
Silica gel is an
amorphous
In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid) is a solid that lacks the long-range order that is a characteristic of a crystal. The terms "glass" and "glassy solid" are sometimes used synonymousl ...
and
porous
Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
form of
silicon dioxide
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , 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 abundan ...
(silica), consisting of an irregular three-dimensional framework of alternating
silicon
Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...
and
oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
atoms with
nanometer
330px, Different lengths as in respect to the Molecule">molecular scale.
The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm), or nanometer (American spelling
Despite the va ...
-scale voids and pores. The voids may contain water or some other liquids, or may be filled by gas or
vacuum
A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...
. In the last case, the material is properly called silica
xerogel.
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
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. Desiccant ...
. It is hard and
translucent
In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through the material without appreciable light scattering by particles, scattering of light. On a macroscopic scale ...
, but considerably softer than massive
silica glass or
quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...
, 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
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation (meteorology), precipitation, dew, or fog t ...
that might cause
spoilage of the food.
"Wet" silica gel, as may be freshly prepared from
alkali
In chemistry, an alkali (; from the Arabic word , ) is a basic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a soluble base has a pH greater than 7.0. The a ...
silicate
A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used ...
solutions, may vary in consistency from a soft transparent
gel, similar to
gelatin
Gelatin or gelatine () is a translucent, colorless, flavorless food ingredient, commonly derived from collagen taken from animal body parts. It is brittle when dry and rubbery when moist. It may also be referred to as hydrolyzed collagen, coll ...
or
agar
Agar ( or ), or agar-agar, is a jelly-like substance consisting of polysaccharides obtained from the cell walls of some species of red algae, primarily from " ogonori" and " tengusa". As found in nature, agar is a mixture of two components, t ...
, to a hard solid, namely a water-logged xerogel. It is sometimes used in laboratory processes, for example to suppress
convection
Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoy ...
in liquids or prevent
settling
Settling is the process by which particulates move towards the bottom of a liquid and form a sediment. Particles that experience a force, either due to gravity or due to Centrifuge, centrifugal motion will tend to move in a uniform manner in the ...
of suspended particles.
History
Silica gel was in existence as early as the 1640s as a scientific curiosity.
It was used in
World War I
World War I or the First World War (28 July 1914 – 11 November 1918), also known as the Great War, was a World war, global conflict between two coalitions: the Allies of World War I, Allies (or Entente) and the Central Powers. Fighting to ...
for the
adsorption
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 ...
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
The Johns Hopkins University (often abbreviated as Johns Hopkins, Hopkins, or JHU) is a private university, private research university in Baltimore, Maryland, United States. Founded in 1876 based on the European research institution model, J ...
.
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
Specific surface area (SSA) is a property of solids defined as the total surface area (SA) of a material per unit mass, (with units of m2/kg or m2/g). Alternatively, it may be defined as SA per solid or bulk volume (units of m2/m3 or m−1).
I ...
around
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.
This moisture can be released upon heating at about
for 1–2 hours.
This makes it reusable multiple times with very little, if any, loss of efficiency.
Preparation
An aqueous solution of
sodium silicate
Sodium silicate is a generic name for chemical compounds with the formula or ·, such as sodium metasilicate (), sodium orthosilicate (), and sodium pyrosilicate (). The anions are often polymeric. These compounds are generally colorless tra ...
is acidified to produce a gelatinous precipitate that is washed, then dehydrated to produce colorless silica gel.
When a visible indication of the moisture content of the silica gel is required, ammonium tetrachlorocobaltate(II) (NH
4)
2 4">oCl4or
cobalt(II) chloride
Cobalt(II) chloride is an inorganic compound, a salt of cobalt and chlorine, with the formula . The compound forms several hydrates ·''n'', for ''n'' = 1, 2, 6, and 9. Claims of the formation of tri- and tetrahydrates have not been confirmed.M. ...
CoCl
2 is added.
This will cause the gel to be blue when dry and pink when hydrated.
Due to a link between the cobalt chloride and cancer, it has been forbidden in Europe for use in silica gel.
