Silicon dioxide is a very hard substance that is resistant against chemicals and alteration. Both crystalline and amorphous SiO2 are nearly insoluble in water and in acids. In aqueous suspensions, however, the very fine-grained forms of the amorphous type slowly transform into silicic acid SiO2 x n H2O. At 25 °C and a pH-value of 7 (neutral), approximately 0.12 g SiO2 per liter water (120 ppm) dissolve that way [6, 7]. The dissolution rate of amorphous SiO2 is about 10 times higher that of quartz. Particularly amorphous SiO2 can be dissolved by aqueous alkaline substances. Being resistant against other acids, SiO2 corrodes when exposed to hydrofluoric acid. Technical Production
High quantities of amorphous SiO2 are produced at a large scale through precipitation or in oxyhydrogen flames. The latter product is often referred to as pyrogenic SiO2 or pyrogenic silicic acid. Pyrogenic SiO2 occurs as powder that consists of primary particles sized 5-50 nm and forms solid aggregates above 100 nm (150-200 nm). The powders are characterized by high specific surface areas (above 50 m²/g).
Natural silicon dioxide is an important base material for the glass industry, the optical industry and building industry. Quartz glasses provide the basis for manufacturing lenses and other optical components as well as temperature-resistant equipment for the chemical industry. Different kinds of SiO2 are used for manufacturing concrete and other building materials. In addition, SiO2 are used as filters and desiccants.Synthetic amorphous SiO2 are used as fillers in plastics, rubber, dyes, and adhesives and serve as adsorbents or trickling agents. They improve the hardness and scratch resistance of surface coatings and varnishes. Although SiO2 has a lower hardness than aluminum oxide, which is used alternatively, clear varnishes that contain nanostructured SiO2 have a much better transparency.
Nano-SiO2 is used increasingly for tire manufacturing. Adding amorphous SiO2 as fillers in addition to carbon black, the tire roll resistance is reduced, and gasoline consumption decreases by up to five percent. Since CO2 emissions are reduced that way, this is not only easy on the wallet but also on the environment .
Amorphous silicon dioxides have been used for more than four decades as food additives (E551). They can be added to certain powdery foods such as table salt, seasonings, dietary supplements, and dry foods [ 3 ] to avoid clogging. Moreover, they are permitted for use as carrier substances in emulsifying agents, colorings, and flavors [ 9 ]. According to the EU Rules on Organic Farming, SiO2 additives are also approved for use in biological food. Since silicon dioxide can neither be absorbed nor salvaged by the human organism, it is excreted in its unchanged form.
Amorphous SiO2 have been approved for use as food additives since they were first tested more than 40 years ago. Since the particle size and structure have remained unchanged, these substances are not considered products of modern nanotechnology [ 4 ].
Highly disperse (nanoscale) amorphous SiO2 are also contained in diverse pharmaceutical products such as tablets, suppositories, gels, and creams. The properties of the approved additives are laid down in the European Pharmacopoeia .
Moreover, amorphous silicon dioxide nanoparticles are used as water repellents for cotton in the textile industry and as abrasives in the electronics industry .
- RÖMPP Online Version 3.6
- Wikipedia: Siliciumdioxid
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- Z. Amjad: Water soluble polymers: solution properties and applications. Verlag Springer, 1998, ISBN 0-306-45931-0
- R. K. Iler, The Chemistry of Silica, John Wiley Sons, 1979
- Nanomaterialien in Textilien: Umwelt-, Gesundheits- und Sicherheits-Aspekte; Fokus: synthetische Nanopartikel; Som, Nowack, Wick, Krug; 2010
- Verordnung über die Zulassung von Zusatzstoffen zu Lebensmitteln zu technologischen Zwecken (Zusatzstoff-Zulassungsverordnung - ZZulV)
- Europäisches Arzneibuch (Pharmacopoea Europaea), 6. Ausgabe, Grundwerk 2008