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Siegel der Universität
Department für Chemie - Arbeitsgruppe Prof. Strey

Supercritical Carbon dioxide-microemulsions as precursor for nanofoams - Preparation, characterization and nanostructure

Lorenz Kramer, april 2008

Supercritical CO2-microemulsions can be used in the production of novel insulating nanofoams. These nanofoams are obtained via POSME (Principle of Supercritical Microemulsion Expansion), a method developed by Strey et al. The focus of this thesis is to achieve efficient supercritical CO2-microemulsions and to study their nanostructure and phase behavior. Starting point was inefficient CO2-microemulsions containing non-ionic, hydrocarbon, technical grade surfactants. By step-wise replacing the hydrocarbon surfactant by a non-ionic, fluorinated, technical grade one, the efficiency of such systems was boosted by around 70%. Insights into the nanostructure of these efficient supercritical CO2-microemulsions were gained by scattering techniques: small angle neutron scattering (SANS) and dynamic light scattering (DLS). This only became possible after the design and construction of two high pressure cells HPSANS and HPDLS for SANS and DLS, respectively. The bicontinuous nanostructure of supercritical CO2-microemulsions was for the first time shown by SANS experiments, which revealed a domain size varying between 2 ? 26 nm. The droplet structure of supercritical CO2-microemulsions was investigated using both high pressure cells HPSANS and HPDLS, which yielded the same droplet diameter for a given system composition. Furthermore, pressure jump experiments performed in HPDLS showed a reversible dependency of the size of the droplet radius on the applied pressure: dropping the pressure increases, while compression decreases the droplet radius. Additionally, the first milestone in realizing fixable microemulsions was set by the successful preparation of a highly viscous microemulsion containing near-critical propane at a pressure of 250 bar and 75% sucrose (as model monomer in the hydrophilic phase). Consequently, the technical realization of the initial two production steps of novel nanofoams via POSME, based on supercritical CO2 microemulsions, was successfully achieved for the first time.