Monomeric silicic acid polymerization

Figure 1 gives results obtained by Alexander et al. (I) and Baumann (2) by dissolving fine particles of commercially available vitreous silica powders in aqueous solutions. Similar data obtained in polymerization and depolymerization experiments by Scheel et al. (15) and Friedberg (10) indicate that the curve shown in Figure 1 represents an equilibrium concentration for oligomeric acid. It can be approached from the supersaturated state of monomeric silicic acid as well as from a solution of pure polymeric silicic acid. 

Silicic acid can be in the form of a monomer or in the form of low molecular weight polymeric units. Monomeric silicic acid Si(OH)4 has not been isolated or obtained in a concentrated solution without considerable polymerization, therefore is not a practical form to use in catalyst slurries to spray dry. Also not very practical is silicic acid formed in a way that does not separate it from the electrolyte products of the forming reaction. Residual electrolytes increase the ionic strength of the solution and result in destabilization followed by premature gelling of the silicic acid. 

Because polymerization of the water-glass solution gives rise to a broad distribution of silicate anions and thus results in poorly resolved Si NMR fines, the application of Si NMR spectroscopy in this type of reaction provides minor information about the different oligomerization steps and the reaction mechanism of monomeric silicic acid. Therefore a different approach to the study of aqueous silicate solutions was apphed. Because of the... 

X = number of OH groups per silicon atom in the polymer, not exceeding 4 m = the number of monomeric silicic acid molecules added to the polymer p - fraction of the hydroxyl groups per monomeric silicic acid molecule that are converted to water during the polymerization reaction... 

The history and use of the silicomolybdate method for analyzing for soluble silica has been discussed in Chapter 1. It is sufficient here to point out that molybdic acid reacts only with monomeric silica to form the yellow silicomolybdic acid. It is fortunate that the reaction with molybdic acid occurs at pH 1-2, where silicic acid polymerizes least rapidly. Thus polymeric silica must first depolymerize before it can react hence the higher the degree of polymerization, the longer the time required for depolymerization and color development. This is reviewed in detail in Chapter 3. 

The reaction of silica with catechol, pyrocatechol, and 2,3-naphthalenediol has been studied by several investigators (158-162), but Bartels and Erlenmeyer appear to have been the only ones to use this reaction to characterize the rate of depolymerization of silica (163a). For example, monomeric silicic acid from ethyl silicate in a standard solution of catechol in 0.8 N HCl reacted rapidly and could be titrated to a constant pH of 8.5 with an equivalent amount of standard NaOH solution in a few minutes. An equivalent amount of silica gel required 2.5 hr, but ignited gel reacted only slightly in 5 hr. The rate of reaction, followed by a constant pH titration, provides a way to estimate the relative degree of polymerization of silica or possibly the specific surface area. 

Monomeric silicic acid, Si(OHL has never been isolated. It is a very weak acid and exists only in dilute aqueous solution, since it polymerizes when it is concentrated. It... 

The rate of polymerization of silicic acid produced by the hydrolysis of tetramethoxysilane (followed by silicon-29 nmr) has turned out to be slow. Indeed at pH 3.5. 0 monosilicic acid may still be detected after several weeks. There is no direct relationship between disappearance of monomeric silicic acid and production of silica gel. ... 

In the case of acid-catalysed polymerization, an equilibrium condition between temporarily positively charged and nentral species of monomeric silicic acid is proposed [13]. This equilibrium is shifted greatly to the right. The cationic monomeric silicic acid is formed due to the addition of an H" " ion [5]. 

Above pHjso 1.7-2, the polymerization proceeds by an OH ions catalysed mechanism. The OH ion deprotonates a nentral monomeric silicic acid. Subsequently, it polymerizes. In the case of a very high pH, the equilibrium between the anion and the siloxane is shifted backwards to the left side, resulting in dissolution of the siloxane (if ever formed). The anionic silicic acid attacks silanol groups of the most acidic silicon atoms, i.e. the silicon atoms with the fewest number of attached OH-groups and the greatest number of siloxane bonds. They are rather middle than end groups. Therefore, formation of highly condensed and branched clusters is promoted [5,10]. 

