E551 SILICON DIOXIDE

E551 Silicon Dioxide occurs almost everywhere on earth. 
E551 Silicon Dioxide is one of the most important and abundant oxides on earth, constituting about 60% weight of the earth’s crust as silica itself or in combination with other metal oxides in silicates. 
E551 Silicon Dioxide commonly is found as sand in the vast ocean and river shores, their beds, deserts, rocks, and minerals.

CAS:    7631-86-9
MF:    O2Si
MW:    60.08
EINECS:    231-545-4

Synonyms
SILICA GEL 60 PF254 FOR PREPARATIVE LAYE;LICHROSORB SI 100 (10 MYM) 10 G;TLC-SILICA GEL 60 GF254 MEAN PARTICLE SI;LICHROSORB SI 100 (10 MYM) 100 G;SEA SAND EXTRA PURE 5 KG;SILICA GEL 60 GF254 FOR THIN-LAYER CHROM;SILICA GEL 60 PF254 + 366 FOR PREPARATIV;SEA SAND EXTRA PURE 25 KG

E551 Silicon Dioxide exists in several structural forms: polymorphic crystalline silica, synthetic quartz crystals, amorphous silica, and vitreous silica. 
E551 Silicon Dioxide is not complete as there are other forms of silica synthesized for specialized applications.
A silicon oxide made up of linear triatomic molecules in which a silicon atom is covalently bonded to two oxygens.
E551 Silicon Dioxide, also known as silica, is an oxide of silicon with the chemical formula SiO2, commonly found in nature as quartz.
In many parts of the world, silica is the major constituent of sand.
Silica is one of the most complex and abundant families of materials, existing as a compound of several minerals and as a synthetic product. 
Examples include fused quartz, fumed silica, opal, and aerogels. 
E551 Silicon Dioxide is used in structural materials, microelectronics, and as components in the food and pharmaceutical industries. 

All forms are white or colorless, although impure samples can be colored.
E551 Silicon Dioxide is a common fundamental constituent of glass.
The additive E551, silicon dioxide, is an anti-caking substance, used for clarification and stabilisation. 
E551 Silicon Dioxide is the chemical formula of a group of inorganic polymers where each silicon atom is surrounded by 4 tetrahedrally arranged oxygen atoms. 
The average stoichiometric composition of the compound is SiO2.
The IUPAC name is silicon dioxide. 
E551 Silicon Dioxide is also known as silicon oxide, aerosil, silica, silicon dioxide.
The molecular mass of silicon dioxide is 60.6 g/mol.

In the food industry, silica is known as food additive E551. 
The food industry uses amorphous silica, i.e. not crystalline, odourless, non-toxic and chemically inert. 
Food grade silica meets the requirements of the US Food and Drug Administration (FDA), the regulations specified in 21 CFR (2) citations and the FCC (3) food additive requirements, and complies with European regulations (E551). 
Food grade silica is a synthetic mixture of precipitated amorphous silica particles which improves the flow properties of powders by facilitating their handling, transport and use. 
The high absorption capacity and adhesive properties of food grade silica make E551 Silicon Dioxide ideal for use with all types of foodstuffs, with exceptional adhesion properties that improve the flow characteristics of the powder. 

E551 Silicon Dioxide is a white, very fluffy powder (1kg~2L) which is insoluble in water and does not absorb it, making it very suitable for use as an anti-stick additive. 
E551 Silicon Dioxide is used in bulk products, spices, freeze-dried products, mixing, coating of basic products to prevent sticking. 
E551 Silicon Dioxide can be found in icing sugar for coating Turkish desserts (Lokum). 
In the food industry, E551 Silicon Dioxide prevents materials from sticking together by spray-drying, resulting in a fine powder, improves milling processes, and facilitates the transport of powders during processing (extends shelf-life, e.g. with spices, table salt), allows precise dosing, e.g. in the form of a powder. 
E551 Silicon Dioxide improves the distribution of powdered products, e.g. milk powder and cream, soups, powdered drinks, grated cheese, etc. 
Due to its high absorption capacity, it facilitates production processes by converting liquids into solids, improves the dosage of high-value or highly viscous products, preserves liquid properties such as color, flavor, essential oils.
In cosmetics E551 is used in creams, scrubs, tooth pastes. 
E551 Silicon Dioxide is useful for its light-diffusing properties and natural absorbency. 
E551 Silicon Dioxide removes dental plaque or dirt from the skin surface through a grinding / polishing effect. 

