Aqueous Solutions of Silicates

The complexity of these solutions is demonstrated by a report, based on the use of Si nmr, that presents definitive evidence for the structures of 11 species present in an aqueous solution of potassium silicate. Five of these involve a three-membered ring containing three siloxy units, which contrary to expectation must therefore be stable at high pH. Other species are a monomer, dimer, and other cyclic or cage compounds that contain four-membered rings. The Si nmr spectrum of tetramethylammonium aluminosilicate has also been discussed.  

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Owing to this activation threshold, the first precipitation product from aqueous solutions of silicic acids will be an amorphous silica of some degree of hydration, while at room temperature the growth of vitreous and crystalline forms of silica from the precipitate (and thus the approach toward the absolute equilibrium) will proceed extremely slowly. With this understanding the data in Figure 1 are said to represent, an equilibrium—i.e., the reversible equilibrium between silicic acids in aqueous solution and metastable hydrated silica or polymeric silicic acid as precipitate. 

The reaction may be applied to aqueous solutions of silicates, but phosphates and arsenates must be absent. 

It is also interesting to note that a similar phenomenon is observed with the growth of quartz in aqueous solution of silicic acid oversaturated with respect to this mineral (Stober, 1967 Wollast, 1974). The solid deposited has a tendency to attain a more increasingly disordered structure as the precipitated layer increases. There is a further slow reorganization of the precipitated layer, leading finally to the formation of relatively well-crystallized quartz (Mackenzie, 1971). An evaluation of the solubility of the disordered phase initially deposited on quartz based on the backward reaction rate indicates that this rather amorphous silica has a solubility 10 times higher than quartz. 

Si NMR Spectroscopy of Colloidal Silica. 29Si NMR spectroscopy has been used extensively to study aqueous solutions of silicates and has provided detailed information on the types of polymeric species in solution (3-5). However, despite the power of the technique it has not been exploited previously to investigate colloidal silica. The usefulness of the technique arises from the successive shift of the 29Si resonance to high field on replacement of Si-OH bonds in silicate species with siloxy bonds, Si-O-Si. Thus monomeric species are observed furthest downfield (Q°) and other resonances are observed at intervals of —10 ppm for end units (Q1), middle units (Q2), branching units (Q3), and tetrafunctional units (Q4). (Q refers to four possible coordination bonds the superscript refers to the actual number.) From the relative intensities of the different... 

Frolov, Shabanova, and co-workers (37-39) studied the transition of a sol into a gel and the aggregate stability of colloidal silica. Their aim was to develop a technology for the production of highly-concentrated silica sols and to use them as binders, catalyst supports, polymer fillers, adsorbents, and so forth. Kinetic studies were made of polycondensation and gel formation in aqueous solutions of silicic acids. At the stage of particle growth, poly condensation proceeds in the diffusion-kinetic region. With changes in pH, temperature, concentration, and the nature of electrolytes,... 

Si NMR spectroscopy has been used extensively to study aqueous solutions of silicates and has provided detailed information on the types of polymeric species in solution [3-5]. However, despite the power of the technique it has not been exploited previously to investigate colloidal silica. The usefulness of the technique arises from the... 

Aqueous solutions of silicates play an utmost important role in geochemistry, inorganic materials processing and chemical engineering [IJ.For practical applications, three main classes of products can be defined by looking at the molar ratio R = Si02/Na20 ... 

The new porosil was synthesized from aqueous solutions of silicic acid and boric acid in the presence of 1,2,2,6,6-pen tamethy In iperi dine,, as guest molecule. 

Typical values for the relaxation times for organosiloxanes are between 35 to 205 s. Lower values are found for aqueous solutions of silicates (0.3-4.6 s), higher ones for silicon compounds without protons, e.g. chlorosiloxanes (53-352 s). 

Engelhard s in-situ FCC catalyst technology is mainly based on growing zeolite within the kaolin-based particles as shown in Figure 3-9A. The aqueous solution of various kaolins is spray dried to form micR)spheres. The microspheres are hardened in a high-temperature l,3f)(TF/704°C) calcination process. The NaY zeolite is produced by digestion of the microspheres, which contain metakaolin, and mullite with caustic or sodium silicate. Simultaneously, an active matrix is formed with the microspheres. The crystallized microspheres are filtered and washed prior to ion exchange and any final treatment. 