By contrast, an unrestricted acceleration of gelation for the base-catalysed polymerization with increasing temperature z cannot be stated [4, 10]. It is reported that the gelation time tg increases for temperatures from i = 15 °C to > = 35 °C, but decreases again with higher temperatures. It is assumed that this peculiar behaviour is due to a preequilibrium step involving an induction period. During this period, small polymeric units are formed initially with which monomeric silicic acid reacts preferentially [4]. 

Sorption reactions of silica in soils were postulated many years ago (Sreenivasan [1935]). In the early work, however, somewhat high concentrations of sodium and potassium silicates were used, and such systems would be subject to hydrolysis and polymerization reactions and also to pH changes. Thus, in recent studies on the sorption of soluble silica by soils (Eliassaf [1962] Beckwith and Reeve [1963] McKeague and Cline [1963]), dilute solutions (100 to 135 ppm) of monomeric silicic acid have been employed, and results indicate that the residual concentration of monosilicic acid is controlled by an adsorption equilibrium which is pH dependent. Sesquioxides make a considerable contribution to the capacity of soils to sorb soluble silica (Nejegebauer [1958] Beckwith and Reeve [1963]), and the apparent increase in solubility of silica in soil suspensions with increased acidity has been discussed in terms of... 

The nature of the silica in silicate ions in any alkaline solution cannot be determined by a chemical measurement that involves any change in the concentration of silica or alkali, electrolyte content, or temperature because these all shift the equilibrium between monomeric and various polymeric ion species. However, if a sample is simultaneously and instantaneously diluted and acidified to pH 2 al less than 30 C, the resulting silicic acid is sufficiently stable to permit characterization. The problem is to ensure that acidification is so sudden that the various silica species do not have time to polymerize or depolymerize as the pH is dropped from the usual region of... 

As will be discussed in detail in Chapter 3, this technique has been shown by Alexander and others to allow the formation of SKOH) from NajSi0,-9H,0. Salt-free solutions can be obtained by ion-exchange techniques. Since conditions have been found for converting monomeric silicate ions to monosilicic acid, which is extremely prone to polymerize, it is evident that higher polysilicates can likewise be converted to the more stable polysilicic acids with even less difficulty. 

Such a case may be involved in the observations of Schwartz and Muller (33), who made a highly purified solution of silicic acid from methyl orthosilicate at concentrations up to 150 ppm. Initially, conductivity measurements indicated that the silica was monomeric, but after half an hour the conductivity, at all concentrations, slowly-decreased to about half the original value. This happened even though in half the samples the concentrations were less than the solubility of amorphous silica. It was assumed that the monomer polymerized slowly at pH 7 to a polymer species that is smaller than usual colloidal dimensions, since it passed through an ultrafiler, yet it must be more insoluble than amorphous silica. 

Polysilicic acids of different molecular weights can be separated and molecular weights estimated by gel chromatography on Sephadex columns, using 0.1 M NaCl solution adjusted to pH 2 with HCl as the eluent. A blue dextran 2000 in 0.2% solution was used as a standard. Tarutani (83) made silicic acid at a concentration of 500 ppm by neutralizing the monomeric solution of sodium metasilicate with acid to pH 7. This solution was aged for various lengths of time and then acidified to pH 2 to stop polymerization. 

Certainly hydrogen bonding may be involved if the silicic acid is polymerized at least to some degree, but in most living tissues where silicon is being metabolized or transported, nothing is known about the monomeric or polymeric state of the silica. 

Earlier, soluble silica was thought to play a role. One possibility was that mono-silicic acid entered into DNA or RNA leading to modification of enzyme systems. The more prevalent idea was that quartz dissolved to soluble monomeric silica. This in itself, was harmless but polymerized to polysilicic acid, which is generally known to be a protein denaturant and a destroyer of cell membranes, that is, it is cytotoxic. ... 

Whether a ligand increases or decreases Al solubility depends on the particular Al-ligand complex and its tendency to remain in solution or precipitate. Ligands that increase the overall solubility of Al include F", oxalate ", citrate ", fulvic acid, and silicate(monomeric). Ligands that decrease the overall solubility of Al include phosphate, sulfate, silicate(polymeric), and hydroxyl. Figure 5.9 summarizes the influence on Al of common soluble ligands that may be encountered in soil solution. 

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