AmericanChemicalSociety.com

    This research project focused 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.  A major part of the work was directed towards the understanding of the formation mechanism of   of templated mesoporous materials, where highly ordered mesoporous materials are prepared using surfactant self-assemblies as templates, is an intriguing process.  It depends on a delicate interplay between several concomitant basic processes; the self-assembly of the surfactant molecules forming structures that serve as templates, the sol-gel chemistry that generates the inorganic silica network, and the specific interaction at the interface between the organic and forming inorganic phases. We have studied this process on the molecular level by introducing nitroxide  spin-probes, designed to examine different regions in the forming mesostructure, into the reaction mixture and applying a variety of EPR techniques. Continuous wave (CW)  EPR measurements, carried out  in-situ, gave information on the polarity and microviscosity in the close environment of the spin-probe. These were complemented by electron-spin echo modulation (ESEEM) experiments that follow the water content, presence of additives and interaction with ions and provide an understanding of their effect  on the final material. Finally double electron-electron resonance (DEER) measurements are used to explore size variation of the micelles during the initial stages of the reaction. Information regarding the mesoscale aspects of the reaction, namely the detection and evolution of intermediate structures in solution and the phase transformations occuring during the reaction were followed using cryogenic electron microscopy microscopy. Here we ombined cryo-TEM (trasmission electron microscopy), cryo-SEM (scanning electron microscopy)  and FFR (freeze fracture replication) micrscopy.

The material we focused on was KIT-6 which has a bicontinous cubic strcture and is synthesized with Pluronic P123 at low acid concentration and n-butanol at 40°C.¹  We found that at the beginning of the reaction, when the silica precursor is added, only spheroidal micelles are present, and condensation of silica oligomers takes place at the micellar/water interface and inside the corona of the micelles. This condensation causes depletion of water and butanol from the micelles core towards their corona, leading to a decrease of curvature, followed by the elongation of the micelles into threadlike micelles (TLMs). At the end of that first stage of the reaction the TLMs form bundles and then precipitation occurs.  An accelerated condensation of silica in the precipitate is detected, associated with the formation of a hexagonal phase. In addition, water and butanol molecules diffuse from the corona to the corona/water interface, thus reducing the polarity of the corona, and promoting a curvature decrease at the last stage of the reaction, when a transition from the hexagonal to the cubic phase takes place. The presence of butanol in the micelle corona is essential in the last stage, 6-24 h, where the cubic phase is formed. This final stage  involves cylinder merging for the hexagonal to a perforated perforated layer (PL) phase. Upon additional polymerization of the silica, the structure relaxes into the stable cubic phase. Another minor mechanism detected involves the direct transition between the hexagonal to the final cubic phase through cylinder branching. The mechanistic studies lead to a modification of the preparation procedure, which gave a better ordered matrial.
  In addition we studied siliceous mesoprours materials prepared with anionic surfactants (AMS)², specifically the hexagonal and cubic phases prepared with alkyl carboxylic acids as templates.   These types of materials require a co-structure-directing agent , an alkoxysilane (N-trimethoxylsilylpropyl-N,N,N trimethylammonium chloride) that mediates the interaction between the polymerizing silica and the precursor micelles of the surfactant. Here we have used two types of spin labels to study the reaction, one that is a nitroxide labeled surfactant molecule and the other is a nitroxide attached to an alkoxysilane that resembles the co-surfactant.  Using the EPR experiments we detected the specific interaction between the surfactant and the co-surfactant and the functionalization of the internal surface of the pores by the co-surfactant tertiary amino groups. The formation of the final material evolves through the formation of a silica-co-surfactant-surfactant hybride  mesophase that can be detected within the first 10 min. Here we have not yet carried out TEM measurements.

 1. T. W. Kim,; F.  Kleitz, B.  Paul,  R. Ryoo, J. Am. Chem. Soc. 2005, 127 (20), 7601-7610.
2. C. Gao, H. Qiu, W. Zeng, Y. Sakamoto,O. Terasaki, K. Sakamoto, Q. Chen, S. CheChem. Mater. 2006, 18, 3904-3914