Silicon dioxide or Silicon dioxide is a very common chemical compound, silicon oxide. The term 'silicon dioxide' is derived from its constituent elements:

• "dioxide" indicates that there are two oxygen atoms for every silicon atom in the compound. The prefix 'di-' comes from the Greek word 'two' and 'oxide' refers to the oxygen component of the compound.
• "Silicon" is a chemical element with the symbol Si and the atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid and semiconductor.

The synthesis of silicon dioxide can occur naturally or be produced industrially. Here are some common methods:

• Natural formation: silicon dioxide forms naturally over time through geological processes. It is the main component of sand and quartz and is also found in many types of rock.
• From silicon: silicon dioxide can be produced from silicon through oxidation. When silicon is exposed to oxygen (in the air), it reacts to form silicon dioxide. This is the principle behind the formation of a thin layer of silicon dioxide on the surface of silicon wafers used in the semiconductor industry.
• From sodium silicate: Silicon dioxide can also be produced from sodium silicate, also known as water glass. 
• From tetraethylorthosilicate (TEOS): In the sol-gel process, a common method for producing silica gels and aerogels, silicon dioxide is synthesised from a silicon alkoxide, such as tetraethylorthosilicate (TEOS). When TEOS is hydrolysed and then condensed, it forms silicon dioxide.

It appears in the form of a white powder with a native particle size of 5nm to 40nm with functions of filling, thickening, strengthening and thixotropy of various materials. 

In industry, it is used in the form of "Fumed silica" synthesized by pyrolysis method in which halogenated silanes (hydrolyzed chlorosilane) react with oxygen and hydrogen at high temperature. 

This type of silica is used in many industries:

• Glass
• Rubber
• Optics
• Construction
• Food
• Medicine

Where it comes from

It is found in the earth's crust, in rocks in crystalline or amorphous form.

What it is used for and where

Food

In particular, in food can be used in foods, food supplements and other as anti-caking agent. Labelled as food additive thickener E551 in the European Food Additives List,  its function is to avoid clumping in sauces, food supplements, cooking salt, dry foods. "Anti-caking agents" are substances that reduce the tendency of individual particles of a food product to adhere to each other.

Chillproofing agent in beer.

Pharmaceuticals

In the pharmaceutical industry, colloidal silica improves powder flow as it acts on breaking the interparticle force of silica particles adhering to the product surface. It is a thickening agent. Food thickeners are normally used to facilitate the ingestion of drugs in tablet form. Some thickeners directly influence the dissolution and disintegration of tablets and may even delay their dissolution.

It should not exceed 2% of the total weight of the product in which it is inserted.

Cosmetics

• Abrasive agent. It contains abrasive particles to remove stains or biofilm that accumulate on the stratum corneum or teeth. Baking soda, kieselguhr, silica and many others have abrasive properties. Peeling or exfoliating products used in dermatology or cosmetic applications contain abrasive agents in the form of synthetic microspheres, however these microspheres or abrasive particles are not biodegradable and create pollution in aquatic ecosystems.
• Absorbent. Absorbs substances dispersed or dissolved in aqueous solutions, water/oil, oil/water.
• Anticaking agent. This compound facilitates free flow and prevents aggregation or clumping of substances in a formulation by reducing the tendency of certain particles to stick together. 
• Bulking agent. It regulates the water content, dilutes other solids, can increase the volume of a product for better flow, acts as a buffer against organic acids, helps to keep the pH of the mixture within a certain level.
• Opacifying agent. It is useful into formulations that may be translucent or transparent to make them opaque and less permeable to light.
• Viscosity control agent. It controls and adapts, Increasing or decreasing, viscosity to the required level for optimal chemical and physical stability of the product and dosage in gels, suspensions, emulsions, solutions. 

Other uses

• controls the rheology and thixotropy of binder systems, polymers, liquids, etc.
• reinforces HTV- and RTV- 2K silicone rubber
• absorbent and carrier in ink-jet printing coatings
• filling for dental materials
• low-temperature and high-temperature insulation as a carrier catalyst, with significant thermal insulation.

Safety

Regarding its toxicity, we must refer to nanoparticles, rather than milligrams (2) since its presence derives, mainly in the environment, from plastics, rubber, ceramics, paints, adhesives and others and it is also the product of coal combustion (3).

The potential risk of nanoparticles from air pollution has recently attracted a great deal of attention. Although the toxicology of nanoparticles has been extensively studied, little work has been reported on the combined effect of silicon dioxide (SiO₂) nanoparticles and cold exposure at the cellular level. 

The haze problem has a major impact on public health, and has been a widespread concern in recent years. Studies have shown that ultrafine particulate matter (PM 0.1 ), which is equivalent to nanoparticles, is harmful to humans. The increasing use of nanoparticles for a wide range of commercial, industrial, and biomedical applications has led to safety concerns (4).

