Silicate oligomers

This, however, is an oversimplification, since these monomeric anions are in rapid equilibrium with oligomers (small polymers) such as the dimer (cf. Fig. 7.2b), cyclic trimer (7.2c), and other small silicate oligomers, 22 of which have been identified by silicon-29 magnetic resonance methods and many of which have no counterparts in solid silicate minerals9 ... 

Under our experimental conditions, no significant influence of the CTAB polar head group on the nature of the silicate oligomers is observed. Indeed, the spectra of experiments 3 and 11 are similar to those of the corresponding silicate solutions free of surfactant (spectra not reported) [10,16],... 

Approximately 85% of the of the peak area shown in Figure 1 can be assigned to 19 specific silicate structures 14.51. A schematic of these structures is presented in Table I, together with the chemical shift for each distinct silicon atom. It is evident from this table that silicate oligomers tend to occur as single and multiple ring structures, rather than as linear chains. 

In the following section the structure of the silicate oligomers will be considered in more detail. 

STRPCTPRE of the SILICATE OLIGOMERS The experimentally identified species (9.10.11) are comprised of a rather small number of silicon tetrahedra (t < 12). They are for instance linear (t = 1 to 4). cyclic (t = 3 and 4), bicyclic oligomers (t = 6,8 and 12) consisting of 2 identical rings. There are, in addition, more complex species related to the cyclic trimer and tetramer and other polycyclic forms ( t = 7 and 8). 

Several recent NMR studies have alloHed the identification of about tnenty silicate oligomers and quantitative or semi-quantitative estimates of their respective concentrations. In spite of the precautions used by some authors, the measured concentrations and in particular the experimental monomer concentration (used as a basis in our calculations) are probably not alaays very accurate expecial-ly in the case of the less alkaline or the more concentrated (in silica) solutions. The use of an inert silicon-containing internal standard could perhaps be useful. To this lack of accuracy is added the uncertainty of the pH measurement in such strongly alkaline solutions. 

Because the structure of silicates oligomers tend to be as condensed as possible, structures with long linear chains and large cyclic rings are less plausible. On the basis of this statement, structures containing three-membered rings and threefold substituted Si sites appear the most likely candidates. Therefore, the most probable structure of the analysed species is 7e. In combination with two other samples... 

B) Ion-exchange of silicate oligomers with Br and OH" to form inorganic-organic aggregates. (C) Self-assembly of SLC meso-phases. D4R = double four ring SLC = silicatropic liquid crystal. 

Linden, M. et al., Solubilization of oil in silicate-surfactant mesostructures, Langmuir, 16, 5831, 2000. Bussian, P. et al., Speciation in solution analysis of silicate oligomers using ESI-MS, Angew. Chem. Int. Ed, 112, 4065, 2000. 

Thus, as the pH is changed from 11 to 8 in 0.15 M salt, silicate steric layers are generated. For these layers to stabilize the sol under these conditions, they must extend farther than the Vi-max = 0 condition of 2 nm. The hypothesis is therefore that a 2-nm steric silicate oligomer-polymer barrier protects a silica sol at pH 8.0 in 0.15 M salt. 

Influencing the gel chemistry, such as the kinetics of gel dissolution by the mineralizing agents, and generating precursors , like silicate oligomers in the case of zeolites. 

In the inhibition period aluminum cations react with the silica oligomers and subcolloidal particles to form isolated patches of alumino-silicates on the surface of the small silica particles. From the review work of Iler (Her 1979), it is known that the reaction of aluminum cations with silicate oligomers is fast compared to the clustering of particles. After the inhibition period, the size of the aggregates slowly increases. Increasing the concentration of aluminum cations in the solution involves the formation of more homogeneous alumino-silicate particles. At a concentration of 0.10 mol% aluminum in the silicate solution, the fractal dimensionality... 

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