Silica soluble form stabilization

Keywords synthesis of silicon organic compounds, biogenic silica, silica solubility, silica soluble form stabilization, vicinal dihydroxy-organic compounds... 

At this point (the same pH as that of Wollast, t al., 1968) sepiolite begins to precipitate. In experiments maintaining pH at values above 9, montmorillonoids and talc were formed. Chemical analysis of the precipitates reveal a greater proportion of magnesium as the pH of the experiment is increased. Recalling the information on amorphous silica solubility, a two-fold increase in solubility of SiC>2 occurs between pH 8 and 10.5 (Krauskopf, 1959)—and thus at higher pH it could be expected that relatively less silicious phases would precipitate where the masses of Mg and Si are fixed. Final concentrations of Mg-Si in solution were not determined by Siffert and therefore thermodynamic calculations of mineral stabilities cannot be made. 

Study of these new catalysts is intensive. Small molecular-weight distribution was demonstrated by Petrova (112) and by Baulin et al. (113). In addition, polymer substrates have been used (114-116) in order to increase lifetime and activity. As shown by Suzuki (36), stabilization is caused by inhibition of reduction by polymeric ligands. Karol (117, 118) described the reaction of chromocene with silica to form highly active catalysts sensitive to hydrogen. An unknown role is played by the structure mt—CH2—CH2—mt which is formed with ethylene and reduced forms of titanium (119). For soluble systems, it has been shown that the mt—CH2—CH2—mt structure is formed in a biomolecular reaction with /3-hydrogen transfer (120). It was considered that this slow, but unavoidable, reaction is the reason for changes in activity during reaction and that the only way to avoid it is to prevent bimolecular reaction of two alkylated species. 

In order to fulfill this aim, we set out to study the solubility of biogenic silica, leading to the possibility of its use in SOC synthesis, and factors influencing this solubility, to investigate the methods of stabilization of the obtained soluble forms of silica, and to synthesize basic organic derivatives of silicon that are stable to hydrolysis and oxidation. 

We determined that vicinal dihydroxy organic reagents stabilize the soluble forms of silica. The stability of monosilicic acid solution is determined by the structure of the stabilizer ethylene glycol and humic acids do not stabilize soluble forms of silica enough, but glycerin and catechol stabilize the silica solution when they are added at 5 - 7% to the solution. This fact is connected with the formation of hydrogen bonds and stable penta- and hexacoordinated compounds, preventing the processes of polycondensation of silica in solution. 

To conclude, it is necessary to note that the reaction ability of biogenic silica depends on its structure and solubility, determined by various factors. It has been found that soluble forms of silica could be stabilized by S - 7% of glycerin and catechol. It has been determined that the amorphous part of the biogenic silica of siliceous rocks formed the complex with triethylphosphate and actively reacts with polyphenols (the simple one, catehol and complex ones, humic acids), with formation of ethers. The silicon organic derivatives and complexes formed are inert to hydrolysis. 

Taking into account solubility and stability boundaries of organic acids and of carbonic acid we may conclude that the organic acids are more efficient on a local scale for the dissolution of carbonate and silica cements. Where the source of the latter, i.e. the carbonic acid, is deep and further removed they may not increase the porosity by solution and removal of cement. The action of carbonic acid formed by decarboxylation on carbonate cement leads to the enrichment, in the dissolved carbonate, of the light carbon isotope inherited from the organic matter whereas the reaction of organic acids with the carbonate cement takes place under dissociation with the formation of secondary carbonate enriched with the heavy carbon isotope inherited from carbonates of mineral origin. 

This deionized colloidal silica is not a strong binder. The removal of the sodium hydroxide, which appears to act as a binder catalyst, and/or the presence of the alumina on the surface, weakens the overall bond strength developed. However, this acidic version of colloidal silica does form stable mixtures with several water soluble polymers, the most important of which is polyvinyl alcohol (PVA). PVA is itself a good film former and imparts this property of much improved film formation to colloidal silica even when present as only about 3-5% of the total solids. The mixture of PVA and acid stabilized colloidal... 

Adsorption behavior and the effect on colloid stability of water soluble polymers with a lower critical solution temperature(LCST) have been studied using polystyrene latices plus hydroxy propyl cellulose(HPC). Saturated adsorption(As) of HPC depended significantly on the adsorption temperature and the As obtained at the LCST was 1.5 times as large as the value at room temperature. The high As value obtained at the LCST remained for a long time at room temperature, and the dense adsorption layer formed on the latex particles showed strong protective action against salt and temperature. Furthermore, the dense adsorption layer of HPC on silica particles was very effective in the encapsulation process with polystyrene via emulsion polymerization in which the HPC-coated silica particles were used as seed. 

Ti-Beta zeolites and, even more, mesoporous Ti-siUcates can be somewhat unstable to aqueous hydrogen peroxide and to strongly chelating agents. A partial collapse of the lattice and the release of Ti, in the form of Ti02 particles or soluble Ti peroxides, was sometimes observed under these conditions (see also Section 18.4.2). The structural instability grows in parallel with the hydrophiUcity of the surface and the defectiveness of the silica matrix Ti-P < Ti,Al-P Ti-MCM-41 [87-89]. For the same reason, the stability of the catalyst is indirectly related to the method of synthesis, as far as this is able to produce materials with a different content of connectivity defects. 

To act as an SDA, a molecule must fulfil certain other prerequisites besides stability [5-7]. A critical factor is its solubility in the solvent used (usually water). The potential SDA must possess at least a limited solubility to take part in the reaction. However, if the solubility is too high or if the SDA forms strong hydrogen bonds with water, then its tendency to co-crystallize with the silica will be low and it will prefer to stay in solution. For this reason, alcohols are only weak SDAs, for instance. The fact that ethylene glycol and ethanolamine are grouped with other SDAs for the synthesis of silica sodalite in Fig. Id seems to contradict this conclusion. However, these molecules act as SDAs only in solvothermal synthesis, in which the SDA simultaneously acts as the solvent. 

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