Xerogel morphology

In general, the structure of sol gel materials evolves sequentially as the product of successive and/or simultaneous hydrolysis and condensation and their reverse reactions (esterification and depolymerization). Thus, in principle, by chemical control of the mechanisms and kinetics of these reactions, namely the catalytic conditions, it is possible to tailor the structure (and properties) of the gels over a wide range. For example, stable silica xerogels of tailored particle dimensions, pore morphology, density and porosity, from relatively... 

Mariscal R., Lopez-Granados M., Fierro J. L. G., Sotelo J. L., Martos C. and Van Grieken R., Morphology and surface properties of titania-silica hydrophobic xerogels, Langmuir 16 (2()00) 9460-9467. 

Finally, visual insight into the aggregation mode and morphology in 46 was obtained by scanning electron microscopy (SEM) of dry samples, or xerogels. While the precursors show only negligible fiber formation, a 3D network was obvious for 46. 

We recently reported the switchable supramolecular self-assemblies on the basis of interaction between melamine group containing photochromic diarylethene unit (DTE) and naphthalimide derivate (33) with similar triple hydrogen bonds (Fig. 3.27). The gel morphology and the contact angels of the xerogel could be facilely tuned by the ratio of DTE and light stimulus [68]. 

Xerogel silica was prepared from tetraethoxysilane in the presence of different porphyrins and a template (e.g. pyridine) in an alcohol/water mixture at between 25 and 120 °C [48]. The following metal-free or Fe complexes of porphyrins were employed 2 with pentafluorophenyl, 2.6-dichlorophenyl-3-sulfonic acid, 4-carboxyphenyl or 4-methylpyridinium instead of -C6H4R. A mixture of 3 mL tetraethoxysilane and 3 mg of the porphyrin was used. It was found that the structure and morphology of the inorganic/organic hybrid materials depended on the kind of porphyrin and the xerogel preparation conditions. Side selection spectra and vibronic analysis were carried out for metal-free 2 (-CeHs, -CeFs... 

More recently, Hao et cd. investigated the influence of various metal catalysts on the morphologies of nanostructmed SiC produced by a sol-gel route, using lanthanum nitrate as an additive to prepare a xerogel [120]. The xerogel was converted, via a VLS mechanism, into bamboo-like SiC nanofibers with diameters of 40-100 nm and lengths of hundreds of micrometers. 

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