Molecular self-assembly for the preparation of novel nanostructured materials

Resumo

In this thesis, the phase behavior in water of a technical grade diglycerol-based surfactant (C41V) and a group of dyes (Quinaldine Red, Pyronin Y, N-Alkylthiacarbocyanines, Oxacarbocyanines, Pinacyanol and Alcian Blue) was studied. Several experimental techniques such as polarized optical microscopy, small and wide X-ray scattering, dynamic light scattering, UV-Vis and NMR spectroscopy, were used to characterize the different phases and get insight into the molecular self-assembly behavior. The phase diagram of the water/C41V system is characterized by a wide region at temperatures lower than 70 ºC in which an inverse hexagonal liquid crystal (H2) coexists with excess water. The H2 phase could be successfully dispersed in water in the form of nanoparticles (hexosomes). A hydrophobic drug (Ketoprofen) was encapsulated in the hexosomes and its release to a receptor solution showed a non-Fickian diffusion profile. Excess of glycerol and surfactants with higher degree of esterification were separated from C41V. It was found that glycerol causes phase segregation at high C41V concentrations. The spontaneous curvature of the surfactant system can be tuned by mixing the purified C41V with the surfactants with higher degree of esterification. Structural characterization of dyes in water reveals that the molecules stack in columnar aggregates, which increase in size with concentration. At relatively high concentrations (ca. > 30 wt%) Quinaldine Red and Pyronin Y form a Nematic chromonic liquid crystal composed by columns with unimolecular cross-section. Additionally, at higher concentrations, Quinaldine Red forms a chromonic rectangular phase, which indicates that the columns cross-section is anisotropic. UV-Vis spectroscopy suggests that N-Alkylthiacarbocyanines stack face to face. The dimer model was used to fit the UV-Vis spectra and to estimate the dimeric constant and the intermolecular dissociation. The intermolecular interactions increase with the alkyl chain length (from 19.2 kBT to 20.5 kBT) suggesting that not only aromatic interactions but also the hydrophobic effect contribute to the aggregation process. From small angle X-ray scattering measurements it can be inferred that the molecular columns are hollow. By applying the exciton theory to the UV-Vis spectra of Oxacarbocyanines it was found that the stacking angle of these dyes is around 54.7 º. It was also found that the molecular interactions can also be tuned by varying the carbon atoms in the poly-methine spacer of cyanine dyes. Alcian Blue showed evidence of stacking in aqueous solution, however, due to the poor solubility of this dye in water no liquid crystal was formed; nevertheless, Alcian blue solubility increases at acidic pH and liquid crystals can be formed. Cyanine dye aggregates were used as templates for silica synthesis by a sol-gel reaction. Silica nanofibers with high surface area (i.e. 230 m2/g) and hierarchical arranged mesopores were obtained. Furthermore, silica nanofibers were used as hard templates to produce carbon materials. The resulting templated carbon nanofibers were used as functional materials in energy storage and sensing applications. Carbon nanofibers showed superior performance as double layer capacitors (capacitance values of ca. 3000 F/g) and presented potential as sensing materials for aromatic compounds, especially pyridine.