An alternative indicator is
methyl violet which is orange when dry and green when hydrated.
Uses
Desiccant
Initial use
Moisture can cause mold and spoilage in many items.
It can also damage electronics by causing condensation and shorten the lifespan of chemicals, like those in vitamins. Silica gel packets help by absorbing moisture and extending the life of these items.
Silica gel may also be used to keep the
relative humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation (meteorology), precipitation, dew, or fog t ...
inside a high frequency radio or satellite transmission system
waveguide
A waveguide is a structure that guides waves by restricting the transmission of energy to one direction. Common types of waveguides include acoustic waveguides which direct sound, optical waveguides which direct light, and radio-frequency w ...
as low as possible (see also ''
humidity buffering'').
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 US, 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
Preservation may refer to:
Heritage and conservation
* Preservation (library and archival science), activities aimed at prolonging the life of a record while making as few changes as possible
* ''Preservation'' (magazine), published by the Nat ...
tool to control relative humidity in museum and library exhibitions and storage.
Other applications include diagnostic test strips, inhalation devices,
syringe
A syringe is a simple reciprocating pump consisting of a plunger (though in modern syringes, it is actually a piston) that fits tightly within a cylindrical tube called a barrel. The plunger can be linearly pulled and pushed along the inside ...
s,
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.
Regeneration
Once saturated with water, the gel may be regenerated by heating it to about
for 1–2 hours.
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.
Regenerating silica gel is important for both economic and environmental reasons.
It can be regenerated thermally or chemically. Chemical methods involve the use of desiccating agents or solvents to desorb water from silica gel, though these are generally less practical for routine regeneration. Thermal regeneration can be applied also to packed silica gel beds without opening them, and can be done by conventional heating in an oven at a temperature around .
Another option for thermal regeneration is by using microwaves to apply heat more evenly and reduce regeneration time compared to conventional ovens, but this process should be tightly controlled to avoid overheating.
Yet another option to apply heat is using a solar heater with a compound parabolic concentrator.
There are also studies on using ultrasonic waves to remove water molecules from silica gel at which are lower temperatures than that of conventional heating.
Chemistry

In chemistry, silica gel is used in
chromatography
In chemical analysis, chromatography is a laboratory technique for the Separation process, separation of a mixture into its components. The mixture is dissolved in a fluid solvent (gas or liquid) called the ''mobile phase'', which carries it ...
as a
stationary phase. In
column chromatography
Column chromatography in chemistry is a chromatography method used to isolate a single chemical compounds, chemical compound from a mixture. Chromatography is able to separate substances based on differential absorption of compounds to the adsorbe ...
, 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
In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water.
Hydrophobic molecules tend to be nonpolar and, thu ...
groups (such as C
18 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
Scavengers are animals that consume dead organisms that have died from causes other than predation or have been killed by other predators. While scavenging generally refers to carnivores feeding on carrion, it is also a herbivorous feeding ...
.
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 metal
The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...
s to form a
M-SG reducing agent. (See
SiGNa chemistry)
Silica gel is not expected to biodegrade in either water or soil.
Cat litter
Silica gel is also used as
cat litter,
by itself or in combination with more traditional materials, such as clays including
bentonite
Bentonite ( ) is an Absorption (chemistry), absorbent swelling clay consisting mostly of montmorillonite (a type of smectite) which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelli ...
. 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
Generally recognized as safe (GRAS) is a United States Food and Drug Administration (FDA) designation that a chemical or substance added to food is considered safe by experts under the conditions of its intended use. An ingredient with a GRAS d ...
(GRAS), meaning it can be added to food products without needing approval. Silica is allowed to be added to food in the US at up to 2% as permitted under 21 CFR 172.480. In the EU, it can be in up to 5% concentrations.
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.
Listed uses include:
anticaking agent
An anticaking agent is an additive placed in powdered or granulated materials, such as table salt or confectioneries, to prevent the formation of lumps ( caking) and for easing packaging, transport, flowability, and consumption. Caking mechanism ...