E551 Silicon Dioxide Chemical Properties
Melting point: >1600 °C(lit.)
Boiling point: >100 °C(lit.)
Density: 2.2-2.6 g/mL at 25 °C
Bulk density: 200-800kg/m3
Vapor pressure: 13.3hPa at 1732℃
Refractive index: 1.46
Fp: 2230°C
Storage temp.: 2-8°C
Solubility: Practically insoluble in water and in mineral acids except hydrofluoric acid. 
E551 Silicon Dioxide dissolves in hot solutions of alkali hydroxides.
Form: suspension
pka: 6.65-9.8[at 20 ℃]
Specific Gravity: 2.2
Color: White to yellow
Odor: at 100.00?%. odorless
PH: 5-8 (100g/l, H2O, 20℃)(slurry)
Resistivity: 1∞10*20 (ρ/μΩ.cm)
Water Solubility: insoluble
Sensitive: Hygroscopic
Hydrolytic Sensitivity    6: forms irreversible hydrate
Crystal Structure: Trigonal
Merck: 14,8493
Exposure limits    NIOSH: IDLH 3000 mg/m3; TWA 6 mg/m3
Stability: Stable.
CAS DataBase Reference: 7631-86-9(CAS DataBase Reference)
NIST Chemistry Reference: Silicon(iv) oxide(7631-86-9)
IARC: 3 (Vol. Sup 7, 68) 1997
EPA Substance Registry System: Silica (7631-86-9)

Colorless amorphous (i.e., fused silica) or crystalline (i.e., quartz) material having a low thermal expansion coefficient and excellent optical transmittance in far UV.
Silica is insoluble in strong mineral acids and alkalis except HF, concentrated H3PO4, NH4 HF2 , concentrated alkali metal hydroxides. 
Owing to its good corrosion resistance to liquid metals such as Si, Ge, Sn, Pb, Ga, In, Tl, Rb, Bi, and Cd, E551 Silicon Dioxide is used as crucible container for melting these metals, while silica is readily attacked in an inert atmosphere by molten metals such as Li, Na, K Mg, and Al. 
Quartz crystals are piezoelectric and pyroelectric. 
Maximum service temperature 1090°C.
Amorphous silica, the noncrystalline form of SiO2, is a transparent to gray, odorless, amorphous powder.
Diatomaceous earth is a transparent to gray, odorless amorphous powder.

Structure
In the majority of E551 Silicon Dioxide, the silicon atom shows tetrahedral coordination, with four oxygen atoms surrounding a central Si atom (see 3-D Unit Cell). 
Thus, SiO2 forms 3-dimensional network solids in which each silicon atom is covalently bonded in a tetrahedral manner to 4 oxygen atoms.
In contrast, CO2 is a linear molecule. 
The starkly different structures of the dioxides of carbon and silicon are a manifestation of the double bond rule.

Based on the crystal structural differences, silicon dioxide can be divided into two categories: crystalline and non-crystalline (amorphous). 
In crystalline form, this substance can be found naturally occurring as quartz, tridymite (high-temperature form), cristobalite (high-temperature form), stishovite (high-pressure form), and coesite (high-pressure form). 
On the other hand, amorphous silica can be found in nature as opal and diatomaceous earth. Quartz glass is a form of intermediate state between these structures.

All of these distinct crystalline forms always have the same local structure around Si and O. 
In α-quartz the Si–O bond length is 161 pm, whereas in α-tridymite E551 Silicon Dioxide is in the range 154–171 pm. 
The Si–O–Si angle also varies between a low value of 140° in α-tridymite, up to 180° in β-tridymite. 
In α-quartz, the Si–O–Si angle is 144°.

Uses    
E551 Silicon Dioxide is also known as silicone dioxide. 
E551 Silicon Dioxide has a variety of applications: to control a product’s viscosity, add bulk, and reduce a formulation’s transparency.
E551 Silicon Dioxide can also function as an abrasive. 
In addition, E551 Silicon Dioxide can act as a carrier for emollients, and may be used to improve a formulation’s skin feel. 
Spherical silica is porous and highly absorbent, with absorption capabilities roughly 1.5 times its weight. 
A typical claim associated with silica is oil control. 
E551 Silicon Dioxide is found in sunscreens, scrubs, and wide range of other skin care, makeup, and hair care preparations. 

