Silica gel

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Silica gel
Silica gel pb092529.jpg
Identifiers
ChemSpider
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ECHA InfoCard 100.065.880 OOjs UI icon edit-ltr-progressive.svg
UNII
Properties
SiO2
Molar mass 60.08 g/mol
AppearanceTransparent beads
Odor Odorless
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Colloidal silica gel with light opalescence Opaleszens Kolloid SiO2.jpg
Colloidal silica gel with light opalescence

Silica gel is an amorphous and porous form of silicon dioxide (silica), consisting of an irregular tridimensional framework of alternating silicon and oxygen atoms with nanometer-scale voids and pores. The voids may contain water or some other liquids, or may be filled by gas or vacuum. In the last case, the material is properly called silica xerogel.

Contents

Silica xerogel with an average pore size of 2.4 nanometers has a strong affinity for water molecules and is widely used as a desiccant. It is hard and translucent, but considerably softer than massive silica glass or quartz, and remains hard when saturated with water.

Silica xerogel is usually commercialized as coarse granules or beads, a few millimeters in diameter. Some grains may contain small amounts of indicator substance that changes color when they have absorbed some water. Small paper envelopes containing silica xerogel pellets, usually with a "do not eat" warning, are often included in dry food packages to absorb any humidity that might cause spoilage of the food.

'Wet' silica gel, as may be freshly prepared from alkali silicate solutions, may vary in consistency from a soft transparent gel, similar to gelatin or agar, to a hard solid, namely a water-logged xerogel. It is sometimes used in laboratory processes, for example to suppress convection in liquids or prevent settling of suspended particles. [4]

History

Silica gel was in existence as early as the 1640s as a scientific curiosity. [5] It was used in World War I for the adsorption of vapors and gases in gas mask canisters. The synthetic route for producing silica gel was patented in 1918 by Walter A. Patrick, a chemistry professor at Johns Hopkins University.

Types

Silica alumina gel - light yellow, chemically stable, flame-resistant, insoluble except in alkali or hydrofluoric acid. Superficial polarity, thermal stability, performance greater than fine-pored silica gel.

Stabilizing silica gel - non-crystalline micro-porous solid powder, nontoxic, flame-resisting, used in brewery of grains for beer to improve taste, clearness, color, and foam and for removal of non-micro-organism impurities.

Properties

Silica gel's high specific surface area (around 750–800 m2/g (230,000–240,000 sq ft/oz)) [6] allows it to adsorb water readily, making it useful as a desiccant (drying agent). Silica gel is often described as "absorbing" moisture, which may be appropriate when the gel's microscopic structure is ignored, as in silica gel packs or other products. However, material silica gel removes moisture by adsorption onto the surface of its numerous pores rather than by absorption into the bulk of the gel.

Silica gel is able to adsorb up to 37% of its own weight in moisture in high-humidity environments. [7] This moisture can be released upon heating at 120 °C for extended periods of time. This makes it reusable multiple times with very little, if any, loss of efficiency.

Regeneration

Once saturated with water, the gel may be regenerated by heating it to 120 °C (248 °F) for 1–2 hours. [7] Some types of silica gel will "pop" when exposed to enough water. This is caused by breakage of the silica spheres when contacting the water. [8]

Preparation

An aqueous solution of sodium silicate is acidified to produce a gelatinous precipitate that is washed, then dehydrated to produce colorless silica gel. [6] When a visible indication of the moisture content of the silica gel is required, ammonium tetrachlorocobaltate(II) (NH4)2[CoCl4] or cobalt(II) chloride CoCl2 is added. [6] This will cause the gel to be blue when dry and pink when hydrated. [6] Due to a link between the cobalt chloride and cancer, it has been forbidden in Europe for use in silica gel. [9] An alternative indicator is methyl violet which is orange when dry and green when hydrated.

Uses

Desiccant

Silica gel, as beads packed in a permeable bag, is a commonly used desiccant. Silica gel bag open with beads.jpg
Silica gel, as beads packed in a permeable bag, is a commonly used desiccant.

