On amorphous silica. Silica.

Author Chemical Encyclopedia b. I.L.Knunyants

SILICON DIOXIDE  (silica) SiO 2, bestsv. crystalline., amorphous or glassy substance.
Structure. SILICA DIOXIDE d. Exists in several polymorphic modifications (see table). Transition temperatures at normal pressure: a-quartz D b -quartz 575 ° C (DH 0 transition 0.41 kJ / mol), p-quartz D p-cristobalite 927 ° C (2.26 kJ / mol), p-quartz D g -tridymite 867 ° С (0.50 kJ / mol), a-tridmmit D b -tridymite 115 ° С (0.27 kJ / mol), b -tridymite D g -tridymite 160 ° С (0.15 kJ / mol), g -dithymite D b-cristobalite 1470 ° С (0.21 kJ / mol), a-cristobalite D b-cristobalite 270 ° С. Melting point of p-quartz is 1610 ° С (D H 0 mp 8.53 kJ / mol), g -tridymite 1680 ° С, b -cristobalite 1723 ° С (D H 0 pl 9.6 kJ / mol). Polymorphic transformed quartz, tridymite and cristobalite are accompanied by a change in volume. The crystalline forms of SILICON DIOXIDE are constructed of SiO 4 tetrahedra, and the a - and b-forms are distinguished by a slight displacement and rotation of the tetrahedra. For low-temperature tridymites, data are also given for other crystalline crystals. modifications, however, all of them contain b -tremitite as the main structural unit, but are differently distorted. For example, triclinic and monoclinic tridymites are described. In nature, also found cubic. SiO 2 modification — melanphlogite (a = 1.3402 nm, z = 48, space group Pt3p). At high pressures, whale (80-130 MPa, 400-500 ° C), coesite (1.5-4 GPa, 300-1700 ° C), stishovite (16-18 GPa, 1200-1400 ° C) are formed. Stishovit is the only modification of the SILICON DIOXIDE, built of SiO 6 octahedra. The stability of tridimite is probably determined by the impurities of Na and Al. The unstable form of silicon DIOXIDd. - rhombo-dipyramidal "fibrous silica". In addition to crystalline crystals, SILICON DIOXIDE may also have other forms of existence. Hidden crystal forms (chalcedony) are similar in structure to quartz. When extracting cations from certain silicates with acid, a hydrate is obtained. crystalline, silica. They inherit the texture of the source minerals, forming fibrous, scaly (lepidoidal) and sheet-like layered structures. Amorphous anisotropic and isotropic (opal) formations, fine natural silica (tripoli, synthetic colloidal silica and silica powders) are known. Hydrate amorphous silica precipitated from solutions of Na silicate, etc., polymerized to spherich. particles with a diameter of less than 100 nm, usually 2-3 nm. Received amorphous silica in the form of leaflets, ribbons and fibers (see Silica gel). At high temperatures fine powders of pyrogenic anhydrous silica — Aerosil and others — are released from the gas phase. About vitreous silica, see. Quartz glass.
Properties  The vapor pressure of SILICON DIOXIDE, 13.3-133 Pa, near the melting point. SILICA DIOXIDED. - dielectric, r 10 12 Ohm * m (20 ° С), 9 * 10 - 1 Ohm * m (1600 ° C). A-quartz single crystals have a chiral structure, which determines their optical activity and piezoelectric. properties. Quartz is transparent to UV and partially IR rays.

   * Refractive indices: N g - more, N p - less. ** The a - and b -quartz have two spaces. groups, as for both forms, left and right optical isomers are possible. *** b -120 ° C.

