Precipitated silica, definition

The precipitated silicas include a wide range of silicas with a variety of structural characteristics. Most of the preparation methods are patented. In general the formation involves a coagulation and precipitation from silica solutions. Properties may therefore be supposed to be similar to those of the gels. For these silicas however, preparation conditions are such as to avoid gel growth and stimulate precipitation. As an overall definition, Barby proposed dry silicas with no long or short distance characteristic structure. 

Silica is one of the most abundant chemical substances on earth. It can be both crystalline or amorphous. The crystalline forms of silica are quartz, cristobalite, and tridymite [51,52]. The amorphous forms, which are normally porous [149] are precipitated silica, silica gel, colloidal silica sols, and pyrogenic silica [150-156], According to the definition of the International Union of Pure and Applied Chemistry (IUPAC), porous materials can be classified as follows microporous materials are those with pore diameters from 3 to 20 A mesoporous materials are those that have pore diameters between 20 and 500 A and macroporous materials are those with pores bigger than 500 A [149],... 

Adsorptive properties of porous silicas compacts of pyrogenic powders, 506, 507f precipitated silicas, 507-509 silica gels, 509, 510-512 zeolitic silicas, 512-514 Adsorptive properties of silicas, challenges for improvement, 505 Aerogel(s) definition, 7, 620 porosities, 379, 380 ... 

Table 12. Definition of Precipitated Silica Structure Silica Structure Level Oil Absorption (ml/IOOg)...

But [said he] it was still necessary to learn the base of the salt. Its solution could not be precipitated either by tartaric acid in excess or by platinum chloride. Consequently it could not be potassium. I mixed another portion of a solution of the same salt with a few drops of pure potash, but without its becoming cloudy. Therefore it contained no more magnesia hence it must be a salt with soda for a base. I calculated the quantity of soda which would be necessary to form it but it always resulted in an excess of about 5 parts in 100 of the mineral analyzed. Therefore, since it seemed probable to me that the different substances might not have been well washed, Or that the analysis might not have been made with sufficient precision in other respects, I repeated it twice more with all the care possible, but always with results very little different. I obtained. Silica. 78 45, 79.85, Alumina 17 20, 17.30 Sulfate 19.50, 17.75. At last, having studied this sulfate more closely, I soon found that it contained a definite fixed alkali, whose nature had not previously been known (21). 

In a definitive series of experimental investigations H. N. Wilson showed that the quinolinium salt, (C isNJ fPCV I2M0O3]3- was anhydrous, contained exactly 12 moles of molybdenum trioxide per mole of phosphate, that the precipitate had a negligible solubility and could be dried to constant weight in two hours at 105 °C. This precipitate also lent itself to a precise alkalimetric titration. In the presence of citric acid interference by silica was inhibited so that the method was admirably suitable for the analysis of basic slags or fertilizers.34... 

We observe again that the formation of hydrosilicates in the co-precipitation catalysts is deleterious for the activity, especially when montmoril-lonite is formed. The interaction causing the decrease in activity proves active even before a definite indication of hydrosilicate formation can be obtained, and it is interesting to note that partial deactivation occurs with a co-precipitation catalyst (8241), but not with a mixture type catalyst (8242). This fact appears to confirm the suggestion (see I, 5) that even a layer of silica on the Ni(OH)2 crystals during co-precipitation is harmful. 

To sum up, the main mode of extraction of dissolved silica coming into the seas and oceans from the weathered layer is biogenic precipitation—a process widespread both in recent and in ancient sedimentary basins. Biogenic precipitation of silica from undersaturated solutions could also have been of definite importance in the formation of the Precambrian iron cherts, but no direct evidence has been obtained as yet. 

When solutions of sodium silicate are acidified, silica is precipitated as a gelatinous mass, which can then be extracted and dried. The resulting silica gel, as it is called, has no definite structure and is non-crystalline. The structural units of silica gel are still silica tetrahedra but they are joined together at random and not according to a pattern . 

Hazel et al. were the first to study the strong interaction between metal ions, specificaUy Fe, and silicate species [69]. Later, when studying corrosion inhibition of zinc by silica. Hazel et al. [70] measured the interaction of Zn ion and silicates and observed slight, but definite, pH value shifts in pH titration inflection points. They suggested that hydroxyl ion was being adsorbed, or occluded, by the amorphous sihceous precipitates. Chemisorption of sihca at a zinc hydroxide surface appeared to occur independent of the surface charge or the exact chemical composition of the surface. Falcone [71] measured the activities of Ca, Mg" and Cu" in the presence of soluble silica. Polymerized sihcate anions also shifted the pMe vs. pH curves to lower pH values... 

There is some question whether true equilibrium can be established for cristobalite and tridymite. There appears to be no instance where these crystals have been grown under hydrothermal conditions in preference to quartz. Fournier and Rowe (137) believed that precipitation of crystalline silica under hydrothermal conditions would generally result in quartz. For this reason, in their study of the solubility of cristobalite, they left water in contact with the solid for long times without changing temperature, so as not to supersaturate the solution any more than neces.sary, and then suddenly quenched the system and measured the concentration of monomeric silica, which was the only type of silica in the water. In Figure 1.4, line <7 strongly suggests that a definite solubility value was established. 

In dilute solutions, colloidal metal silicates are precipitated at a pH slightly below that at which the metal hydroxide alone would be precipitated, according to Britton (198, 199). This suggests that no definite compounds are formed and that the hydroxides are precipitated along with silica by mutual coagulation. 

The most commonly met regular nano-fillers are silicas and titanias. These are generally made by controlled precipitation in the presence of surfactants, to prevent agglomeration and fusion of particles. Sol-gel processes starting from metal alkoxides are often utilised. Some very fine particles produced by more conventional gas phase processes are just within the definition used for this discussion. For example, fumed silicas, with diameters of under 10 nm are available. 

The chemical structure and physicochemical properties of asphaltenes and resins are not well understood. The operational definitions of asphaltenes and resins are based on their solubility in different diluents. Asphaltenes are defined as the fraction of crude oil insoluble in excess normal alkanes such as n-pentane but soluble in excess benzene and toluene at room temperature. Resins are defined as the fraction of crude oil insoluble in excess liquid propane at room temperature. Resins are adsorbed on silica, alumina, or other surface-active material. Figure 5.8 shows the precipitation when a bitumen oil is mixed with various diluents. The normal alkanes used are n-pentane, n-hexane, n-heptane, n-octane, n-nonane, and n-decane. According to the operational definition stated above, the asphaltene content of the bitumen... 

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