Moisture can cause mold and spoilage in many items. [10] [11] It can also damage electronics by causing condensation and shorten the lifespan of chemicals, like those in vitamins.[ citation needed ] Silica gel packets help by absorbing moisture and extending the life of these items. [12] [13] [14]

Silica gel may also be used to keep the relative humidity inside a high frequency radio or satellite transmission system waveguide as low as possible (see also humidity buffering ). [15] Excessive moisture buildup within a waveguide can cause arcing inside the waveguide itself, damaging the power amplifier feeding it. Also, the beads of water that form and condense inside the waveguide change the characteristic impedance and frequency, degrading the signal. It is common for a small compressed air system (similar to a small home aquarium pump) to be employed to circulate the air inside the waveguide over a jar of silica gel.

Silica gel is also used to dry the air in industrial compressed air systems. Air from the compressor discharge flows through a bed of silica gel beads. The silica gel adsorbs moisture from the air, preventing damage at the point of use of the compressed air due to condensation or moisture. The same system is used to dry the compressed air on railway locomotives, where condensation and ice in the brake air pipes can lead to brake failure.

Prior to widespread use of air-conditioning, salt shakers with caps containing silica gel beads to keep the salt dry enough to prevent clumping were marketed in the USA, replacing the practice of including a few grains of rice in salt shakers to effect the same drying.

Silica gel is sometimes used as a preservation tool to control relative humidity in museum and library exhibitions and storage.

Other applications include diagnostic test strips, inhalation devices, syringes, drug test kits, and hospital sanitation kits.

Silica gel is commonly described as a way to remove moisture from phones and electronics accidentally exposed to water, but there are no known controlled studies that compare its actual efficacy compared to simple air or fan exposure. The average person is also unlikely to have a sufficient quantity of ready-to-use activated or regenerated silica gel.

Chemistry

Chromatography column P1000495.JPG
Chromatography column

In chemistry, silica gel is used in chromatography as a stationary phase. In column chromatography, the stationary phase is most often composed of silica gel particles of 40–63 μm. Different particle sizes are used for different kinds of column chromatography as the particle size is related to surface area. The differences in particle size dictate if the silica gel should be used for flash or gravity chromatography. In this application, due to silica gel's polarity, non-polar components tend to elute before more polar ones, hence the name normal phase chromatography. However, when hydrophobic groups (such as C18 groups) are attached to the silica gel then polar components elute first and the method is referred to as reverse phase chromatography. Silica gel is also applied to aluminium, glass, or plastic sheets for thin layer chromatography.

The hydroxy (OH) groups on the surface of silica can be functionalized to afford specialty silica gels that exhibit unique stationary phase parameters. These so-called functionalized silica gels are also used in organic synthesis and purification as insoluble reagents and scavengers.

Chelating groups have also been covalently bound to silica gel. These materials have the ability to remove metal ions selectively from aqueous solutions. Chelating groups can be covalently bound to polyamines that have been grafted onto a silica gel surface producing a material of greater mechanical integrity. Silica gel is also combined with alkali metals to form a M-SG reducing agent. (See SiGNa chemistry)

Silica gel is not expected to biodegrade in either water or soil. [16]

Cat litter

Silica gel is also used as cat litter, [17] by itself or in combination with more traditional materials, such as clays including bentonite. It is non-tracking and virtually odorless.

