Ordered Mesoporous Silica: Control of Morphology and Exploration with Single Molecules

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.