Controlling interfacial curvature in nanoporous silica films formed by evaporation-induced self-assembly from nonionic surfactants. I. Evolution of nanoscale structures in coating solutions
The double-gyroid phase of nanoporous silica films formed by evaporation-induced self-assembly (EISA) has been shown to possess facile mass-transport properties and may be used as a robust template for the nanofabrication of metal and semiconductor nanostructures. Recently, we developed a new synthesis of double-gyroid nanoporous silica films where the aging time of the coating solution prior to EISA was the key parameter required to control the interfacial curvature that results upon self-assembly of the film. Here, we use Si-29 nuclear magnetic resonance (NMR) and small-angle X-ray scattering (SAXS) to investigate the nanoscale structure of the coating solutions used to obtain double-gyroid nanoporous silica films. NMR and SAXS were carried out on the water, ethanol, silica, and poly(ethylene oxide)-b-poly(propylene oxide)-b-alkyl (EO17-PO12-C-14) surfactant coating solutions as well as similar solutions that excluded either the silica or the surfactant. NMR data reveal that the silica monomers in the coating solution condense very rapidly to form rings and connected ring species. After 1 day of aging, all monomers and dimers have disappeared, and the distribution is dominated by Q(2) and Q(3) species, where the superscript in Q(n) describes the number of silicon atoms in the second coordination shell of the central silicon. Over the course of the next 9 days, the Q(3) population slowly rises at the expense of the Q(2) and Q(3t) populations. Absolute intensity SAXS measurements reveal that the size of the silica clusters increases steadily during this aging period, reaching an average radius of gyration of 9.0 A after 9 days of aging. Longer aging results in the continued growth of clusters with a mass fractal dimension of 1.8. Absolute intensity SAXS data also reveals that micelles are not present in the coating solution. At 9% volume fraction of surfactant, the coating solution is far above the aqueous critical micellar concentration. However, even a small amount of ethanol inhibits micellization. SAXS data also shows that when surfactant is present the radius of gyration is larger but increases more slowly. This indicates that there are weak associative interactions between the silica clusters and surfactant in solution that reduce the cluster-cluster growth rate. In part II of this work, we use the results discovered here to interpret the effects of aging on interfacial curvature in the nanostructured films that self-assemble from these solutions.
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