46302-AC5
Structure Directing by Nanoaggregates from the Molecular to the Mesoscopic Scale
This research project focuses on structure directing by nanoaggregates where we follow the process on both the molecular and the mesoscopic scales in order to understand the intricate relation between the two. We first investigated the formation mechanism of the bicontinuous cubic (Ia3d) mesoporous material KIT-6. KIT-6 is synthesized at 40°C with Pluronic P123 (PEO20PPO70PEO20) as template, low acid concentration and n-butanol. Through in-situ EPR (electron paramagnetic resonance) and freeze-quench electron spin echo envelope modulation (ESEEM) measurements on a series of spin-labeled Pluronic probe molecules we followed the evolution of the hydrophobicity and the mobility of the polymer chains as well as the water and butanol content within the P123 micellar structures during the reaction. The evolution of the solution nanostructures was followed by cryogenic-transmission electron microscopy (cryo-TEM) and at the very early stages of the reaction by double-electron-electron- resonance (DEER). At these early stages of the reaction, the micellar structures are not well resolved in the cryo-TEM micrographs. Initially, the aqueous solution, held at 40°C, consists of Pluronic P123 micelles, which are characterized by a hydrophilic corona, from the polyethylene oxide (PEO) chains, and a hydrophobic core, consisting of the polypropylene oxide (PPO). The variations in the volume of the hydrophobic core of the micelles were evaluated using a hydrophobic spin-probe, 4-hydroxy-tempo-benzoate (4HTB), which is localized in the hydrophobic core of the micelles. We observed swelling of the hydrophobic core due to penetration of the silica source (TEOS, tetra-ethoxy-orthosilane) and its hydrolysis products into the micelles. It reached a maximum at ~10 min., then the swelling decreased, and the micelles reached almost their original size at ~ 20 min due to the migration of fully hydrolyzed TEOS into the core and the solvent.
After these initial stages the following reaction stages were detected: during the first 140 min of the reaction a depletion of water and butanol molecules from the core-corona interface, concomitant with the restriction of the mobility of the ends of the Pluronic chains located at the corona-water interface, was observed. This was ascribed to the condensation of the silica oligomers at the micellar/water interface. This in turn lead to a transition from spheroidal micelles to threadlike micelles and to their aggregation towards the end of this stage. Next, precipitation (140-160 min), reorganization in the micellar structure, and a change in the relative sizes of core and corona take place. The next stage that ends around 6 hrs involves the formation of a hexagonal phase through accelerated condensation of silica oligomers in the corona, accompanied by extensive depletion of water and butanol molecules.
We found that presence of butanol in the micelle corona is essential only in the last stage, 6-24 hrs, when the cubic phase is formed. We showed that butanol can be added to the original reaction mixture of of SBA-15, after the formation of hexagonal phase leading to the transformation to the cubic phase. This last stage of the reaction, where the transformation to the final phase takes place, was studied in more detail for both preparation. We found that the addition of the butanol allows curvature decrease that causes the transition from hexagonal phase, probably through lamellar phase to the final cubic phase. The synthesis of SBA-15 with added butanol gave a more regular homogeneous cubic product. The reason for this is still to be determined.
Currently we are studying the formation mechanism of mesoporous materials prepared with anionic surfactants as a template and a co-structure-directing agent, that is an aminosilane or a quaternized aminosilane. Here we have started with the sodium salt of long-chain alkyl acids and follow the formation of hexagonal and cubic structures that form at different pH. We have selected a silane based and fatty acid based spin-probes to follow the reaction and currently we are searching for the conditions of synthesis within the EPR cavity.
