Properties
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).

Application
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 ^[5].

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 ^[10].
Moreover, amorphous silicon dioxide nanoparticles are used as water repellents for cotton in the textile industry and as abrasives in the electronics industry ^[8].


Literature 

1. RÖMPP Online Version 3.6
2. Wikipedia: Siliciumdioxid
3. Datenbank zusatzstoffe-online.de: Siliciumdioxid 
4. NanoTrust Dossier Nr. 004 Mai 2008: Nanopartikel und nanostrukturierte Materialien in der Lebensmittelindustrie
5. Hessen-Nanotech NEWS 4/2006
6. Z. Amjad: Water soluble polymers: solution properties and applications. Verlag Springer, 1998, ISBN 0-306-45931-0
7. R. K. Iler, The Chemistry of Silica, John Wiley Sons, 1979
8. Nanomaterialien in Textilien: Umwelt-, Gesundheits- und Sicherheits-Aspekte; Fokus: synthetische Nanopartikel; Som, Nowack, Wick, Krug; 2010
9. Verordnung über die Zulassung von Zusatzstoffen zu Lebensmitteln zu technologischen Zwecken (Zusatzstoff-Zulassungsverordnung - ZZulV)
10. Europäisches Arzneibuch (Pharmacopoea Europaea), 6. Ausgabe, Grundwerk 2008