels on the droplet surface. Furthermore, transmission x-ray microscopy (TXM) and small-angle neutron scattering (SANS) are used to shed light on the protrusion of microgels into the oil phase. Flat microgel covered interfaces are examined using a special cryo-SEM setup, namely freeze-fracture shadow-casting (FreSCa) cryo-SEM, that gives information about the arrangement and protrusion height of the microgels. Furthermore, pronounced differences compared to hard particles are observed when the microgel layer is compressed in a Langmuir trough. The interfacial microgel layer is then transferred to solid substrates using the Langmuir-Blodgett method and subsequent imaging with atomic force microscopy (AFM) reveals a dependence of microgel structure and size on the compression. Additional experiments of interfacial shear rheology show that the viscoelastic properties of the interface depend on the microgel charge. Thus, the viscoelasticity of the interface may be a crucial parameter governing emulsion stability