, defoaming agent, stabilizer, adsorbent, carrier, conditioning agent, chill proofing agent, filter aid, emulsifying agent, viscosity control agent, and anti-settling agent.
Silica can be found commonly in foods including baked goods, spices and herbs, dairy products, cocoa products, and more.
Water filtration
Given the water adsorption properties of silica gel, it is used in domestic water filters.
The surface structure of silica gel allows the adsorption of some minerals that are dissolved in the water,
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.
Humidity indicator (color-changing silica gel)

Silica gel may be doped with a moisture indicator that gradually changes its color when it transitions from the
anhydrous
A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achie ...
(dry) state to the hydrated (wet) state. Common indicators are
cobalt(II) chloride
Cobalt(II) chloride is an inorganic compound, a salt of cobalt and chlorine, with the formula . The compound forms several hydrates ·''n'', for ''n'' = 1, 2, 6, and 9. Claims of the formation of tri- and tetrahydrates have not been confirmed.M. ...
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
The European Union (EU) is a supranational union, supranational political union, political and economic union of Member state of the European Union, member states that are Geography of the European Union, located primarily in Europe. The u ...
in July 2000 as a toxic material.
Methyl violet may be formulated to change from orange to green, or orange to colorless. It also is toxic and potentially carcinogenic,
but is safe enough to have medicinal uses.
Ferric
In chemistry, iron(III) or ''ferric'' refers to the chemical element, element iron in its +3 oxidation number, oxidation state. ''Ferric chloride'' is an alternative name for iron(III) chloride (). The adjective ''ferrous'' is used instead for i ...
and
ferrous
In chemistry, iron(II) refers to the chemical element, element iron in its +2 oxidation number, oxidation state. The adjective ''ferrous'' or the prefix ''ferro-'' is often used to specify such compounds, as in ''ferrous chloride'' for iron(II ...
salts, sometimes combined with small amounts of
sodium hydroxide
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula . It is a white solid ionic compound consisting of sodium cations and hydroxide anions .
Sodium hydroxide is a highly corrosive base (chemistry), ...
, provide a better alternative. In particular,
ferric sulfate
Iron(III) sulfate or ferric sulfate (British English: sulphate instead of sulfate) is a family of inorganic compounds with the formula Fe2(SO4)3(H2O)n. A variety of water of crystallization, hydrates are known, including the most commonly encount ...
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.
[ ]
Hazards
Silica gel is non-toxic, and non-reactive and stable with ordinary usage. It will react with
hydrogen fluoride
Hydrogen fluoride (fluorane) is an Inorganic chemistry, inorganic compound with chemical formula . It is a very poisonous, colorless gas or liquid that dissolves in water to yield hydrofluoric acid. It is the principal industrial source of fluori ...
,
fluorine
Fluorine is a chemical element; it has Chemical symbol, symbol F and atomic number 9. It is the lightest halogen and exists at Standard temperature and pressure, standard conditions as pale yellow Diatomic molecule, diatomic gas. Fluorine is extre ...
,
oxygen difluoride,
chlorine trifluoride, strong acids, strong bases, and oxidizers.
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.
Crystalline silica dust can cause
silicosis
Silicosis is a form of occupational lung disease caused by inhalation of crystalline silica dust. It is marked by inflammation and scarring in the form of Nodule (medicine), nodular lesions in the upper lobes of the lungs. It is a type of pneum ...
, but synthetic amorphous silica gel is
indurated
In materials science, friability ( ), the condition of being friable, describes the tendency of a solid substance to break into smaller pieces under stress or contact, especially by rubbing. The opposite of friable is indurate.
Substances tha ...
, so it does not cause silicosis. Additional hazards may occur when doped with a
humidity indicator.
References
External links
* {{CPID, id=2845
UK Material Safety DatasheetDrying Seed with Color Indicating Silica Gel Southern Exposure Seed Exchange
Silica Gel Dehumidifier FAQsHelpful information on rechargeable silica packs for home use
Information on development and tables illustrating reactivity
General notes and Tables that summarize scavenging capacity
Conservation and restoration materials
Desiccants
Gels
Glass types
Silicon dioxide
American inventions