Molecular weight silicates

Yamaguchi, H. (2002). Low molecular weight silicic acid - inorganic compound complex as wood preservative. Wood Science and Technology, 36(5), 399-417. 

The most plausible interpretation of the above results is as follows Al in aluminate solutions is present in the form of AKOH) anions irrespective of the Al concentration or the Na/Al ratio (150,181,182). When solution A, containing low molecular weight silicates and thus a high proportion of reactive SiOH groups, is mixed with the aluminate solution, a predominantly aluminous intermediate results. When, by contrast, solution B, richer in silicon and with fewer silanol groups, is used, a silca-rich intermediate is formed. But the composition of zeolite A (with Si/Al = 1.00) is achieved via structural rearrangements in the amorphous precursors and is dictated by the final structure itself. 

DnR silicates in solution (4). Absolute amounts of silica present in the form of the various silicates are mentioned, together with (in brackets) their relative amounts, i.e. as if no polymeric silicates were present. These polymeric silicates, i.e., silicates consisting of more than 10 Si atoms, cannot be characterized by the chemical trapping method since the silyl esters are not volatile enough to be detected by GLC/flame ionization detection (FID) (14). Moreover, Si-NMR spectroscopy studies have never been successful in positively identifying higher molecular weight silicates than Si] ones (9). 

The year 1941 was the turning point toward commercial silica sol production methods. Bird [10], at Nalco, found that low-molecular-weight silicic acids could be prepared by ion exchange. If a small amount of sodium hydroxide was added to these materials, particles were formed. These materials could then be concentrated with conventional evaporator techniques to about 27.5% silica. Nalco... 

It can be seen that even if the low molecular weight silicic acid counterparts were initially produced, condensation of silanol groups between adjacent molecules, forming siloxane bonds and water molecules, could occur rapidly. However, the water is still held so tenaciously that the overall chemical analysis is not changed, while the mass becomes cross-linked and insoluble. 

Low molecular weight silicic acids were separated by Baumann (82) with paper chromatography using a mixture of isopropyl alcohol, water, and acetic acid as the moving liquid and the molybdic acid reaction to locate the separate species. 

The conversion of a sol of spherical particles to a uniform gel coctaiuing all the liquid in the sol is not easily understood. When particles collide it is assumed that adhesion can occur but in the case of silica particles there is reason to believe that the attachment is through the formation of Si-O-Si bonds. One reason for thinking so is that the same factors that promote polymerization of moncrr.er and low molecular weight silicic acids also promote the conversion of a sol of coUoidal silica particles to a gel. Thus sols consisting of well-defined spherical particles form gel least rapidly at about pH 2 and the process is accelerated by fluoride ior.s at low pH. 

Knowing the templating role of sodium ions in the TPA-free synthesis of MFI-type zeolites [13, 22, 56-58], these findings indicate that the rate-determining factor is concentration of "free" low-molecular weight silicate species, in the short synthesis duration of this seed-induction system... 

From the above results, it could be concluded that the addition of excess amount of sodium ions into the crystallization system has apparent effect on the particulate properties of the product. At low batch alkalinity, the additional sodium ions causes de-aggregation of the final products, rendering the particles with more uniform size distributions. At high batch alkalinity, the excess amount of sodium ions triggers the surface condensation reactions on the crystalline end products. However, the crystallization rate is not enhanced by the increase of batch sodium ion content, indicating that the determining factor of crystallization of ZSM-5 zeolites in SDA-free system is concentration of low molecular weight silicate species, determined by batch alkalinity. 

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