The most important type of mixed solution is a buffer, a solution in which the pH resists change when small amounts of strong acids or bases are added. Buffers are used to calibrate pH meters, to culture bacteria, and to control the pH of solutions in which chemical reactions are taking place. They are also administered intravenously to hospital patients. Human blood plasma is buffered to pH = 7.4 the ocean is buffered to about pH = 8.4 by a complex buffering process that depends on the presence of hydrogen carbonates and silicates. A buffer consists of an aqueous solution of a weak acid and its conjugate base supplied as a salt, or a weak base and its conjugate acid supplied as a salt. Examples are a solution of acetic acid and sodium acetate and a solution of ammonia and ammonium chloride. 

The phosphate bonded cements described in this chapter are the products of the simple acid-base reaction between an aqueous solution of orthophosphoric acid and a basic oxide or silicate. Such reactions take place at room temperature. Excluded from this chapter are the cementitious substances that are formed by the heat treatment of aqueous solutions of acid metal phosphates. 

Dental silicate cement was also variously known in the past as a translucent, porcelain or vitreous cement. The present name is to some extent a misnomer, probably attached to the cement in the mistaken belief that it was a silicate cement, whereas we now know that it is a phosphate-bonded cement. It is formed by mixing an aluminosilicate glass with an aqueous solution of orthophosphoric acid. After preparation the cement paste sets within a few minutes in the mouth. It is, perhaps, the strongest of the purely inorganic cements when prepared by conventional methods, with a compressive strength that can reach 300 MPa after 24 hours (Wilson et al, 1972). 

Dental silicate cement liquids are concentrated aqueous solutions of orthophosphoric acid generally containing aluminium and zinc (Wilson, Kent Batchelor, 1968 Kent, Lewis Wilson, 1971a,b Wilson et al., 1972). The optimum orthophosphoric acid concentration is 48 to 55 % by mass (Wilson et al, 1970a), although higher concentrations are encountered. Aluminium is present as phosphate complexes and zinc as a simple ion (see Section 6.1.2). Examples are given in Table 6.6. 

P 33] The catalyst bed was manually positioned in the micro channel (300 pm wide 115 pm deep) at room temperature using a 10% (v/v) solution offormamide and potassium silicate [6, 7]. Micro reactor bottom and top plates were thermally bonded thereafter. Then, 75% THF (aqueous) solutions of 4-bromobenzonitrile and phenylboronic acid having equimolar concentrations were placed in the two reservoirs of a micro-mixing tee chip. In the collection reservoir, 30 pi of the THF solution was placed. Voltages ranging from 100 to 400 V were used, but kept constant only for one reservoir. The other one was switched on and off at 200 V for given time periods. 

Siroc [Silicate rock] Also known as the one-shot system. A chemical grouting system for hardening ground formations. Aqueous solutions of sodium silicate and formamide are mixed and injected into the ground the formamide slowly reacts with the silicate, precipitating hydrated silica, which binds the soil particles together. Invented in 1961 by the Diamond Alkali Company. See also Joosten. 

Due to the very time consuming of the above procedure, the latter was modified as follows (synthesis B) equal volumes of silicate and Al-Pr4N+-H2SO, solutions were added at the same rate to an aqueous solution of NaCl (Merck, anal, grade). The gel which is formed (pH 3.5), is stirred for 2 h before its pH is adjusted to about 9-9.5 by further adding of the remaining excess of Na silicate solution. The so obtained gel is stirred for 3 h before heating. 

Chirica and Remcho first created the outlet frit, packed the column with ODS beads, and then fabricated the inlet frit. The column was filled with aqueous solution of a silicate (Kasil) and the entrapment achieved by heating the column to 160 °C [105,106]. The monolithic column afforded considerably reduced retention times compared to the packed-only counterpart most likely due to a partial blocking of the pores with the silicate solution. This approach was recently extended to the immobilization of silica beads in a porous organic polymer matrix [107]. 

Some of the compounds described in this chapter were studied for specific physical properties. Surface tension measurements with solutions of 9-16 in 0.01 M hydrochloric acid demonstrated that these zwitterionic X5Si-silicates are highly efficient surfactants.21 These compounds contain a polar (zwitterionic) hydrophilic moiety and a long lipophilic z-alkyl group. Increase of the n-alkyl chain length (9-15) was found to result in an increase of surface activity. The equilibrium surface tension vs concentration isotherms for 9 and 16 were analyzed quantitatively and the surface thermodynamics of these surfactants interpreted on the molecular level. Furthermore, preliminary studies demonstrated that aqueous solutions of 9-16 lead to a hydrophobizing of glass surfaces.21... 

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