Silicon dioxide (SiO2) is the most common silicon compound and a major constituent of the Earth’s crust.
Properties and Applications
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 0,12g SiO2 per liter water (120ppm) dissolve that way [6,7]. The dissolution rate of amorphous SiO2 is about 10 times higher than 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.
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  to avoid clogging. Moreover, they are permitted for use as carrier substances in emulsifying agents, colorings, and flavors . 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 particles 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 .
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.
Silicon dioxid is not self-inflammable as nanometer-sized powder. Also as a mixture with air (dust) under the influence of an ignition source, it is not inflammable, so there is no possibility of a dust explosion.
Occurrence and Production
It occurs in nature in the crystalline (mostly quartz) and amorphous forms. Being a major constituent of sand, it is found in numerous types of rock and occurs, in addition, in precious stones and gemstones such as rock crystal.
The so-called amorphous non-crystalline silicon dioxides may be of biological origin or are formed by nature whenever rock is subjected to high temperatures (volcanoes, meteorite impacts, lightning strokes, geysers). Opals, which are very popular due to their ”opalescent“ colors, are an amorphous form of SiO2.
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 5-50nm and forms solid aggregates above 100 nm (150-200nm). The powders are characterized by high specific surface areas (above 50m²/g).
- Roempp Online (DE): Silizium (last access date: Jun 2010).
- Wikipedia (EN): Silicon Dioxide (last access date: Jun 2010).
- Zusatzstoffe-online.de:Siliziumdioxid(last access date: Jun 2010).
- NanoTrust Dossier No.004en (May 2008). Nanoparticles and nanostructured materials in the food industry, NanoTrust,Institute of Technology Assessment (ITA), Vienna.
- Hessen-Nanotech NEWS 4/2006. Nano-Produktion – Herstellung von und mit Nanotechnologie, Band 9, 01.09.2006.
- Amjad, Z (1998). Water soluble polymers: solution properties and applications, Kluwer Academic Publishers, New York, ISBN 0-306-45931-0.
- Iler, RK (1979). The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties and Biochemistry of Silica, John Wiley Sons,ISBN 978-0471024040.
- Som, C et al. (Mar 2010). Nanomaterialien in Textilien: Umwelt-, Gesundheits- und Sicherheits-Aspekte, Fokus: synthetische Nanopartikel. Empa und TVS Textilverband Schweiz, St. Gallen 2010. (in German).
- Zusatzstoff-Zulassungsverordnung (ZZulV) (1998). Verordnung über die Zulassung von Zusatzstoffen zu Lebensmitteln zu technologischen Zwecken.gesetze-im-internet.de (last access date: Mar 2010). (in German)
- Europäisches Arzneibuch (Pharmacopoea Europaea) (2008), 6. Ausgabe, Grundwerk, Deutscher Apotherker Verlag Stuttgart. ISBN 978-3769253832. (in German).
- European Council Regulation (EC) No 834/2007 (28.06.2007). On organic production and labelling of organic products and repealing Regulation (EEC) No 2092/91.