The solubility of α-quartz in water is 10–3% by weight (25 ° C), amorphous forms of silica are 0.007–0.015% (for aqueous solutions of SiO 2, see Silicic Acids). The solubility of SILICON DIOXIDE in acid and alkaline media is determined by the nature of the solvent. SILICA DIOXIDE d. Insoluble in most organic solvents. Reacts with hydrofluoric acid to form fluorosilicic acid. At 250-400 ° C, the interaction with gaseous HF, F 2 (giving SiF 4), at 200-250 ° C - with NH 4 HF 2. In a mixture with coal it reacts with Cl 2 when heated, forming SiCl 4. Amorphous silica is slowly soluble in aqueous solutions of alkali, Na 2 CO 3 (giving silicates) and NH 4 F (forming fluorosilicates), the dissolution rate increases with increasing pressure and temperature. When mixtures of powdered SILICON DIOXIDE are heated with various oxides, silicates are formed, and when fused with Na 2 CO 3 and Na 2 SO 4 it is water-soluble sodium silicate (see also soluble glass).
Distribution in nature. Content free SILICON DIOXIDE in the earth's crust 12%; it is also included in the rocks in the form of various silicates or in the form of mixtures with other minerals (granites). Quartz is one of the most common minerals; tridymite, cristobalite, lschatelit (natural quartz glass), chalcedony, opals are much less common. Small, differently oriented quartz crystals form a "gangue" quartz. With the destruction of rocks, quartz sands arise, the compaction of which leads to the formation of sandstones and quartzites. Haib pure quartz is a rock crystal, the crystals of which can reach several meters and weigh tens of tons. Single crystals of quartz are transparent, colorless (rock crystal) or colored with impurities in violet (amethyst), black (marion), yellow (citrine), smoky (rauch-topaz) colors. Types of cryptocrystalline. quartz forms: pink-red carnelian, bluish sapphirine, apple-green chrysoprase, banded agates and onyxes, finely colored jasper, silicon and hornfelses. Wiikalen amorphous "noble" opal, consisting of homogeneous colloidal particles with a diameter of 0.1-0.3 μm, tightly packed into ordered agglomerates; its water content is less than 1% by mass (for most ordinary opals, 4–9%). Prir. silica deposits also form tripoli, diatomite, etc. The shells of diatoms and skeletons of some sponges are built from silica; it strengthens the stems of plants - horsetails, bamboo, cane, is contained in the straw. SILICON DIOXIDE is responsible for the silicification of the forms of living plant organisms. Silica concentration in human blood and plasma is 0.001% by weight.
Receipt. Synthetic SILICON DIOXIDE d. Receive: the action of acids (H 2 SO 4, HCl, CO 2) on Na silicate, less often on other soluble silicates (the main mode of production in capitalist countries); from colloidal silica by coagulation under the action of ions Na +, NH 4, F - or by freezing; hydrolysis of SiCl 4, SiF 4, (NH 4) 2 SiF 6, (C 2 H 5 O) 4 Si in aqueous, ammonia-water solutions (sometimes with the addition of ethanol or organic bases) and in the gas phase. Amorphous SILICON DIOXIDE D. Is also obtained from tripoli and diatomaceous earth by burning rice husks and grinding melted quartz sand. Silicon Dioxide anhydrous powders with a high specific surface are obtained by chemical vapor deposition by burning SiCl 4 vapors in a mixture of H 2 and O 2 (aerosil, in the USA - Cabosil), oxidizing and hydrolyzing vapors of Si esters (fumed silica), and also SiF 4 (fluosyl). The initially condensed particles of SILICON DIOXIDE, 1 nm in diameter, are densely packed in secondary conglomerates, combined into a loose structure with a specific surface of 200-400 m 2 / g. A-quartz single crystals are grown from alkaline solutions of SILICON DIOXIDE in high pressure autoclaves (35-120 MPa) at 300-420 ° С (see Hydrothermal processes). In the production of materials used in the technique on the basis of SILICON DIOXIDE, the following processes are used:
   - chemical deposition from the gas phase during high-temperature combustion of SiCl 4, previously purified by distillation. Particles of SILICON DIOXIDE, formed in an oxygen-hydrogen flame or plasma, are precipitated giving massive quartz glass or layers of optical fiber preforms;
   - single-crystal surface oxidation. Si with the formation of heterostructures (in the manufacture of integrated circuits);
   - sintering fine SILICON DIOXIDE in quartz ceramics (see also Ceramics);
   - sol-gel process, including hydrolysis of organic compound Si, slow dehydration of the formed gel and moderate heating. Used to produce quartz and high-silica glass;
   - obtaining high-purity SILICON DIOXIDE and porous glasses (Vicore type) by thermodynamically treating the leading borosilicate glass, leaching with acid and washing the silica framework.
Prir. Silica is used in the manufacture of silicate glasses, porcelain and earthenware, abrasives, concrete, silicate bricks, Dinas, and ceramics. Synthetic SILICON DIOXIDE d. ("White soot") - filler in the production of rubber (up to 70% of the produced SILICON DIOXIDE d.). We won. use precipitated hydrate. silica (containing 85-95% SiO 2) with a specific surface area of ​​60-300 m 2 / g, to a lesser extent - anhydrous silica of aerosil type. Aerosil is also an adsorbent in chromatography, a thickener of lubricants, adhesives, paints. Quartz single crystals are used in radio engineering (piezoelectric. Frequency stabilizers, filters, resonators, etc.), in acoustooptics and acoustoelectronics (see Acoustic materials), in optical instrumentation (prisms for spectrographs, monochromators, lenses for UV optics, etc.), in jewelry (transparent, beautifully colored varieties - semi-precious stones). Silica gels with an effective pore diameter of 2-15 nm are used as prom. sorbents and catalyst carriers. Synthetic SILICON DIOXIDE and rock crystal are the raw material for the production of single crystals of quartz, quartz glass, ceramics and quartz fibers. Quartz glass and ceramics - konstrukts. material in aviats. industry (for example, for windows and window covers of aircraft), in optics (for input windows, optical devices of the UV and IK ranges), in electronics (delay lines), etc. Quartz fabric is a heat-insulating material. Quartz fibers are used to create fiber-optic (light guide) communication lines and information transmission systems. Manufacturing synthetic SILICON DIOXIDE d. In capitalistich. countries 600-700 thousand tons / year (1980). When SILICON DIOXIDE gets into the living tissue, granulomas slowly develop. Silica dust causes irritation top. breathing ways and bronchi, a disease of the gastrointestinal tract, with prolonged inhalation - a serious disease - silicosis of the lungs. MPC at the content of SiO 2 in dust of St. 70% - 1 mg / m 3, 10-70% - 2 mg / m 3, 2-10% - 4 mg / m 3.