In this study, the genotoxic effects of SiO₂^EN20(-) and SiO₂^EN100(-) were elucidated using four genotoxicity assays in standardized Good Laboratory Practice system protocols. Although the different exposure routes in this study may induce genotoxicity from SiO₂^EN20(-) and SiO₂^EN100(-) in different organs in in vivo systems, the data suggest that SiO2 are not genotoxic substances based on OECD test guidelines (5).

The results of this research confirmed that the addition of colloidal silica in microemulsion simultaneously loaded with vitamins C and E improved the skin bioavailability of the vitamins due to its dual influence on the delivery characteristics of the microemulsion and the properties of the skin (6).

Silicon dioxide studies

General features:

• refractive index    n20/D 1.46 (lit.)
• surface area    395 m2/g±25 m2/g
• density    2.3 lb/cu.ft at 25 °C (bulk density)(lit.)

• Molecular Formula : SiO₂  O₂Si
• Molecular Weight : 60.08
• CAS : 112926-00-8   7631-86-9
• UNII  ETJ7Z6XBU4
• EC Number  215-683-2
• DSSTox Substance ID  DTXSID1029677      DTXSID4029852     DTXSID5041801 e molti altri
• MDL number  MFCD00011232
• PubChem Substance ID 24866359
• InChI=1S/O2Si/c1-3-2
• InChI Key    VYPSYNLAJGMNEJ-UHFFFAOYSA-N
• SMILES    O=[Si]=O
• IUPAC dioxosilane
• ChEBI 30563
• MDL number  MFCD00011232
• PubChem Substance ID   57654622
• ISC    0248   0807    0808    0809 
• RTECS    VV7325000    VV7330000    VV7335000
• NCI    C29853
• RXCUI    9771    314826    1362874

Synonyms: 

• Silicon dioxide
• Silicon dioxide amorphous
• Silicic anhydride

References_______________________________________________________________________

(1) Tran DT, Majerová D, Veselý M, Kulaviak L, Ruzicka MC, Zámostný P. On the mechanism of colloidal silica action to improve flow properties of pharmaceutical excipients. Int J Pharm. 2019 Feb 10;556:383-394. doi: 10.1016/j.ijpharm.2018.11.066.

(2) Petrick L, Rosenblat M, Paland N, Aviram M. Silicon dioxide nanoparticles increase macrophage atherogenicity: Stimulation of cellular cytotoxicity, oxidative stress, and triglycerides accumulation. Environ Toxicol. 2016 Jun;31(6):713-23. doi: 10.1002/tox.22084.

(3) Dai C, Huang Y, Zhou Y. Research progress about the relationship between nanoparticles silicon dioxide and lung cancer. Zhongguo Fei Ai Za Zhi. 2014 Oct 20;17(10):760-4. Chinese. doi: 10.3779/j.issn.1009-3419.2014.10.09.

Hassankhani R, Esmaeillou M, Tehrani AA, Nasirzadeh K, Khadir F, Maadi H. In vivo toxicity of orally administrated silicon dioxide nanoparticles in healthy adult mice. Environ Sci Pollut Res Int. 2015 Jan;22(2):1127-32. doi: 10.1007/s11356-014-3413-7.

(4) Zhang Y, Li X, Lin Y, Zhang L, Guo Z, Zhao D, Yang D. The combined effects of silicon dioxide nanoparticles and cold air exposure on the metabolism and inflammatory responses in white adipocytes. Toxicol Res (Camb). 2017 Jul 6;6(5):705-710. doi: 10.1039/c7tx00145b.

(5) Kwon JY, Kim HL, Lee JY, Ju YH, Kim JS, Kang SH, Kim YR, Lee JK, Jeong J, Kim MK, Maeng EH, Seo YR. Undetactable levels of genotoxicity of SiO2 nanoparticles in in vitro and in vivo tests. Int J Nanomedicine. 2014 Dec 15;9 Suppl 2(Suppl 2):173-81. doi: 10.2147/IJN.S57933. Erratum in: Int J Nanomedicine. 2015;10:4621. 

(5) Kwon JY, Kim HL, Lee JY, Ju YH, Kim JS, Kang SH, Kim YR, Lee JK, Jeong J, Kim MK, Maeng EH, Seo YR. Undetactable levels of genotoxicity of SiO2 nanoparticles in in vitro and in vivo tests. Int J Nanomedicine. 2014 Dec 15;9 Suppl 2(Suppl 2):173-81. doi: 10.2147/IJN.S57933. Erratum in: Int J Nanomedicine. 2015;10:4621. 

(6) Rozman B, Gosenca M, Gasperlin M, Padois K, Falson F. Dual influence of colloidal silica on skin deposition of vitamins C and E simultaneously incorporated in topical microemulsions. Drug Dev Ind Pharm. 2010 Jul;36(7):852-60. doi: 10.3109/03639040903541187.