E551 Silicon Dioxide has been successfully used in hypoallergenic and allergy-tested formulations.
Silica (SiO2) (RI: 1.48) is mined from deposits of diatomaceous soft chalk-like rock (keiselghur). 
This is an important group of extender pigments, which is used in a variety of particle sizes. 
They are used as a flatting agent to reduce gloss of clear coatings and to impart shear thinning flow properties to coatings. 
They are relatively expensive.
E551 Silicon Dioxide, amorphous is used as carriers, processing aids, anti-caking and free-flow agents in animal feed. 
Defoamer applications such as paint, food, paper, textile and other industrial applications. 
Synthetic E551 Silicon Dioxide is used as a rheology control agent in plastics. 
E551 Silicon Dioxide is also used to manufacture adhesives, sealants and silicones.

Structural use
About 95% of the commercial use of E551 Silicon Dioxide is in the construction industry, e.g. in the production of concrete (Portland cement concrete).
Certain deposits of silica sand, with desirable particle size and shape and desirable clay and other mineral content, were important for sand casting of metallic products.
The high melting point of silica enables it to be used in such applications such as iron casting; modern sand casting sometimes uses other minerals for other reasons.
Crystalline silica is used in hydraulic fracturing of formations which contain tight oil and shale gas.

Precursor to glass and silicon
Silica is the primary ingredient in the production of most glass. 
As other minerals are melted with silica, the principle of freezing point depression lowers the melting point of the mixture and increases fluidity. 
The glass transition temperature of pure SiO2 is about 1475 K.
When molten E551 Silicon Dioxide is rapidly cooled, it does not crystallize, but solidifies as a glass.
Because of this, most ceramic glazes have silica as the main ingredient.

The structural geometry of silicon and oxygen in glass is similar to that in quartz and most other crystalline forms of silicon and oxygen, with silicon surrounded by regular tetrahedra of oxygen centres. 
The difference between the glass and crystalline forms arises from the connectivity of the tetrahedral units: Although there is no long-range periodicity in the glassy network, ordering remains at length scales well beyond the SiO bond length. 
One example of this ordering is the preference to form rings of 6-tetrahedra.
The majority of optical fibers for telecommunications are also made from silica. 
E551 Silicon Dioxide is a primary raw material for many ceramics such as earthenware, stoneware, and porcelain.

Food, cosmetic, and pharmaceutical applications
Silica, either colloidal, precipitated, or pyrogenic fumed, is a common additive in food production. 
E551 Silicon Dioxide is used primarily as a flow or anti-caking agent in powdered foods such as spices and non-dairy coffee creamer, or powders to be formed into pharmaceutical tablets.
E551 Silicon Dioxide can adsorb water in hygroscopic applications.
Colloidal silica is used as a fining agent for wine, beer, and juice, with the E number reference E551.
In cosmetics, silica is useful for its light-diffusing properties and natural absorbency.
Diatomaceous earth, a mined product, has been used in food and cosmetics for centuries. 
E551 Silicon Dioxide consists of the silica shells of microscopic diatoms; in a less processed form it was sold as tooth powder.
Manufactured or mined hydrated silica is used as the hard abrasive in toothpaste.

Agricultural Uses    
E551 Silicon Dioxide is one of the most abundant materials on the earth's crust. 
Quartz is an example of silica. 
E551 Silicon Dioxide is used as a filler in fertilizers, and also, in the manufacture of glass, ceramics, abrasives, rubber and cosmetics.

Health effects
E551 Silicon Dioxide ingested orally is essentially nontoxic, with an LD50 of 5000 mg/kg (5 g/kg).
A 2008 study following subjects for 15 years found that higher levels of E551 Silicon Dioxide in water appeared to decrease the risk of dementia. 
An increase of 10 mg/day of silica in drinking water was associated with a reduced risk of dementia of 11%.
Inhaling finely divided crystalline E551 Silicon Dioxide dust can lead to silicosis, bronchitis, or lung cancer, as the dust becomes lodged in the lungs and continuously irritates the tissue, reducing lung capacities.
When fine E551 Silicon Dioxide particles are inhaled in large enough quantities (such as through occupational exposure), it increases the risk of systemic autoimmune diseases such as lupus and rheumatoid arthritis compared to expected rates in the general population.