Monolithic gels

Chenite et al. reported on thermosensitive chitosan gels for encapsulating living cells and therapeutic proteins they are liquid below room temperature but form monolithic gels at body temperature [220-223]. 

Replacement of the hydrophilic acrylamide by the more hydrophobic N-iso-propylacrylamide, in combination with the pre-functionalization of the capillary with (3-methacryloyloxypropyl) trimethoxysilane, afforded a monolithic gel covalently attached to the capillary wall. A substantial improvement in the separations of aromatic ketones and steroids was observed using these fritless hydrogel columns, as seen by the column efficiencies of 160,000 found for hydrocortisone and testosterone [92]. The separations exhibited many of the attributes typical of reversed-phase chromatography and led to the conclusion that, in contrast to the original polyacrylamide-based gels, size-exclusion mechanism was no longer the primary mechanism of separation. 

These reactions are responsive to both acid and base catalysis, and can be manipulated to give a variety of silica products, e.g., discrete particles, monolithic gels, films, and fibers. This technique of materials synthesis via alkoxide hydrolysis has become known as sol-gel processing (17). It should be noted, however, that under certain conditions, gelation may be confined only to the interior of discrete particles (base-catalyzed systems), while the sol may consist of polymeric networks rather than individual particles (acid-catalyzed systems). 

It can now be said that the microemulsion-mediated silicon alkoxide sol-gel process has come of age. The ability to form monodisperse spherical silica particles (20-39) and monolithic gels (40-53) by this method has been amply demonstrated. Recipes are available to prepare materials with predetermined characteristics, especially particle size and polydispersity. Potential applications of these microemulsion-derived... 

Substitution of methanol by isopropyl or n-butyl alcohol also prevented formation of opaque monolithic gels composed of micrometer-sized particles (7). TMOS solutions with mole ratios TMOS H2O HCl alcohol of... 

There are two families of monolithic gels obtained from organic solutions. The first is basically identical to those obtained in aqueous medium. Similarities come from the mineral compound, which undergoes gelation the solvent and additives are organic compounds. The second refers to gels prepared from alkoxides in organic solvent. This kind of gel is the most popular and has been extensively reported in the literature. 

When completely dense materials are to be obtained, porous monolithic gels must be densified by increasing the temperature. Residual organic removal and crystallization of amorphous gel are well-known phenomena and may be accomplished or avoided by using specific thermal schedules. 

Matsui et al. (141) showed the luminescence changes upon the sol-gel conversion of monolithic gels that are doped with [Ru(bpy)3] at different pH values and various H2O/TEOS ratios. A similar relative red shift of Xjnax, as with the study of Innocenzi, is observed after gelation of a sample made at pH 1 (Fig. 12) (141). 

Unified Interpretation of the Dq-Di and D2 Raman Bands of Disperse Silicas, Monolithic Gels, and Fused Silicas... 

We have developed such a multiphase composite using sol-gel processing which produces large size bulks of excellent optical quality (d). In this approach, we prepare highly porous monolith gel (in the present case silica) and thermally process it. The pores in the silica monolith under our processing conditions are in the nanoscale region ( 5 nm). This allows various molecules (such as fullerene) to be adsorbed in the pores by diffusion of a solution followed by evaporation of the solvent. Then the pores are filled with a polymerizable liquid such as methyl methacrylate (MMA) which is then... 

Propylene oxide (and other epoxides) were used to promote gelation of metal salts in alcoholic solutions. It was postulated that a proton from the [M(H20)] " ions is transferred to the epoxide, which then undergoes an irreversible ring-opening reaction with the anion of the metal salt used. The net effect is to add HX to the epoxide and thus to raise the pH of the solution slowly. It was found that this method only applies to metal ions with z S 3 the reason for this may be the lower acidity of the aquo complexes of mono- and divalent metals. Monolithic gels were obtained from lanthanide, Al, In, Ga, Zr, Hf, Ta, Nb, Fe , Cr , and w " salts. ... 

Zarzycki J, Prassas M, Phalippou J (1982) Synthesis of glasses from gels the problem of monolithic gels. 

Monolithic gels were obtained in 20 min at room temperature. After aging at 50°C for 1 h, the wet gels were immersed into polystyrene solution. Immersion conditions are the following the concentration of polystyrene (PS, M.W. 230,000, Aldrich Chemical Co.) was 0.1-11 g in 30 ml of THF, and immersion time was 1-100 h. The wet gels were also aged and dried without immersion to obtain the xerogels. 

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