Oligomers, polysilicic acids

The study of alkaline silicate and aluminosilicate solutions is of fundamental importance for a better understanding of the mechanism of zeolite synthesis (1,2). Real progress has been realized durinq the last fifteen years thanks to the use of new techniques such as the trimethylsilylation (3,4) of polysilicic acids (followed by a chromatographic separation of the derivatives) and, even more, 29Si and 27Ai NMR spectroscopy (5-11). The latter techniques have enabled the identification of about twenty types of oligomers in silicate solutions and in some cases estimates of their respective concentrations. The investigation of alkaline aluminosilicate solutions is... 

Polysilicic acid (oligomers). Polymers with molecular weights (as SiOj) up to about 100,000, whether consisting of highly hydrated active" silica or dense spherical particles less than about 50 A in diameter. 

It is obvious that a few percent of monomer in water would not contribute appreciably to viscosity, for example, any more than a small molecule like glycerol would. It is probable that below a certain particle size, probably between 1 and 1.5 nm, the polymer no longer acts as a particle, but rather like liquid oligomers. Thus Her (116) found that the viscosity of polysilicic acid solution containing 6.34% SiO, at pH 1.7, derived from 3.25 ratio sodium silicate, had a viscosity of 1.11 relative to the sodium sulfate solution medium. This increased to 1.15 as the degree of polymerization (cryoscopic method) increased from 4 to 23, the latter corresponding to a 1.25 nm... 

In the case of polysilicic acid of very low molecular weight, such as cyclic tetramer and similar oligomers no liquid phase can be salted out. It appears necessary to let the silica polymerize to three-dimensional units or small particles before it can be covered by oriented organic molecules to form a hydrophobic complex that will form a second phase. On the other hand, if the particles further polymerize by aggregation to form microgel, then only a gelatinous emulsion is formed. 

Examples of liquid coaicervates or complexes of polysilicic acid oligomers have been disclosed in previous publications (89) and in the patent literature (166-168), from which the following examples are taken. 

The above remarks apply to systems where the silica is added as active silica prepared as a separate solution, usually by. ion exchange. More rapid growth occurs when the silica is added directly as sodium silicate to a heel of nuclei from which sodium ions are being constantly removed by hydrogen ion-exchange resin, as discussed later. In this case, the silica is initially present as monomer and oligomers and thus no time is required to depolymerize higher polysilicic acids which are invariably present when the silica is prepared first as a separate solution of active silica. 

When the solution concentration of Si(OH)4 is sufficiently large, the reaction of Eq. 15 runs in reverse, and silicic acid condenses back into a solid (Eq. 16). In the course of this condensation reaction, various polysilicic acids with the general formula [SiOx(OH)4 2x]n where 2 < x < 0, are present in solution (Greenwood and Eamshaw 1984). In neutral or acidic solutions, these oligomers can condense to the point of precipitation. 

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