Colloidal silicas, silica sources

Photomultipliers are used to measure the intensity of the scattered light. The output is compared to that of a second photocell located in the light trap which measures the intensity of the incident beam. In this way the ratio [J q is measured directly with built-in compensation for any variations in the source. When filters are used for measuring depolarization, their effect on the sensitivity of the photomultiplier and its output must also be considered. Instrument calibration can be accomplished using well-characterized polymer solutions, dispersions of colloidal silica, or opalescent glass as standards. 

Silica is normally present in most sources of MU water, although not to any significant level as soluble Si02 but rather as both colloidal amorphous silica and silicate salts. In any particular water, such... 

Note Colloidal silica (glidant) and sodium starch glycolate (disin-tegrant) were fixed at 0.1% and 5%, respectively, in all batches. Source. Ref. 100. 

Dissolved titanium method. Hydrothermal synthesis using tetrapropylammonium peroxytitanate (prepared from TEOT, distilled water, 30% aqueous H2O2, and 25% aqueous TPAOH) as the source of Ti and colloidal silica (Ludox AS-40) as the source of Si and TPAOH as template. All additions done at 278 K Preparation using TiCl2, 14% aqueous TPAOH, 30% colloidal silica, and demineralized water Preparation at low pH using fluoride ions as mineralizing agent... 

Silica is normally present in most sources of makeup water, both as colloidal amorphous silica and as silicate salts, typically at total silica concentrations of 5 to 15 ppm Si02, but much higher in some waters (notably in arid areas), where the silica content may rise to 50 to 70 ppm Si02 or more. 

Zeolite ZSM-5. Zeolite samples were crystallized from a gel containing colloidal silica (Ludox AS-40), deionized water, aluminiumtriisopropylate (ATIP, Merck), as source of aluminium, and tetrapropylammonium bromide, (TPABr, Aldrich). Ammonia solutions were prepared by saturating a thermostated aqueous solution with... 

The largest area of ignorance and therefore the most promising field far investigation lies in nucleation. Breck s synthesis required SiC>2 in die form of colloidal silica sol to make silica-rich type Y. We have since learned to seed the batch with a freshly prepared slurry (18), and use cheaper silica sources. But we are far from a chemical definition of the nucleating agent... 

When thermal volcanic waters react with aerated surface waters, the appearance of ferric iron colloids is quite permissible. However, there are no organic compounds of fulvic acid t)q)e in volcanic waters and only colloidal silica could act as a stabilizer. At the same time there often is sulfur in thermal solutions, the stable form of which in the presence of free oxygen is the S04 ion—the main coagulant of colloidal iron. For this reason the possibilities of colloidal transport of iron from volcanic sources to sedimentary basins are limited. A high COj content in the hydrosphere and atmosphere does not exert a stabilizing effect on Fe(OH)3 colloids. 

Periodic nanoporous silicates have been prepared in a wide variety of conditions. Different sources of molecular, and non molecular silica have been used. This includes TEOS, TMOS, fumed, colloidal and precipitated silicas. Depending on the synthesis conditions, particularly on the nature of the silica source, crystallization may take place in seconds at subambient temperatures [82], or at room temperature [60,61,69,72,83]. However, in most cases the crystallization temperature was set in the 80 - 120 °C range. Liu et al. [84,85] found that the use of small amounts of colloidal particles (silica or titania) promotes the formation of ordered structures by providing nucleation seeds. The pH conditions varied from extremely acidic [60,61], to neutral [69,72] to very basic [48,49]. Ryoo and Kim [86]... 

The synthesis of mesoporous silica films typically begins with the preparation of precursor solutions. These solutions contain a silica source (typically an alkoxide, although chloride and colloidal precursors can be used), a surfactant molecule used to template the mesostructure, an acid or base catalyst, and solvents. The nanoscale structure is then formed by a cooperative self-assembly of monomeric or partially... 

The symposium was exceptional because it gathered together for the first time many of the world s outstanding silica scientists, who presented close to 200 selected papers. Forty of the most prominent silica researchers from the former Soviet republics, who could not attend the symposium, sent papers to be discussed at the meeting. On this basis Iler s book and the symposium can be considered landmarks in the history of exchange of information on colloidal silica systems. A most valuable source of information published before the symposium was the excellent textbook by Brinker and Scherer printed in 1990 (8). 

Many of these applications are practiced today. Other applications exist only for specialized situations. For example, silica sols are a desirable source of silica for catalyst substrates. For catalyst preparations, however, colloidal silica is generally used only for specialized applications. 

Stiles, McClellan, and Sowards (25-28) showed that colloidal silica is a good source of raw material for making the base catalyst. Silica sols have the advantage of a uniform distribution and known particle size. Thus, the catalyst manufacturer can predict the pore size and volume of the final product. Colloidal silicas also offer the advantage of low levels of sodium (compared to sodium silicate), a known catalyst poison, and therefore require less washing and processing to remove the unwanted cation. In... 

Where will future applications come from One source is universities, and a second is extension of existing applications. Some applications of colloidal silica originating in universities include the following ... 

Figure 1. Partial reaction composition diagram. Projection of the NazO, Al2Os, Si02> P205, H20 system at 100°-125°C. Na90 4-Si02 + P2Os = 100 mole %. Al2Os = 12-20 mole % of the total anhydrous gel composition mole H20/Al203 is constant (= 110). A solid colloidal silica ( Cab-O-Sil ) is the silica source.

Figure 2. Same as Figure 1 with a crystallization temperature of 125°-150°C and an aqueous colloidal silica source ( Ludox ). Al2Os =... 

From the obtained results, CsOH supported on a Zr-promted silica gel (CsOH/Zr-SiOa) with Cs/Zr/Si atmic ratio of 22/10/1000 was used in this study as the typical catalyst for the aldol-lype-condensation reaction. As the source of the silica gel, colloidal silica Cataloid S-20LE which was offered kindly by Mitsubishi Rayon Co., showed relatively good performances. 

Most commercially viable manufacturing processes of aqueous colloidal silica use an alkali silicate, such as sodium or potassium silicate as the source of silica. Most, or all, of the alkali is removed from the solution by ion exchange or electro-dialysis and is replaced by hydrogen ions thus forming silicic acid. The silicic acid is then polymerized to give particles of colloidal silica. This is not to imply that other processes cannot make colloidal silica, many of which are described in Her [1], but these have not been found to be useful on a commercial scale for the most part. 

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