Physical and chemical properties of aerosil (silicon dioxide)

Aerosil  (from the Latin word - Aerosilum), oxyls (from the Latin word - Oxylum) silicon dioxide, Silica colloidalis anhydrica (Ph. Eur.), Colloidal silicon dioxide (USP), Colloidal anhydrous silica (BP), Silica (CAS No. 7631-86-9) - anhydrous amorphous silica, belongs to the group of synthetic active highly dispersed mineral fillers. In pharmacy, Aerosil (silicon dioxide) is used as an auxiliary substance, a stabilizer, a gelling agent, an adsorbent, improves the fluidity of tablets, ointment, gel and other mixtures. Sometimes silicon dioxide is used as an active pharmacological ingredient (it has antibacterial properties, detoxifier, sorbent).

Get silica by hydrolysis of silicon tetrachloride vapor in a hydrogen flame at a temperature of\u003e 1000 ° C (1100-1400 ° C). The resulting product is white, amorphous, non-porous, indifferent powder sprayed, contains 99.3% SiO2; has a high dispersion (particle diameter 4-40 microns, have a spherical or almost spherical shape), the specific adsorption surface is 50-450 m2 / g; bulk volume is about 50 g / l, density - 2.36 g / cm3; the pH of the aqueous suspension is 4.0; refractive index n20D = 1.46. Aerosil does not dissolve in water, acids and dilute alkalis. At a concentration of Aerosil in water in an amount of 10-12%, a low-viscous fluid suspension is formed, at 17% - a semi-rigid mass, at 20% - a coarse mass, which, when rubbed, turns into a homogeneous ointment-like mass. Due to the high affinity for water, aerosil is referred to as hydrophilic substances. But silicon dioxide (aerosil) brand R972 has hydrophobic properties.

