In situ SAXS/XRD on mesoscopically ordered surfactant-silica mesophases; What can we learn?
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In situ SAXS/XRD on mesoscopically ordered surfactant-silica mesophases; What can we learn? Mika Lindén,1,* Cilaine V. Teixeira,1 Heinz Amenitsch,2 Viveka Alfredsson, 3 Freddy Kleitz4 1
Dept. Phys. Chem., Abo Akademi University, Porthansgatan 3-5, 20500 Turku, Finland Austrian Academy of Science, Schmiedlstrasse 6, 8043 Graz, Austria 3 Phys. Chem. 1, Lund University, 221 00 Lund, Sweden 4 Dept. Heterogeneous Catalysis, Max-Planck Institute for Coal Research, Kaiser-Wilhelm-Platz 1, 45470 Mülheim/Ruhr, Germany 2
*e-mail: [email protected]
Abstract In situ investigations have proven to be a very useful means of understanding the different processes involved in the formation of mesoporous materials. In this communication, we demonstrate the potential of in situ small angle x-ray scattering, SAXS and x-ray diffraction, XRD, measurements for giving both qualitative and quantitative results on the structural evolution during the early stages of the surfactant-silicate composite formation. The examples given are based on results obtained for 2D hexagonal structures, synthesized both under acidic and alkaline conditions. Careful analysis of both the scattering and diffraction patterns allows the different stages of the formation to be described in some detail. Thus, new synthesis approaches can be foreseen that allow the structure of the final hybrid mesophase to be rationally controlled. Introduction The determination of the mechanism of formation for surfactant-assisted formation of mesoporous silica in solution is a challenging task. There are pronounced changes in the interactions between the scattering objects and in the distribution of matter in the sol during the reaction. The initial concentration of the amphiphile is often within the region of the micellar phase, but the product achieves a highly ordered meso-structure that corresponds to higher concentrations in the phase diagram. Different experimental techniques probe different lengthscales and therefore a combination of techniques is often needed in order to get a complete picture of the nucleation and growth of the hybrid mesophase. For example, electron paramagnetic resonance (EPR),1,2 (synchrotron) low angle X-ray scattering/diffraction,3-9 fluorescence quenching,10 (cryo)-TEM,11-13 infrared spectroscopy,14 and 1H-NMR13 have all been used for in situ studies of developing silicate-surfactant mesophases. There is now some agreement that the development of the composite mesophase involves the following steps, at least in the case of 2D hexagonal silicate-surfactant mesophases, a) hydrolysis of the siliconalkoxide, (not in effect if inorganic silica precursors are used) b) attractive interaction between oligomeric silicate species and surfactant molecules leading to the formation of silicatesurfactant micelles, c) clustering of poorly condensed silica-surfactant hybrid micelles into flocs,
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d) elongation of the surfactant micelles and parallel formation of domains exhibiting hexagonal order, e) growth of the domains. In the following we will,
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