Silica microcrystalline

Silica, Microcrystalline - ml-kro- kris-to-lin n Si02 Pigment White 27 (77811). Extender pigment obtained from extensive geologic formations of hard, compact, homogeneous, microcrystalline silica located in the vicinity of Hot Springs, Arkansas. Can be considered as a very finely divided or micro form of quartz in a bound state of subdivision. Excellent dielectric and low abrasive properties. Density, 2.65 g/cm (22.1 lb/ gal) O.A., 20.0 mean particle size, 4.0 pm. Novaculite is a generic name. Syn Novacite. 

VAcA C microspheres Na, Li, Mg silicate (laponite) silica microcrystalline wax 19,600 50,000 surface coating two-piece packaging static-dissipating adhesive tape packaging for electronic devices semiconductor devices... 

MicrocrystaUine Silicas. Various microcrystalline (cryptocrystalline) materials such as flint, chert, and diatomaceous earth are found ia nature (see Diatomite). These may arise from amorphous silica, often of biogenic origin, which undergoes compaction and microcrysta11i2ation over geologic time. 

Crystalline free silica (Si02, including microcrystalline forms)... 

A microcrystalline form of silica. An impure form of flint used in abrasives. 

A microcrystalline impure form of silica similar to chert. Used for decorative purposes. Rare in industry. 

Diatomaceous earth, widely-known and long-used as a filteraid in process and waste filtrations, has a high microcrystalline silica content. As well as being a respiratory hazard in the workplace, the silica is being scrutinized in some jurisdictions as a potentially hazardous dust in landfills in which spent filter cakes are deposited. 

MesSiNs 19 (0.34 rtiL, 2.,4 mmol) and of Et2NCOCl (0.32 mL, 2.4 mmol) are added, in this sequence, under argon, to a solution of 3-aminopyrazine-l-oxide 934 (1 mmol) in abs. MeCN (8 mL) and the reaction mixture is heated under reflux for 18 h with exclusion of humidity. After evaporation in vacuo the residue is chromatographed with hexane-ethyl acetate (10 1 to 3 1) on a column of 20 g silica gel to give almost quantitative yield of microcrystalline 2-amino-3-azidopyrazine 935, m.p. 225 °C (dec.) [55] (Scheme 7.56). 

Paper chromatography (PC) and thin layer chromatography (TLC) have been used since the 1940s. Preparative PC on Whatman 3 paper, analytical PC on Whatman 1 paper, and analytical TLC on microcrystalline cellulose, silica gel, or polyamide have been applied with a variety of solvents and the behaviors of anthocyanins have been similar in all media. Two-dimensional TLC allows the separation of several compounds and has been nsed to clarify the anthocyanin compositions of different commodities. ... 

These aspects of solvent property similarly apply to precoated impregnated silica gel plates, e.g., by ammonium sulfate, silver nitrate, or magnesium acetate, as well as to microcrystalline cellulose precoated plates. On preparative RP phases, water has the lowest elution power. Therefore, more polar or aqueous solvents should be preferred. In contrast to HPTLC RP-18 layers, on which such aqueous solutions remain as a drop on the surface and are not able to penetrate through the lipophilic layer, on preparative RP phases, pnre aqneons application solutions can be apphed owing to the minor degree of C18 modification. 

Adsorbents are natural or synthetic materials of amorphous or microcrystalline structure. Those used on a large scale, in order of sales volume, are activated carbon, molecular sieves, silica gel, and activated alumina [Keller et al., gen. refs.]. 

Leamnson, R. N., J. Thomas, Jr. and H.P Ehrlinger, III, 1969, A study of the surface areas of particulate microcrystalline silica and silica sand. Illinois State Geological Survey Circular 444. 

Figure 3.12 Enantioseparation of TrOger s base on (a) microcrystalline cellulose triacetate (CTA I) and (b) 21 coated on silica gel. Column, 25 x 0.46 (i.d.) cm eluent ethanol-HoO (7/3) flow rate, 0.5 ml/min).

We also include in this class of quasi-2D nanostructured materials Titania deposited inside ordered mesoporous silica (because an inner coating of mesoporous silica may be realized), or nano-dot type Titania particles well dispersed in the ordered porous matrix. We do not consider here solids which contain linear or zig-zag type TiOTiO-nanowires in a microcrystalline porous framework, such as ETS-4 and ETS-10, notwithstanding the interest of these materials also as photocatalysts,146-151 because these nanowires are located inside the host matrix, and not fully accessible from the gas reactants (the reactivity is essentially at pore mouth). 

Microcrystallin cellulose triacetate (MCTA) has also been employed in the TLC analysis of flavanones. Plates were home-made by mixing 3 g of silica with 9 g of MCTA in 35 ml of ethanol and the suspension was spread on to 20 X 10 cm plates. A considerable number... 

RETENTION (hRFli hRmf AND RESOLUTION (a, Rs)h DATA FOR ENANTIOMERIC COMPOUNDS ON MICROCRYSTALLINE CELLULOSE TRIACETATE PLATES WITH SILICA GEL 60 GF254 AS BINDER (TEMPERATURE 25°C)... 

The same principles that are valid for the surface of crystalline substances hold for the surface of amorphous solids. Crystals can be of the purely ionic type, e.g., NaF, or of the purely covalent type, e.g., diamond. Most substances, however, are somewhere in between these extremes [even in lithium fluoride, a slight tendency towards bond formation between cations and anions has been shown by precise determinations of the electron density distribution (/)]. Mostly, amorphous solids are found with predominantly covalent bonds. As with liquids, there is usually some close-range ordering of the atoms similar to the ordering in the corresponding crystalline structures. Obviously, this is caused by the tendency of the atoms to retain their normal electron configuration, such as the sp hybridization of silicon in silica. Here, too, transitions from crystalline to amorphous do occur. The microcrystalline forms of carbon which are structurally descended from graphite are an example. 

Chromatographic resolution is also dependent on column efficiency (i). Column efficiency is directly dependent on the nature of the support matrix and how well that support is packed in its column. Available chromatographic supports are based on dextran, agarose, polystyrene, acrylic, cellulose, silica gel and a variety of other polymers. Althou cellulosic supports are manufactured in both microcrystalline and leaded forms, most supports are beaded. Newer supports may use hybrid bead construction where the base support is coated with a second materid (e.g., dextran or silica coated with agarose). 

Microscopically, the BIF is composed mainly of iron oxide rich bands that alternate with microcrystalline silica bands, quartz, chert and jasper. The ratios of these constituents are different from one sample to another. Occasionally, the samples contain sulfides, garnet and graphite. 

Because the forces of attraction prevail when molecules are brought into sufficiently dose proximity under normal conditions, release is best effected if both the strength of the interaction and the degree of contact are minimized. Aliphatic hydrocarbons and fluorocarbons achieve the former effect, finely divided solids the latter. Materials such as microcrystalline wax [64742 42-3] and hydrophobic silica [7631-86-9] combine both effects. Some authors refer to this combined effect as the ball bearing mechanism. A perfluoroalkylated fullerene nanosphere would perhaps be the ultimate example of this combined effect (17). These very general mechanistic remarks can be supplemented by publications on the mechanism of specific classes of release agents such as metallic stearates (18), fatty acids and fluorinated compounds (19), and silicone-coated rdease papers (20,21). The mechanism of release of certain problem adherents, eg, polyurethanes, has also been addressed (22,23). 

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