Transparent gels

Example. The Pechini method for fuel cell electrode preparation. La, Ba, Mn niU ates - - CgHgO — citrate complex - - C2FI6O2 — gel. Metal nitrates are complexed with citric acid, and then heated with ethylene glycol to form a transparent gel. This is then heated to 600 K to decompose the organic content and then to temperatures between 1000 and 1300K to produce tire oxide powder. The oxide materials prepared from the liquid metal-organic procedures usually have a more uniform particle size, and under the best circumstances, this can be less than one micron. Hence these particles are much more easily sintered at lower temperatures than for the powders produced by tire other methods. 

Thiourea will react with neutralised formalin at 20-30°C to form methylol derivatives which are slowly deposited from solution. Heating of methylol thiourea aqueous solutions at about 60°C will cause the formation of resins, the reaction being accelerated by acidic conditions. As the resin average molecular weight increases with further reaction the resin becomes hydrophobic and separates from the aqueous phase on cooling. Further reaction leads to separation at reaction temperatures, in contrast to urea-formaldehyde resins, which can form homogeneous transparent gels in aqueous dispersion. 

Vinylphenyl-terminated PBE dendrons were prepared as polymerizable den-drons from 4-vinylbenzyl chloride [37]. The vinylphenyl-terminated PBE dendrons are useful to make the lanthanide-cored dendrimer complexes polymerizable. The Ist-generation Tb +-cored dendrimer complex bearing the vinyl-phenyl terminal on the dendron subunits (Fig. 5) was copolymerized with N-iso-propylacrylamide in the presence of methylene bis-acrylamide (as crosslinker) in DMSO to give a green-luminescence transparent gel. The DMSO gel was con-... 

Candy starch jellies include sugar and (modilied) starch boiled to a certain viscosity and poured into a starch mold to form semi-solid jelly. Water-soluble synthetic colorants are generally added at concentrations of approximately 6% before the mixture is placed in gel-forming blocks. The shape and thickness of the final semi-transparent gel and subsequent coating with sugar sand may cause the color to become shaded. Natoal colorants are rarely used for such applications due to their low stability to temperature and pH. 

Typical characterization of the thermal conversion process for a given molecular precursor involves the use of thermogravimetric analysis (TGA) to obtain ceramic yields, and solution NMR spectroscopy to identify soluble decomposition products. Analyses of the volatile species given off during solid phase decompositions have also been employed. The thermal conversions of complexes containing M - 0Si(0 Bu)3 and M - 02P(0 Bu)2 moieties invariably proceed via ehmination of isobutylene and the formation of M - O - Si - OH and M - O - P - OH linkages that immediately imdergo condensation processes (via ehmination of H2O), with subsequent formation of insoluble multi-component oxide materials. For example, thermolysis of Zr[OSi(O Bu)3]4 in toluene at 413 K results in ehmination of 12 equiv of isobutylene and formation of a transparent gel [67,68]. 

B. Jaillard, L. Ruiz, and J. C. Arvicu, pH mapping in transparent gel using colour indicator video-densitometry. Plant Soil /83 85 (1996). 

A colorimetric assay for lecithin and choline was described by Kotsira and Klonis (1998) using two enzymes (phospholipase and choline oxidase) and an indicator dye conjugate (bromothymol blue-glutathione) co-immobilised on a glutaraldehyde-activated polyacrylamide transparent gel. The change of the... 

Characteristic microstructural properties of TiOj membranes produced in this way are given in Table 2.5. Mean pore diameters of 4-5 nm were obtained after heat treatment at T < 500°C. The pore size distribution was narrow in this case and the particle size in the membrane layer was about 5 nm. Anderson et al. (1988) discuss sol/gel chemistry and the formation of nonsupported titania membranes using the colloidal suspension synthesis of the type mentioned above. The particle size in the colloidal dispersion increased with the H/Ti ratio from 80 nm (H /Ti = 0.4, minimum gelling volume) to 140 nm (H /Ti " — 1.0). The membranes, thus prepared, had microstructural characteristics similar to those reported in Table 2.5 and are composed mainly of 20 nm anatase particles. Considerable problems were encountered in membrane synthesis with the polymeric gel route. Anderson et al. (1988) report that clear polymeric sols without precipitates could be produced using initial water concentrations up to 16 mole per mole Ti. Transparent gels could be obtained only when the molar ratio of H2O to Ti is < 4. Gels with up to 12 wt.% T1O2 could be produced provided a low pH is used (H /Ti + < 0.025). 

In addition to calcium, we note that divalent manganese and zinc and trivalent aluminium can also induce a-lactalbumin nanotube formation. By using these ions instead of calcium at R = 3, transparent gels (see... 

More that twenty years ago, Doi and Kitabatake (1989) established that the transition between transparent gels and opaque gels occurs over a relatively narrow range of salt concentrations or pH for different types of... 

The friction is expected to be small for such an inhomogeneous gel, since water flows choose mainly the dilute regions that serve as the more open and free paths, while avoiding the denser regions that block the flow. Namely, the opaque gel behaves as a porous sponge for water flow, and the more opaque, the larger the pore size. The opaque gel, therefore, has a much smaller friction coefficient than a transparent gel. 

Few data are available on the hydrolysis of simple metal alkoxides of these elements. Alkoxides of alkaline and alkaline earth metals are mostly used as precursors for the preparation of complex oxides or solid oxide solutions. Commercial production of pure magnesium oxide by hydrolysis of Mg(OMe)2 with formation of transparent gel has been described [715], as well as hydrolysis of Mg(OC5H11i)2 with the following thermal treatment to produce a fine MgO powderthat sinters at low temperatures [1766]. Solutions prepared by dissolving magnesium in methoxyethanol are by far the most convenient precursors for preparation of magnesium oxide films. 

Another facet of the Me3SiN=C=NSiMe3 chemistry is that it can be reacted with chlorosilanes to obtain transparent gels (equation 17). 

The properties of the gels were investigated in detail [102] and it was found that well-defined cylinders built up from 78 are present in the transparent gels offering the possibility of ion transport. Closer investigation of the membranes [98, 99] revealed that MNO3, MCI, and MCIO4 (M = Li, Na, K) can pass the membranes. 

Even if the networks of gels have similar length scales, the kinetics of gel formation can be quite different both with regard to the molecular composition and the conditions during gel formation. This will be demonstrated by a comparison of kappa-carrageenan and pectin. They both form transparent gels at very low polysaccharide concentrations and water can apparently be solidified yet without changing its physical properties by gel formation at a polysaccharide concentration below 1%. 

It is known from Table 1 that gel time increases with the increase of water volume for hydrolysis. Transparent gel body can be obtained under 10 ml or 20 ml of water, whereas it would become opaque and show light white when water volume increases to 40 ml. The structure of gel body will become loosen and a little solution will bleed under the following drying, gel can not be formed when water volume rises up to 50 ml, white... 

The experimental result reveals that pH value has great effect on gel time. From the result shown in Table 3, it can be seen that gel time increases with the fall of pH value. When pH value is equal or down to 3, it is not gel body but white precipitate that is obtained when mixing solution is heated in water-bath at 343K for 8 h. When the pH value is 4 5, transparent gel can be obtained. 

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