There are several trademarks of Aerosil (silicon dioxide)which differ mainly in the specific surface area, degree of hydrophilicity or hydrophobicity, as well as the presence of other filler substances. According to the definition of the nomenclature commission, amorphous silicon dioxide is called oxide. In Ukraine, a chemical and metallurgical plant licensed by Degussa produces unmodified standard aerosil of 175 grades; 300 380 with a hydrophilic surface; Methyl Arosyl AM-1/175 and AM-1/300, modified dimethyldichlorosilane; ether organosilicus of the brands ADEG-175 and ADEG-300, modified with ethylene glycol and diethylene glycol, and AM-2, modified by amino alcohols. In the United States, modified aerosil is produced - organosil and kebosil (Cabot), in Russia, butosil, aerosil-K, which is a combination of 85% silica and 15% starch, COK-84 aerosil, which is a coagulant of 85% silica and 14% ammonium oxide. In Germany, the company "Degussa" produces hydrated brands of aerosilcontaining bound water (durosil, vulkasin, sifloks, ultrasil, etc.) that differ in SiO2 content, particle diameter, density and properties), aerosil in the form of suspensions (K-314, contains 14% A., K-328, contains 28% A.). In Japan, microsil is produced and worn, in France - Francil, in England - Manosil. For cosmetics can be made in the form of a paste. Recently, aerosil was introduced into the pharmacopoeia of various countries (Hungary, Denmark, Austria, etc.). In the USA, silicon dioxide (aerosil) is also allowed as an additive to food products in the amount of 2%.

Aerosil  attributed to the theory of "pure" substances that release the active ingredients without energy costs. Electron microscopic studies have shown that each basic particle of aerosil consists of four separate layers (figure). The core of this particle is a three-dimensional polymer of SiO2 elements. Having Silane Si-OH and siloxane Si-O-Si groups on the surface of the particles, Aerosil is capable of creating a pattern-like framework due to hydrogen bonds, which allows limiting the thermal expansion of the gelled liquid. Siloxane and silane groups in aerosil are functional, and the silicon-oxygen bond is characterized by high strength (reaches 372.5 J / mol), which is explained by its polarity, due to which the covalent bond approaches the ionic bond.

Table - The main properties of silicon dioxide (Aerosil)

Picture. The structure of the spatial grid of Aerosil in hydrogel

Silanol groups are unevenly distributed. There are surface silane groups, which can be free or connected by hydrogen bridges, and silanol groups within the molecule, which can also be interconnected by hydrogen bridges. As a result, a branched bulk structure is created, with the result that aerosil is referred to as inorganic polymers. Siloxane groups have hydrophobic properties, they are stable (OH silanol groups split off at a temperature of\u003e 300 ° C), cause an acidic reaction; have hydroxyl groups both on the surface and inside aerosil molecules. With a uniform distribution, every second silicon atom has a hydroxyl group on the surface.

This causes three types of aerosil interaction: physical adsorption, chemical adsorption (formation of hydrogen bridges by silanol groups with water, alcohols, acids and other substances) and chemical reactions on the surface of the molecule. So, silanol groups interact with alcohols to form ethers.

Aerosil (silicon dioxide) has good sorption properties, absorbs from 15 to 60% of various liquids depending on their nature, without changing the appearance and flowability of the powder. The first layer of water is absorbed by aerosil due to the creation of hydrogen bridges (chemical adsorption), and the subsequent layers are due to physical adsorption. Physically adsorbed water is released at a temperature of 25-150 ° С, whereas chemically adsorbed water is released at 800 ° С.

Aerosil, which is used for the production of drugs, must have high purity. Table 1 shows the chemical composition of various trademarks of Aerosil, which may have certain impurities formed during production processes, for example, traces of hydrochloric acid, which causes a pH of 4% of an aqueous suspension of the polymer (3.6-4.3). So, Aerosil (silica) behaves like a weak acid.

Table 1 - The chemical composition of various grades of aerosil (in terms of dry matter, according to MM Astrakhanov)

Content%	Aerosil brands, composition,%
	200; 300; 380	0	SOK84	R972	Combined
SiO2	>99,87	>99,8	82–86	>98,3	85
Al2O2			14–18		–
Fe2O3					–
TiO2			–	0,03	–
Na2O		–	–	–	–
As		–	–	–	–
B2O3		–	–	–	–
Bi2O3		–	–	–	–
P2O5		–	–	–	–
HCl				–	0,05
Starch	–	–	–	–	–

Table 2 shows the most important physicochemical properties of different brands of aerosilwhich are widely used in the manufacture of medicines

Table 2. Physical and chemical properties of different brands of aerosil

Indicator	Stamps of aerosil
	200	300	380	0	SOK-84	R-972
Specific surface, m2 / g	200 ± 25	300 ± 30	380 ± 30	200 ± 25	170 ± 30	120 ± 30
Losses during ignition,%	1	2	2,5	1	1	2
Average particle size	12	7	7	12	–	16
Moisture content,% (dried at 105 ° С)	–	1,5	1,5	1,5	0,5	2,5
Saturated weight, g / l, standard (non-pressed) pressed	≈60≈120
Porosity	Porous			Non-porous
pH 4% water dispersion	3,6–4,3		–	–	3,6–4,3 3.5–4.1 (in methanol)

