Surfactant mesophases

Recently, a detailed study was reported for a molecularly ordered layered silicate surfactant mesophase [125, 126]. A subtle change in the surfactant which is used for the synthesis of such mesophases appears to induce dramatic modifications in the silica ordering. When cetyltrimethylammonium (C16NMe3+) cations are employed, then only two 29Si NMR resonances at —102 ppm (Q3) and... 

Gibaud, A. Grosso, D. Smarsly, B. Baptiste, A. Bardeau, J. F. Babonneau, F. Doshi, D. A. Chen, Z. Brinker, C. I Sanchez, C. 2003. Evaporation-controlled self-assembly of silica surfactant mesophases. J. Phys. Chem. B 107 6114-6118. 

Directed Synthesis of Micro-Sized Nanopiateiets of Goid from a Chemicaiiy Active Mixed Surfactant Mesophase... 

Figure 16.2 Possible mechanisms of the formation of mesoporous silica materials, by generation of hexagonal silica-surfactant mesophases horn the interaction of silica precursor with (a) spherical micelles, (b) rod-like micelles, or (c) a preformed liquid crystalline phase of the surfactant.

Although PMOs and related materials have many potential applications, including adsorbants,49,50 sensors,51 and catalysts,52,53 we focus here on two specific classes of functional materials, nanovalves and polymer-silica nanocomposites formed via the co-assembly of hybrid precursors (or polymer-monomer species) with surfactant mesophases. 

An example of the first approach is the integration of hydrogels into nanostructured silica films by addition of a suitable monomer (e.g., methyl methacrylate, /V-isopropyl acrylamide, etc.) and an initiator for radical polymerization to a solution containing a structure-directing surfactant and a prehydrolyzed silica precursor. During self-assembly, the monomers partition within the hydrophobic core of the surfactant mesophase postsynthesis polymerization (for instance, by UV treatment) followed by solvent washing to remove the surfactant template yields a polymer-silica nanohybrid. 

Davis S. A., Burkett S. L., Mendelson N. FI. and Marm S., Bacterial templating of ordered macrostructures in silica and silica-surfactant mesophases. Nature 385 (1997) pp. 420-423. 

While there have been efforts to polymerize other surfactant mesophases and metastable phases, bicontinuous cubic phases have only very recently been the subject of polymerization work. Through the use of polymerizable surfactants, and aqueous monomers, in particular acrylamide, polymerization reactions have been performed in vesicles (4-8). surfactant foams ), inverted micellar solutions (10). hexagonal phase liquid crystals (111, and bicontinuous microemulsions (121. In the latter two cases rearrangement of the microstructure occured during polymerization, which in the case of bicontinuous microemulsions seems inevitable b ause microemulsions are of low viscosity and continually rearranging on the timescale of microseconds due to thermal disruption (131. In contrast, bicontinuous cubic phases are extremely viscous in genei, and although the components display self-diffusion rates comparable to those... 

Recently reported meso- and macroscale self-assembly approaches conducted, respectively, in the presence of surfactant mesophases [134-136] and colloidal sphere arrays [137] are highly promising for the molecular engineering of novel catalytic mixed metal oxides. These novel methods offer the possibility to control surface and bulk chemistry (e.g. the V oxidation state and P/V ratios), wall nature (i.e. amorphous or nanocrystalline), morphology, pore structures and surface areas of mixed metal oxides. Furthermore, these novel catalysts represent well-defined model systems that are expected to lead to new insights into the nature of the active and selective surface sites and the mechanism of n-butane oxidation. In this section, we describe several promising synthesis approaches to VPO catalysts, such as the self-assembly of mesostructured VPO phases, the synthesis of macroporous VPO phases, intercalation and pillaring of layered VPO phases and other methods. 

S.A. Davis, S.L. Burkett, N.H. Mendelson, and S. Mann, Bacterial Templating of Ordered Macrostructures in Silica and Silica-Surfactant Mesophases. Nature (London), 1997, 385, 420-423. 

Packing parameter Micellar shape Surfactant Mesophase example... 

S.A. El-Safty and T. Hanaoka, Monolithic Nanostructured Silicate Family Templated by Lyotropic Liquid-crystalline Nonionic Surfactant Mesophases. Chem. Mater., 2003, 15, 2892-2902. 

Si - Solid-state Si NMR, including 2D heteronuclear and homonuclear NMR measurements, has been employed to study molecularly ordered inorganic frameworks in layered silicate surfactant mesophases. " ... 

Despite that the silicate-surfactant mesophase formation resembles the phase separation normally observed in surfactant-polyelecholyte systems, it is interesting to note that it is stiU possible to make qualitative predictions about the influence of inorganic-surfactant phase behavior based on models developed for dilute surfactant systems. The packing parameter concept - is based on a geomettic model that relates the geomehy of the individual surfactant molecule to the shape of the supramolecular aggregate structures most likely to form. N, is defined as... 

FIGURE 8.33 Schematic representation of the experimental setup used for in situ synchrotron SAXS measurements on the formation of mesoscopically ordered silicate-surfactant mesophases. The reactant solution is pumped through a thin quartz capillary and a diffractogram is recorded every 300 ms. The reaction is initiated by emulsifying a macroscopically phase-separated system of tetraethoxysUane and an aqueous phase, respectively. See text for details. 

FIGURE 8.34 Development of the in situ XRD pattern as a function of time during the formation of a hexagonal silicate-surfactant mesophase in the system TEOS-water-CTAB-ammonia. (From Agren, P. et al., Langmuir, 16, 8809, 2000. With permission of the American Chemical Society.)... 

O Brien, S. et al.. Formation of silica-surfactant mesophases studied by real-time in situ x-ray powder diffraction, J. Chem. Soc. Chem. Commun., 2423, 1995. 

Denoyel, R. and Sabio Rey, E., Solubilization in conhned surfactant mesophases, Langmuir, 14, 7321, 1998. 

Lind, A. et al., Controlled oil solubilization by silicate-catanionic surfactant mesophases as studied by in situ and ex situ XRD, Langmuir, 18, 1380, 2002. 

Rheology is increasingly being coupled to other analytical test methods for more comprehensive material characterizations. Many of these developments are driven by research needs for broadened characterization capability. For fundamental studies of detergent systems this offers a broad suite of methods to probe surfactant mesophases and internal microstructure. 

Fowler, CE Burkett, SL Mann, S Synthesis and characterization of ordered organosilica— surfactant mesophases with functionalized MCM-41-type architecture. Chem. Commun. 

Even for polymerizable surfactants, curvature control is still very important, and Eastoe et al. describe mixtures of polymerizable surfactants that retain the same preferred curvature before and after cross-linking [44, 45]. Current approaches to the one-to-one replication of bicontinuous or other surfactant mesophases without the use of polymerizable surfactants involve the use of surfactants of high molecular weight [46] or highly viscous cubic phases featuring slower rearrangement dynamics. 

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