Colloidal silica various forms

Solubility. An important aspect of silica chemistry concerns the silica— water system. The interaction of the various forms of silica with water has geological significance and is applied in steam-power engineering where the volatilization of silica and its deposition on turbine blades may occur (see Power GENERATION), in the production of synthetic quartz crystals by hydrothermal processes (qv), and in the preparation of commercially important soluble silicates, colloidal silica, and silica gel. 

There are a variety of other silicas. These include the metastable silica-W, which has a very low density (1.97 gcm ), and whose structure consists of edge-sharing chains of Si04 tetrahedra, the microcrystaUine diatomaceous earths, various forms of noncry stalline silica inclnding vitreous silica (glassy silica), and the finely divided amorphous silicas such as dry silica gel, fumed silicas, and colloidal silicas. 

There are numerous industrial products of colloidal silicas in various forms. The methods of industrial synthesis and main applications are briefly surveyed as follows. Ultra-fine particles of nanometer order such as aerosil (commercial name) are produced by the hydrolysis of tetrachlorosilane in air. A similar material can be obtained from oxidation of tetralkoxysilane in a gas phase. The former is used as a thixotropic additive of composite materials, heat insulator and etc. 

The past two decades saw a rapid growth in those fields of science and technology that deal with production and utilization of various colloid and microheterogeneous forms of silica with developed surfaces, such as sols, gels, and powders. The colloid chemistry of silica embraces a wide range of diverse scientific and applied problems. It is an important, independent, and progressive field of colloid chemistry that is closely interwoven with a number of physicochemical and other sciences. Research has led to the... 

However, 1 have discovered that effective mixing and binding effect is obtained with sand if the concentrated silica sol is mixed first with the sand to form a uniform and continuous film on the surface of the sand grains. The concentrated sodium silicate solution is then added to the sand mass in a second, separate step and the sodium silicate then mixed with the colloidal silica film on the surface of the sand, gelling in situ to form an intimately and uniformly mixed binder within the sand mass. The sand mix thus formed in the mixer can be molded by any of the various processes available in foundry practice and hardened to form strong molds or cores. 

Numerous patents have been issued on antireflection coatings in which silica in various forms is deposited as a thin, adherent layer, generally on the surface of glass or plastic sheets and lenses. Coating with a low-hydrated oxide of silica was described in 1945 (698). Films of colloidal silica are deposited in various way (699-704). 

Stdber surveyed the history of silicosis and the three main theories involving (1) solubility of silica, (2) surface interactions of silica, and (3) immune reactions resulting from the presence of silica. He then presented a summary of extensive tests made with particles of all known forms of silica in which surface properties and dissolution characteristics were correlated with their biological reactions (309). He pointed out that the simple solubility theory cannot be correct. The fact is that if one arranges the various forms of silica in order of increasing solubility (crystalline silicas, opal, fine-particle amorphous silica, and colloidal silica) the fibrogenic activity decreases whereas the toxicity to tissues increases. 

Silica, in its various forms, can cause serious problems for an RO system. Silica scaling and fouling can occur via a number of ways and is not well understood due to the number of different mechanisms that can take place. The concentration of silica, the speciation of silica, and temperature, pH, and the general chemistry of the water all affect the chemistry of silica scaling and fouling. In general, the silica issues that affect RO systems can be summarized as deposition of silicates, polymerization of silicic acid to amorphous silica, and the accumulation of amorphous colloidal particles. To understand the potential problems, it is first necessary to understand a little about the chemistry of silica. 

Colloidal silica from various origins or chalk or other fine-particle fillers are normally added. If the particles of antiblocking agent incorporated are too large, they can impede surface sliding. This effect is used for antislip modification. The products used here are high molecular weight thermoplastics or elastomers [33] predispersed in a suitable carrier, they form evenly distributed particles on the film surface [34]. 

Preparation of X-ray amorphous ZSM-5 crystallites according to procedure BT It is important that the gel formation takes place as homogeneously as possible. Because of the particular sensitivity of various silica and alumina species to the pH (63,64), the pH range between 4.5 and 8.5 was avoided. Nucleation was performed at pH 3.5-4, where a low viscous gel containing essentially monomeric silica species is rapidly formed (65).The, pH is theii raised to about 9, in order to form tetrahedral A1(0H) entities and to favour the further A1 incorporation within the zeolitic framework. NaCl was used to increase the (super)saturation of the gel, which will flocculate into a macromolecular colloid (V) and initiate the nucleation. This procedure yields 100 % crystalline zeolite after... 

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