SILICON OXIDE (SIO2): STRUCTURE, PROPERTIES, USES, OBTAINING - PHYSICAL - 2024

The silicon oxide is an inorganic solid formed by bonding of a silicon atom and two oxygen. Its chemical formula is SiO ₂. This natural compound is also called silica or silicon dioxide.

SiO ₂ is the most abundant mineral in the earth's crust, since sand is composed of silica. Depending on its structure, silica can be crystalline or amorphous. It is insoluble in water, but dissolves in alkalis and HF hydrofluoric acid.

Sand is a source of silicon dioxide SiO ₂. ರವಿಮುಂ. Source: Wikimedia Commons.

SiO ₂ is also present in the structure of certain plants, bacteria and fungi. Also in skeletons of marine organisms. Besides sand, there are also other types of stones that are made of silica.

Silica is widely used, fulfilling a variety of functions. The most widespread use is as a filter material for liquids such as oils and petroleum products, beverages such as beer and wine, as well as fruit juices.

But it has many other applications. One of the most useful and important is in the manufacture of bioactive glasses, which make it possible to make “scaffolds” where bone cells grow to produce bone pieces missing due to accident or disease.

Structure

Silicon dioxide SiO ₂ is a three-atom molecule, in which the silicon atom is bound to two covalently bonded oxygen atoms.

Chemical structure of the SiO ₂ molecule. Grasso Luigi. Source: Wikimedia Commons.

The structural unit of solid silica as such is a tetrahedron where one silicon atom is surrounded by 4 oxygen atoms.

Structural unit of solid silica: gray = silicon, red = oxygen. Benjah-bmm27. Source: Wikimedia Commons.

Tetrahedra bond together by sharing oxygen atoms from their contiguous vertices.

That is why a silicon atom shares each of the 4 oxygen atoms in half and this explains the relationship in the compound of 1 silicon atom to 2 oxygen atoms (SiO ₂).

Tetrahedra share the oxygens in SiO ₂. Benjah-bmm27. Source: Wikimedia Commons.

SiO ₂ compounds are divided into two groups: crystalline silica and amorphous silica.

Crystalline silica compounds have repeating pattern structures of silicon and oxygen.

Crystalline silica has repeating units. Wersję rastrową wykonał użytkownik polskiego projektu wikipedii: Polimerek, Zwektoryzował: Krzysztof Zajączkowski. Source: Wikimedia Commons.

All silica crystal can be considered as a giant molecule where the crystal lattice is very strong. Tetrahedra can be linked in various ways, giving rise to various crystalline forms.

In amorphous silica, the structures are attached randomly, without following a defined regular pattern between the molecules and these are in a different spatial relationship with each other.

In amorphous silica the bonds are not repetitive or uniform. Silica.svg: * Silica.jpg: en: User: Jdrewittderivative work: Matt. Source: Wikimedia Commons.

Nomenclature

-Silicon oxide

-Silicon dioxide

-Silica

-Quartz

-Tridimita

-Christobalite

-Dioxosilane

Properties

Physical state

Colorless to gray solid.

Sample of pure SiO ₂. LHcheM. Source: Wikimedia Commons.

Molecular weight

60.084 g / mol

Melting point

1713 ºC

Boiling point

2230 ºC

Density

2.17-2.32 g / cm ³

Solubility

Insoluble in water. Amorphous silica is soluble in alkalis, especially if it is finely divided. Soluble in hydrofluoric acid HF.

Amorphous silica is less hydrophilic, that is, less related to water than crystalline.

Chemical properties

SiO ₂ or silica is essentially inert to most substances, it is very little reactive.

Resists attack from chlorine Cl ₂, bromine Br ₂, hydrogen H ₂ and most acids at room temperature or slightly higher. It is attacked by fluorine F ₂, hydrofluoric acid HF and by alkalis such as sodium carbonate Na ₂ CO ₃.

SiO ₂ can combine with metallic elements and oxides to form silicates. If silica is melted with alkali metal carbonates at about 1300 ° C, alkali silicates are obtained and CO ₂ is evolved.

It is not combustible. It has low thermal conductivity.

Presence in nature

The main source of silica in nature is sand.

SiO ₂ or silica is in the form of three crystalline varieties: quartz (the most stable), tridymite, and cristobalite. Amorphous forms of silica are agate, jasper, and onyx. Opal is an amorphous hydrated silica.

There is also the so-called biogenic silica, that is, that generated by living organisms. Sources of this type of silica are bacteria, fungi, diatoms, sea sponges and plants.

The shiny, hard parts of bamboo and straw contain silica, and the skeletons of some marine organisms also have a high proportion of silica; however, the most important are diatomaceous earths.

Diatomaceous earths are geological products of decayed single-celled organisms (algae).

Other types of natural silica

In nature there are also the following varieties:

- Vitreous silicas that are volcanic glasses

- Lechaterielites that are natural glasses produced by the fusion of siliceous material under the impact of meteorites

- Fused silica which is silica heated to the liquid phase and cooled without allowing it to crystallize

Obtaining

Silica from sands is obtained directly from quarries.

Sand quarry in California. Ruff tuff cream puff. Source: Wikimedia Commons.

Diatomite or diatomaceous earth is also obtained in this way, using excavators and similar equipment.