Food additive

Silica gel, also referred to as silicon dioxide or synthetic amorphous silica (SAS), is listed by the FDA in the United States as generally recognized as safe (GRAS), meaning it can be added to food products without needing approval. Silica is allowed to be added to food in the USA at up to 2% as permitted under 21 CFR 172.480. In the EU, it can be in up to 5% concentrations. [18] In 2018, a re-evaluation by the EFSA Panel on Food Additives and Nutrient Sources added to Food found no indications of toxicity even at the highest estimates of exposure level. [19]

Listed uses include: anticaking agent, defoaming agent, stabilizer, adsorbent, carrier, conditioning agent, chill proofing agent, filter aid, emulsifying agent, viscosity control agent, and anti-settling agent. [20] Silica can be found commonly in foods including baked goods, spices and herbs, dairy products, cocoa products, and more. [19]

Water filtration

Given the water adsorption properties of silica gel, it is used in domestic water filters. [21] The surface structure of silica gel allows the adsorption of some minerals that are dissolved in the water, [22] or "Ion-exchange" as it is marketed. Due to the lack of regulations for domestic water filtration products, no studies validate the manufacturer claims regarding the effectiveness of the filtration system. [ citation needed ]

Humidity indicator (color-changing silica gel)

Indicating silica gel Indicating-silica-gel.png
Indicating silica gel

Silica gel may be doped with a moisture indicator that gradually changes its color when it transitions from the anhydrous (dry) state to the hydrated (wet) state. Common indicators are cobalt(II) chloride and methyl violet. Cobalt (II) chloride is deep blue when dry and pink when wet, but it is toxic and carcinogenic, and was reclassified by the European Union in July 2000 as a toxic material. [23] Methyl violet may be formulated to change from orange to green, or orange to colorless. It also is toxic and potentially carcinogenic, [24] but is safe enough to have medicinal uses. Ferric and ferrous salts, sometimes combined with small amounts of sodium hydroxide, provide a better alternative. In particular, ferric sulfate and double salts like ammonium iron(III) sulfate (iron alum), ammonium iron(II) sulfate, and potassium iron(III) sulfate all result in a color change from amber/yellow when dry to colorless/white when saturated. [25] [26]

Hazards

Silica gel is non-toxic, and non-reactive and stable with ordinary usage. It will react with hydrogen fluoride, fluorine, oxygen difluoride, chlorine trifluoride, strong acids, strong bases, and oxidizers. [16] Silica gel is irritating to the respiratory tract and may cause irritation of the digestive tract. Dust from the beads may cause irritation to the skin and eyes, so precautions should be taken. [27] Crystalline silica dust can cause silicosis, but synthetic amorphous silica gel is indurated, so it does not cause silicosis. Additional hazards[ example needed ] may occur when doped with a humidity indicator.

Related Research Articles

<span class="mw-page-title-main">Silicon dioxide</span> Oxide of silicon

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. It 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.

<span class="mw-page-title-main">High-performance liquid chromatography</span> Technique in analytical chemistry

High-performance liquid chromatography (HPLC), formerly referred to as high-pressure liquid chromatography, is a technique in analytical chemistry used to separate, identify, and quantify specific components in mixtures. The mixtures can originate from food, chemicals, pharmaceuticals, biological, environmental and agriculture, etc., which have been dissolved into liquid solutions.

<span class="mw-page-title-main">Adsorption</span> Phenomenon of surface adhesion

Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. This process differs from absorption, in which a fluid is dissolved by or permeates a liquid or solid. While adsorption does often precede absorption, which involves the transfer of the absorbate into the volume of the absorbent material, alternatively, adsorption is distinctly a surface phenomenon, wherein the adsorbate does not penetrate through the material surface and into the bulk of the adsorbent. The term sorption encompasses both adsorption and absorption, and desorption is the reverse of sorption.

<span class="mw-page-title-main">Desiccant</span> Substance used to induce or sustain dryness

A desiccant is a hygroscopic substance that is used to induce or sustain a state of dryness (desiccation) in its vicinity; it is the opposite of a humectant. Commonly encountered pre-packaged desiccants are solids that absorb water. Desiccants for specialized purposes may be in forms other than solid, and may work through other principles, such as chemical bonding of water molecules. They are commonly encountered in foods to retain crispness. Industrially, desiccants are widely used to control the level of water in gas streams.

<span class="mw-page-title-main">Desiccation</span> State of extreme dryness or process of thorough drying

Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic substance that induces or sustains such a state in its local vicinity in a moderately sealed container. The word desiccation comes from Latin de- 'thoroughly' and siccare 'to dry'.

