Silicic condensation

The particle size of a fission aerosol, and the distribution of fission products between particulate and vapour phases, depends on the mechanism of release to the atmosphere. In a weapons explosion, some physicochemical fractionation of radionuclides may occur, particularly if the explosion is near the ground. Everything in the vicinity is vapourised by the heat of the explosion, but within less than a minute the fireball cools to a temperature in the range 1000-2000°C, and refractory materials such as metal oxides and silicates condense to form particles (Glasstone Dolan, 1977). Refractory fission products, and plutonium, are incorporated in these particles. 

For these reasons we have come to the tentative conclusion that the D5R silicate condensation mechanism is not generally operative in the synthesis of MFI-type zeolites. 

One may summarize the various observations as follows. In strongly acid solutions, A1(H20)63+ is dominant. From pH = 6 upwards, Al3+ becomes wholly insignificant, polymeric ions are no longer important, and Al(OH)4 or A102 are dominant. The important Al(OH)4 is tetrahedral in structure and so should favor tectosilicate formation with silicates condensed into a 3-D framework. 

In this respect, the recently discovered family of materials synthetized by silicate condensation around surfactant micelles (Micelle-Templated Silica, MTS) provides unique inorganic support [11,12] due to their regular arrays of uniformly sized channels in the mesopore range of 20-100 A. This solid was recently impregnated with cesium oxides with the aim to obtain superbase catalysts [13]. Nevertheless, the leaching of the basic particles cannot be excluded. In view to avoid such possible phenomena, we have studied the covalent attachment of basic functions such as amino groups on the MTS surface... 

Synonyms/Trade Names Ethyl orthosilicate, Ethyl silicate (condensed), Tetraethoxysilane,... 

Figure 4.7 Silicate condensation reactions simulated computationally by Lewis et Free energy calculations, taking the effect of solvent...

The subsequent addition of tetralkoxysilane (TEOS) in acidic (or even neutral) conditions leads to the formation of a silica core, due to the fact that, especially before hydrolysis, alkoxysilane are rather apolar species and tend to migrate and accumulate in the central part of the polymeric micelles, the more hydrophobic area where the silicate condensation is promoted. This induces the formation of the silica nanoparticles only inside the micelles, as the condensation proceeds, leading to the entrapment of the surfactants molecules and to the final silica core-PEG shell architecture (Fig. 13). 

ETHYL SILICATE (C.HJ iO, Ethyl silicate, condensed Tetraethyl sllk Ethyl ortho Uicatc Tct net boxy-silane Strong oxidizers, water 99 1.3 23... 

Opacity, 363, 380-382, 465, 471, 474, 475, 773. See also Phase separation Opal, 97, 266-267 Opalescence, 266, 273 Optical film absorption, 841 antireflective, 842-847, 852 ferroelectric, 846 graded refractive index, 844-847 interference, 842 laser damage resistant, 842 multilayer, 844, 846 optoelectric, 846, 847 porosity, 842 problems, 846 reflectance, 841 refractive index, 842-848 solar energy applications, 844 transmission, 841 Optically active molecules, 760 Optical waveguide, 288, 292-295, 695, 701 See also Fiber optic preform Ordered domains. See Domain, ordered Ordering, 259, 266-268, 270, 831 of liquid layers, 411-412, 422-424, 460 of particle sols, 821-823 Order of reaction silicate condensation, 146 silicate hydrolysis, 123, 154 silicon alkoxide hydrolysis, 118, 130... 

The poisoned Eden mode) is relevant to silicate condensation under neutral conditions where the hydrolysis rate is minimized. Ether-forming (ROR) condensation is forbidden, and the rate of the alcohol-forming condensation reaction is lower than that of the water-forming condensation reaction [6,95]. Therefore, unhydrolyzed alkoxide sites, which are most abundant at neutral pH, act as poisons by inhibiting condensation (at least temporarily). Keefer argues that once unhydrolyzed sites are incorporated in the cluster, their hydrolysis may be impeded due to steric factors [136], If so, the poisoned Eden model would be physically and chemically relevant. 

MCM-41 mimicked the surfactant s liquid crystal mesophase in aqueous solutions. As a consequence, it was advanced that the siufactant forms a liquid crystal mesophase and the siUca species organize in the space around the surfactant micellar rods interacting with the polar heads of the surfactant molecules. The silicate condensation occurs in the second step once this organization has been set up and traps it. Alternatively, they have postulated that the liquid crystal phase does not pre-exist but forms upon the addition of the inorganic species. It was later evidenced that the MCM-41 synthesis takes place at concentrations lower than that necessary to form the liquid crystal phase, so the second pathway is more plausible when working in dilute media while the first one is true in more concentrated syntheses [11]. 

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