La sílice amorfa se prepara partiendo de soluciones acuosas de silicato de metal alcalino (como el sodio Na) mediante neutralización con ácido, como ácido sulfúrico H₂SO₄, ácido clorhídrico HCl o dióxido de carbono CO₂.

Si el pH final de la solución es neutro o alcalino se obtiene sílice precipitada. Si el pH es ácido se obtiene sílice gel.

La sílice pirogénica se prepara mediante la combustión de un compuesto de silicio volátil, usualmente tetracloruro de silicio SiCl₄. La sílice precipitada se obtiene de una solución acuosa de silicatos a la que se le añade ácido.

La sílice coloidal es una dispersión estable de partículas de tamaño coloidal de sílice amorfa en una solución acuosa.

Usos

En varias aplicaciones

Silica or SiO ₂ has a wide variety of functions, for example it serves as an abrasive, absorbent, anti-caking, filler, opacifier and to promote the suspension of other substances, among many other modes of use.

It is used for example:

-In the manufacture of glass, ceramics, refractories, abrasives and water glass

-Decoloration and purification of oils and petroleum products

-In casting molds

-As an anti-caking agent for powders of all kinds

-As defoamer

-To filter liquids such as dry cleaning solvents, swimming pool water, and municipal and industrial wastewater

-In the manufacture of heat insulation, fire-retardant bricks, and fire- and acid-resistant packaging materials

-As filler in the manufacture of papers and cardboard, to make them more resistant

-As a filler for paints to improve their flow and color

-In materials for polishing metals and wood, as it imparts abrasiveness

-In chemical analysis laboratories in chromatography and as an absorbent

-As an anti-caking agent in insecticide and agrochemical formulas, to help grind up waxy pesticides and as a carrier of the active compound

-As a catalyst support

-As filler to reinforce synthetic rubbers and rubbers

-As a carrier of liquids in animal feed

-In printing inks

-As desiccant and adsorbent, in the form of silica gel

-As an additive in cement

-Like pet sand

-In insulators for microelectronics

-On thermo-optic switches

Silica gel. KENPEI. Source: Wikimedia Commons.

In the food industry

Amorphous silica is incorporated into a variety of food products as a multifunctional direct ingredient in various types of food. It should not exceed 2% of the finished food.

For example, it serves as an anti-caking agent (to prevent certain foods from sticking), as a stabilizer in beer production, as an anti-precipitant, to filter wine, beer, and fruit or vegetable juices.

Equipment for filtering wine with diatomaceous earth (SiO ₂). Fabio Ingrosso. Source: Wikimedia Commons.

It acts as an absorbent for liquids in some foods and a component of microcapsules for flavoring oils.

In addition, the amorphous SiO ₂ is applied through a special process on the surface of the plastics of food packaging articles, acting as a barrier.

In the pharmaceutical industry

It is added as an anti-caking, thickening, gelling agent and as an excipient, that is, as a tableting aid to various drugs and vitamins.

In the cosmetics and personal care industry

It is used in a multitude of products: in face powders, eyeshadows, eyeliners, lipsticks, blushes, makeup removers, powders, foot powders, hair dyes and bleaches.

Also in oils and bath salts, foam baths, hand and body creams, moisturizers, deodorants, face creams or masks (except shaving creams), perfumes, lotions and cleansing creams.

Also in night moisturizing creams, nail polishes and paints, skin refreshing lotions, hair toners, toothpaste, hair conditioners, tanning gels and creams.

In therapeutic applications

SiO ₂ is present in bioactive glasses or bioglasses whose main characteristic is that they can react chemically with the biological environment that surrounds them, forming a strong and lasting bond with living tissue.

This type of material is used to make bone substitutes such as those in the face, as "scaffolds" on which bone cells will grow. They have shown good biocompatibility with both bones and soft tissues.

These bioglasses will make it possible to recover bones from the face of people who have lost them by accident or illness.

Risks

Very fine silica particles can become airborne and form non-explosive dusts. But this dust can irritate the skin and eyes. Its inhalation causes irritation of the respiratory tract.

In addition, inhalation of silica dust causes long-term progressive damage to the lungs, called silicosis.

References

1. US National Library of Medicine. (2019). Silicon dioxide. Recovered from pubchem.ncbi.nlm.nih.gov.
2. Cotton, F. Albert and Wilkinson, Geoffrey. (1980). Advanced Inorganic Chemistry. Fourth Edition. John Wiley & Sons.
3. Da Silva, MR et al. (2017). Green Extraction Techniques. Silica-Based Sorbents. In Comprehensive Analytical Chemistry. Recovered from sciencedirect.com.
4. Ylänen, H. (editor). (2018). Bioactive Glasses: Materials, Properties and Applications (Second Edition). Elsevier. Recovered from books.google.co.ve.
5. Windholz, M. et al. (editors) (1983) The Merck Index. An Encyclopedia of Chemicals, Drugs and Biologicals. Tenth Edition. Merck & CO., Inc.
6. Mäkinen, J. and Suni, T. (2015). Thick-Film SOI Wafers. In Handbook of Silicon Based MEMS Materials and Technologies (Second Edition). Recovered from sciencedirect.com.
7. Sirleto, L. et al. (2010). Thermo-optical switches. Silicon nanocrystals. Recovered from sciencedirect.com.