<span class="mw-page-title-main">Desiccator</span> Sealable enclosures containing desiccants to preserve moisture-sensitive items

Desiccators are sealable enclosures containing desiccants used for preserving moisture-sensitive items such as cobalt chloride paper for another use. A common use for desiccators is to protect chemicals which are hygroscopic or which react with water from humidity.

<span class="mw-page-title-main">Column chromatography</span> Method to isolate a compound in a mixture

Column chromatography in chemistry is a chromatography method used to isolate a single chemical compound from a mixture. Chromatography is able to separate substances based on differential absorption of compounds to the adsorbent; compounds move through the column at different rates, allowing them to be separated into fractions. The technique is widely applicable, as many different adsorbents can be used with a wide range of solvents. The technique can be used on scales from micrograms up to kilograms. The main advantage of column chromatography is the relatively low cost and disposability of the stationary phase used in the process. The latter prevents cross-contamination and stationary phase degradation due to recycling. Column chromatography can be done using gravity to move the solvent, or using compressed gas to push the solvent through the column.

<span class="mw-page-title-main">Molecular sieve</span> Filter material with homogeneously sized pores in the nanometer range

A molecular sieve is a material with pores of uniform size which link the interior of the solid to its exterior. These materials embody the molecular sieve effect: "With respect to porous solids, the surface associated with pores communicating with the outside space may be called the internal surface. Because the accessibility of pores may depend on the size of the fluid molecules, the extent of the internal surface may depend on the size of the molecules comprising the fluid, and may be different for the various components of a fluid mixture." The specification for the pores is that they not only communicate from the exterior to the interior, but the pores are uniform in size. Many kinds of materials exhibit some molecular sieves, but zeolites dominate the field. Zeolites are almost always aluminosilicates, or variants where some or all of the Si or Al centers are replaced by similarly charged elements.

<span class="mw-page-title-main">Activated alumina</span>

Activated alumina is manufactured from aluminium hydroxide by dehydroxylating it in a way that produces a highly porous material; this material can have a surface area significantly over 200 m2/g. The compound is used as a desiccant (to keep things dry by adsorbing water from the air) and as a filter of fluoride, arsenic and selenium in drinking water. It is made of aluminium oxide (alumina; Al2O3). It has a very high surface-area-to-weight ratio, due to the many "tunnel like" pores that it has. Activated alumina in its phase composition can be represented only by metastable forms (gamma-Al2O3 etc.). Corundum (alpha-Al2O3), the only stable form of aluminum oxide, does not have such a chemically active surface and is not used as a sorbent.

<span class="mw-page-title-main">Pressure swing adsorption</span> Method of gases separation using selective adsorption under pressure

Pressure swing adsorption (PSA) is a technique used to separate some gas species from a mixture of gases under pressure according to the species' molecular characteristics and affinity for an adsorbent material. It operates at near-ambient temperature and significantly differs from the cryogenic distillation commonly used to separate gases. Selective adsorbent materials are used as trapping material, preferentially adsorbing the target gas species at high pressure. The process then swings to low pressure to desorb the adsorbed gas.

An atmospheric water generator (AWG), is a device that extracts water from humid ambient air, producing potable water. Water vapor in the air can be extracted either by condensation - cooling the air below its dew point, exposing the air to desiccants, using membranes that only pass water vapor, collecting fog, or pressurizing the air. AWGs are useful where potable water is difficult to obtain, because water is always present in ambient air.

<span class="mw-page-title-main">Moisture sorption isotherm</span>

The relationship between water content and equilibrium relative humidity of a material can be displayed graphically by a curve, the so-called moisture sorption isotherm. For each humidity value, a sorption isotherm indicates the corresponding water content value at a given temperature. If the composition or quality of the material changes, then its sorption behaviour also changes. Because of the complexity of sorption process the isotherms cannot be determined explicitly by calculation, but must be recorded experimentally for each product.

