Colloidal Silica consists of tiny particles of amorphous silicon dioxide, the common silica, dispersed in a water solution and are obtained by polymerising silica nuclei in a highly basic alkaline containing environment (pH value between 8.0 and 10.5). These particles are nanometric, ranging from 5 nm to 40 nm.

The name describes the structure of the molecule:

• Colloidal. A colloid can be a mixture of solids, liquids or gases. In the case of colloidal silica, tiny particles of silica are dispersed in a liquid medium, usually water. The particles are so small that they do not settle out of solution, but form a stable dispersion.
• Silica. This is the common name for silicon dioxide (SiO2), a compound consisting of silicon and oxygen. Silica is the main component of sand and quartz and is one of the most abundant compounds in the earth's crust.

Silicon Dioxide or dioxosilane is a very common component. 

The synthesis process takes place in different steps:

• Preparation. The main raw material is silicon dioxide (SiO₂), which is abundant in nature in the form of quartz or sand.
• Dissolution. Silicon dioxide is dissolved in an alkaline solution, usually sodium hydroxide (NaOH), at high temperature and pressure. Sodium silicate is thus formed.
• Acidification. The sodium silicate solution is acidified, usually with sulphuric acid (H₂SO₄), to a pH where the silica begins to polymerise and form particles.
• Particle growth. The solution is kept at specific conditions to allow the silica particles to grow to the desired size. This is a critical stage that determines the properties of colloidal silica.
• Stabilisation. The pH and ionic strength of the solution are adjusted to stabilise the silica particles and prevent them from aggregating.
• Purification. The colloidal silica obtained is purified to remove any chemicals and by-products not reacted with by processes such as ultrafiltration or dialysis.
• Quality control. The final product is tested to ensure that it meets the necessary quality standards.

Colloidal Silica 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. It is also found in the form of a transparent liquid.

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.

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

Thickening agent. 

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).

Colloidal silica 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 TJ7Z6XBU4
• EC Number: 231-545-4      238-455-4
• DSSTox Substance ID: DTXSID1029677      DTXSID4029852     DTXSID5041801 and others
• MDL number  MFCD00011232
• PubChem Substance ID 24866359
• InChI Key    VYPSYNLAJGMNEJ-UHFFFAOYSA-N
• SMILES    O=[Si]=O
• IUPAC dioxosilane
• ChEBI 30563
• MDL number  MFCD00011232
• PubChem Substance ID   57654622

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.