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Platschek, Barbara (2007): Ordered Mesoporous Silica: Control of Morphology and Exploration with Single Molecules. Dissertation, LMU München: Faculty of Chemistry and Pharmacy
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Abstract

This thesis is focused on the formation of highly ordered mesoporous structures with adjustable orientation within the ordered, vertical channels of anodic alumina membranes (AAMs) under the conditions of the ‘evaporation-induced self-assembly’ (EISA) method. It is shown that three of the most often used structure directing agents (CTAB, Pluronic123 and Brij56) can be employed for the synthesis of ordered silica mesophases embedded in AAM-channels. When using the ionic surfactant CTAB, the hexagonal mesostructures were solely oriented along the AAM-channels (columnar orientation). With the non-ionic surfactants the occurrence of two different orientations (circular or columnar) can be tuned via changing the surfactant concentration and the humidity in the adjacent gas phase. The existence of a previously postulated circular lamellar phase could be proven. In situ scattering experiments show that when ionic CTAB was used as structure directing agent, the columnar hexagonal structure was observed to form directly from the beginning. In the samples synthesized with the non-ionic surfactants, the circular hexagonal structure was found to form first and to directly transform into the columnar hexagonal, or a mixture of the columnar hexagonal and the curved lamellar phase. By correlation of the structure formation with the weight loss during the drying process of the samples it was demonstrated that the structure formation starts after the solvent evaporation. Therefore the absence of an evaporation induced solvent gradient within the AAM channels is postulated, and a homogeneous nucleation and phase formation irrespective of the height within the AAM channels is proposed. The synthesis of pure columnar mesostructures in confined space was achieved by salt addition and temperature control, even when non-ionic surfactants were used, thus leading to salt-induced phase transformations (SIPT) that have not been previously reported. In a collaborative project, single dye molecules were used as nanoscale probes to map out the structure of mesoporous silica channel systems that are prepared as thin films. The dye molecules act as beacons while they diffuse through different structural phases of the host. While measurement of ensemble diffusion provides information about the overall behaviour of the guest in a porous host, tracking of individual molecules provides insight into both the heterogeneity and the mechanistic details of molecular diffusion as well as into the structure of the host. The structure of the trajectories, the diffusivities and the orientation of single molecules are distinctive for molecules travelling in different mesophases. Transitions between the different types of surroundings can be observed for the same individual dye molecule. These experiments reveal unprecedented details of the structure of the host, its domains and the accessibility as well as the connectivity of the channel system.