AT drug manufacturing Aerosil  used as a stabilizer for suspension oil liniments. It has a high ability to adsorb various liquids. The addition of aerosil to the composition of oil and water-alcohol-glycerol suspension liniments enhances the sedimentation, aggregation and aggregative stability of these systems, creating a sufficiently strong spatial structure capable of retaining immobilized liquid phase with suspended particles of pharmaceutical substances in cells. In aqueous and aqueous-alcoholic suspensions, the stabilizing effect of aerosil is mainly due to electrostatic forces. The sedimentation of particles of the solid phase in stabilized aqueous-alcoholic suspensions occurs 3 times slower than in unstabilized systems. The stabilizing effect of Aerosil increases in the presence of a small amount of surfactant, for example, Tween-80. The presence of aerosil (in a concentration of 1.0–5.0%) in emulsions and suspensions increases their stability, the ability to better fixate on the skin and the effectiveness of the therapeutic action. With water and alcohol silicon dioxide at a concentration of 3%  creates turbid suspensions, the shares of which have a negative charge.

One of the properties of Aerosil is its thickening (thickener) ability (Table 3).

Table 3. The thickening effect of Aerosil-200

This property is used to obtain aerosil-containing gels and ointment bases, which include aerosil. As a liquid phase for their compositions, a polyethylsiloxane liquid is used under the name Esilon-5 or Esilon-4, fish oil, PEG-400, castor oil, fatty oils, and the like. Esilon-5 thickened 16% of Aerosil known as Vaseline KV-E / 16, does not irritate the skin and does not prevent the absorption of active substances. As a thickener liquid phase is used aerosil (silicon dioxide)  in the amount of 8-16%. The resulting gels have a soft plastic consistency, are well distributed and fixed on the skin. They have good colloidal stability at elevated temperatures (≥40 ° C), retain the necessary consistency, which does not change even at 100 ° C, and therefore aerosil-containing gels and ointments can be used in tropical climates. According to the structural and mechanical properties of the gels, which include aerosil, constitute thixotropic plastic thermostable systems and have antimicrobial resistance. The quantitative content of Aerosil in the system affects the rheological and biopharmaceutical properties of gels. The increase in silicon dioxide in the composition of the gels leads to an increase in their plastic viscosity and thixotropic properties, while significantly slowing the release of the active pharmacological ingredient.

The use of aerosil (silicon dioxide) in the medical and veterinary pharmaceutical industry

In the manufacture of tablets Aerosil is used  in a concentration of 0.1-0.5% as a sliding and loosening (0.1-2.0%) substance, which reduces the time of their disintegration, facilitates the process of granulation, improves the fluidity of the tablet mass. The adsorption properties of Aerosil are used in the production of powders, extracts and other pharmaceutical preparations.

Numerous pharmacological, toxicological and biopharmaceutical studies have confirmed that when used internally, aerosil is indifferent, well tolerated by patients, has healing properties for diseases of the gastrointestinal tract and other inflammatory processes, and can be a source of supply of silicon in the body. There is evidence that aerosil (silicon dioxide)  can contribute to the reduction of smooth muscles and blood vessels, and also has antibacterial properties.

Aerosil-containing pharmaceutical systems do not exhibit irritating and toxic effects. The same properties are inherent in ointments when using epsilon and aerosil as a basis (composition of epsilon-5, thickened with 15% aerosil in the manufacture of ointments with antibiotics and corticosteroids). Ointments with Aerosil (silica)  easily squeezed out of the tubes, well fixed on the skin, have a prolonged effect.

Literature

Zhoglo F., Wozniak V., Popovich V., Bogdan J. Auxiliary substances and their use in the technology of medicinal forms. - Lviv, 1996; Pertsev I.M., Kotenko A.M., Chueshov A.V., Khaleeva E.L. Pharmaceutical and biological aspects of ointments: Monograph. - H., 2003; Pechkovskaya K.A. Filling rubber. In the book: Encyclopedia of polymers. In 3 t. - M., 1974.