<span class="mw-page-title-main">Glass disease</span>

Glass disease, also referred to as sick glass or glass illness, is a degradation process of glass that can result in weeping, crizzling, spalling, cracking and fragmentation. Glass disease is caused by an inherent instability in the chemical composition of the original glass formula. Properties of a particular glass will vary with the type and proportions of silica, alkali and alkaline earth in its composition. Once damage has occurred it is irreversible, but decay processes can be slowed by climate control to regulate surrounding temperature, humidity, and air flow.

<span class="mw-page-title-main">Tetraethyl orthosilicate</span> Chemical compound

Tetraethyl orthosilicate, formally named tetraethoxysilane (TEOS), ethyl silicate is the organic chemical compound with the formula Si(OC2H5)4. TEOS is a colorless liquid. It degrades in water. TEOS is the ethyl ester of orthosilicic acid, Si(OH)4. It is the most prevalent alkoxide of silicon.

<span class="mw-page-title-main">Polyethylenimine</span> Chemical compound

Polyethylenimine (PEI) or polyaziridine is a polymer with repeating units composed of the amine group and two carbon aliphatic CH2CH2 spacers. Linear polyethyleneimines contain all secondary amines, in contrast to branched PEIs which contain primary, secondary and tertiary amino groups. Totally branched, dendrimeric forms were also reported. PEI is produced on an industrial scale and finds many applications usually derived from its polycationic character.

<span class="mw-page-title-main">Nitrogen generator</span>

Nitrogen generators and stations are stationary or mobile air-to-nitrogen production complexes.

<span class="mw-page-title-main">Thermal wheel</span> Type of energy recovery heat exchanger

A thermal wheel, also known as a rotary heat exchanger, or rotary air-to-air enthalpy wheel, energy recovery wheel, or heat recovery wheel, is a type of energy recovery heat exchanger positioned within the supply and exhaust air streams of air-handling units or rooftop units or in the exhaust gases of an industrial process, in order to recover the heat energy. Other variants include enthalpy wheels and desiccant wheels. A cooling-specific thermal wheel is sometimes referred to as a Kyoto wheel.

Synthetic magnesium silicates are white, odorless, finely divided powders formed by the precipitation reaction of water-soluble sodium silicate and a water-soluble magnesium salt such as magnesium chloride, magnesium nitrate or magnesium sulfate. The composition of the precipitate depends on the ratio of the components in the reaction medium, the addition of the correcting substances, and the way in which they are precipitated.

<span class="mw-page-title-main">Flower preservation</span> Preservation techniques

Flower preservation has existed since early history, although deliberate flower preservation is a more recent phenomenon. In the Middle East, the bones of pre-historic man were discovered with delicate wild flowers probably as a tribute to a passing loved one. Evidence of deliberate use of specific flowers is indicated by the pollen grains that were present. Brightly colored and vivid flowers were also found in Egyptian tombs. These flowers were approximated to be 4,000 years old. In the sixteenth century medicinal nosegays began to give way to ornamental ones. Flowers essentially started to be used for decorative purposes such as jewels, fans and gloves. During the Elizabethan Age the once familiar ruff was replaced by soft lacy collars, and bosom flowers also became popular.

<span class="mw-page-title-main">Aerogel</span> Synthetic ultralight solid material

Aerogels are a class of synthetic porous ultralight material derived from a gel, in which the liquid component for the gel has been replaced with a gas, without significant collapse of the gel structure. The result is a solid with extremely low density and extremely low thermal conductivity. Aerogels can be made from a variety of chemical compounds. Silica aerogels feel like fragile styrofoam to the touch, while some polymer-based aerogels feel like rigid foams.

