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Siegel der Universität
Department für Chemie - Arbeitsgruppe Prof. Strey

Microemulsions as compartmentalised reaction media: structural characterisation of water-in-oil microemulsions

Tobias Foster, august 2006

In this study the different nanostructures in water-in-oil-microemulsions, like spheres, cylinders or networks of branched cylinders, and their transitions were systematically characterised. These complex fluids provide compartmentalised environments with variable connectivity to study biologically relevant, chemical reactions on a single-molecule level. To solubilize biologically relevant macromolecules like DNA, microemulsion droplets with a radius of up to 50 nm are required. Thus, highly efficient water-in-oil microemulsions were prepared containing water, cyclohexane and the non-ionic surfactant C12E6. Systematic small angle neutron scattering (SANS) experiments were conducted to determine the type and the size of the microstructure. The data was analysed with a newly derived scattering form factor for Gaussian shells with droplet-core scattering contribution, yielding droplet radii of up to 35 nm. In an alternative approach, highly efficient oil-rich microemulsions were formulated by adding small amounts of amphiphilic block copolymers of the type poly(ethylenepropylene)-co-poly(ethyleneoxide) to water ? n-decane ? C10E4 microemulsions. To analyse the SANS-data from these systems, a form factor for core-shell particles with the polymer-chain scattering contribution was derived, yielding droplet radii of up to 30 nm. Moreover, the hydrophobic polymer blocks that surround the microemulsion droplets were shown to be stretched with increasing polymer surface density and/or decreasing droplet radius. Further temperature-dependent SANS-experiments have been conducted to follow the structural transitions, namely the droplet-to-network transition in water-in-oil-microemulsions. In order to describe the scattering from the cylindrical network, two novel form factors have been derived in collaboration with Samuel Safran: (i) for the surface scattering from a single network junction and (ii) for the scattering from a network junction to which three rods with spherical cross-section are attached. Using the second form factor (ii) it was demonstrated that with increasing water content of the microemulsion, both the radius of the cross-section and the length of the cylindrical network-branches increase.