References

  1. ↑ Silica gel, site jtbaker.com
  2. ↑ Silica gel, site chemcas.org
  3. ↑ Silicon dioxide, site echa.europa.eu
  4. ↑ Henisch, H.K. (1988). Crystals in Gels and Liesegang Rings. Cambridge: Cambridge University Press. ISBN   0521345030.
  5. ↑ Feldman, Maryann; Desrochers, Pierre (March 2003). "Research Universities and Local Economic Development: Lessons from the History of the Johns Hopkins University" (PDF). Industry and Innovation. 10 (1): 5–24. doi:10.1080/1366271032000068078. S2CID   154423229. Archived from the original (PDF) on 2005-11-12.
  6. 1 2 3 4 Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN   978-0-08-037941-8.
  7. 1 2 Chandradhas, Susheel (27 August 2016). "How To Recharge Silica Gel Crystals". Beyond Photo Tips. Retrieved 2 November 2022.
  8. ↑ Spence Konde, "Preparation of High-Silica Zeolite Beads From Silica Gel" Archived 2016-03-04 at the Wayback Machine , retrieved 2011-09-26
  9. ↑ "Blue Silicagel & Conclusions: Safety information on blue silicagel". Archived from the original on 2016-01-05.
  10. ↑ "Prevent bacteria from feasting on your fresh produce, causing spoilage". MSU Extension. 2013-01-30. Retrieved 2024-03-08.
  11. ↑ Command, United States Naval Education and Training (1978). Mess Management Specialist 3 & 2. Department of Defense, Navy Department, Office of the Chief of Naval Operations], Naval Education and Training Command. p. 91.
  12. ↑ Machinery and Production Engineering. Machinery Publishing Company. 1951.
  13. ↑ Anderson, Gary A. (1988). Floral Design and Marketing. Ohio Agricultural Education Curriculum Materials Service, the Ohio State University.
  14. ↑ Varlamoff, Marie-Thérèse; Kremp, Virginie; Conservation, IFLA Programme on Preservation and; Resources, Council on Library and Information (1998). IFLA Principles for the Care and Handling of Library Material. International Federation of Library Associations and Institutions, Core Programme on Preservation and Conservation. ISBN   978-2-912743-00-8.
  15. ↑ Sabry, Fouad (2022-01-16). Aerogel: Want to Colonize Mars? Aerogel could help us farm and survive on Mars "in our lifetimes". One Billion Knowledgeable.
  16. 1 2 Environmental Health and Safety (2007-09-10). "Silica Gel" . Retrieved 2008-01-12.
  17. ↑ Andrew Kantor (2004-12-10). "Non-Tech High Tech Litters the Landscape". USA Today. Retrieved 2008-03-02.
  18. ↑ "Notification of the GRAS Determination of Silicon Dioxide When Added Directly or Indirectly to Human Food" (PDF). Archived from the original (PDF) on April 18, 2013.
  19. 1 2 Younes, M.; Aggett, P.; Aguilar, F. (2018). "Scientific Opinion on the re-evaluation of silicon dioxide (E 551) as a food additive". EFSA Journal. 16 (1): 5088–5158. doi:10.2903/j.efsa.2018.5088. PMC   7009582 . PMID   32625658. S2CID   79503431.
  20. ↑ "GRAS Notice (GRN) No. 298" (PDF). Archived from the original (PDF) on April 9, 2011.
  21. ↑ ZeroWater
  22. ↑ Peri, J. B.; Hensley Jr., A. L. (1968). "The surface structure of silica gel". The Journal of Physical Chemistry. 72 (8): 2926–2933. doi:10.1021/j100854a041.
  23. ↑ "Classifications - CL Inventory".
  24. ↑ "Methyl Violet Safety Data Sheet" (PDF). labchem.
  25. ↑ WOpatent WO2000065339A1,Stephen Moreton&Graham James Earl,"Humidity indicators",published 2000-11-02
  26. ↑ "SORBSIL CHAMELEON Safety Data Sheet" (PDF). OkerChemie.
  27. ↑ Fisher Scientific (1997-02-09). "Material Safety Data Sheet: Silica Gel Dessiccant" . Retrieved 2008-01-12.
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