Silica gel is a granular, vitreous, porous form of silicon dioxide made synthetically
from sodium silicate. Silica gel is tough and hard; it is more solid than common household
gels like gelatin or agar. It is a naturally occurring mineral that is purified and processed
into either granular or beaded form. As a desiccant, it has an average pore size of
2.4 nanometers and has a strong affinity for water molecules.
Silica gel is most commonly encountered in everyday life as beads in a small paper packet.
In this form, it is used as a desiccant to control local humidity to avoid spoilage or
degradation of some goods. Because silica gel can have added chemical indicators and
absorbs moisture very well, silica gel packets usually bear warnings for the user not to
eat the contents.
History Silica gel was in existence as early as the
1640s as a scientific curiosity. 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
by chemistry professor Walter A. Patrick at Johns Hopkins University in 1918.
In World War II, silica gel was indispensable in the war effort for keeping penicillin dry,
protecting military equipment from moisture damage, as a fluid cracking catalyst for the
production of high octane gasoline, and as a catalyst support for the manufacture of
butadiene from ethanol, feedstock for the synthetic rubber program.
Types Type A - clear pellets, approximate pore diameter:
2.5 nm, drying and moistureproof properties, can be used as catalyst carriers, adsorbents,
separators and variable-pressure adsorbent. Type B - translucent white pellets, pore diameter:
4.5-7.0 nm, liquid adsorbents, drier and perfume carriers, also may be used as catalyst carriers,
cat litter. Type C - translucent, micro-pored structure,
raw material for preparation of silica gel cat litter. Additionally dried and screened,
it forms macro-pored silica gel which is used as drier, adsorbent and catalyst carrier.
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, removal of non-micro-organism impurities.
Properties Silica gel's high surface area allows it to
adsorb water readily, making it useful as a desiccant. 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 chemically, material silica gel removes
moisture by adsorption onto the surface of its numerous pores rather than by absorption
into the bulk of the gel. Regeneration
Once saturated with water, the gel can be regenerated by heating it to 120 °C for
two 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.
Preparation A solution of sodium silicate 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)2CoCl4 or cobalt chloride CoCl2 is added. This will
cause the gel to be blue when dry and pink when hydrated. An alternative indicator is
methyl violet which is orange when dry and green when hydrated.
Applications Desiccant
In many items, moisture encourages the growth of mold and spoilage. Condensation may also
damage other items like electronics and may speed the decomposition of chemicals, such
as those in vitamin pills. Through the inclusion of silica gel packets, these items can be
preserved longer. 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. 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 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.
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.
Chemistry
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 achieving a desired separation of certain molecular sizes. 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 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 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 media. 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. 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. It is trackless and virtually odorless.
Purchasing silica in the form of cat litter can be an easy and cost-effective way for
retail consumers to purchase silica gel for use in other applications, such as maintaining
the desired relative humidity in humidors, keeping tools or other materials rust-free
in damp environments or long-term storage, and preserving dried food.
Food additive Silica gel, also referred to as silica aerogel
or hydrated silica, has FDA GRAS status, 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.
Listed uses include: anticaking agent, defoaming agent, stabilizer, adsorbent, carrier, conditioning
agent, chillproofing agent, filter aid, emulsifying agent, viscosity control agent, and anti-settling
agent. Hazards
Silica gel is non-toxic, non-flammable, 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. Silica gel is irritating to the respiratory tract
and may cause irritation of the digestive tract, and dust from the beads may cause irritation
to the skin and eyes, so precautions should be taken. Crystalline silica dust can cause
silicosis but synthetic amorphous silica gel is indurated, and so does not cause silicosis.
Humidity indicator
Silica gel may be doped with a moisture indicator that gradually changes its color when it transitions
from the anhydrous state, to the hydrated state. Common indicators are cobalt(II) chloride
and methyl violet. Cobalt chloride is deep blue when dry and pink when wet. It is toxic
and carcinogenic, and was reclassified by the European Union in July 2000 as a toxic
material. Methyl violet can be formulated to change from orange to green, or orange
to colorless. It is also toxic and potentially carcinogenic, but is safe enough to have medicinal
uses. References
External links Record in the Household Products Database
of NLM UK Material Safety Datasheet
Silica Gel with Moisture indicator: Instructions for Use Detailed use and re-drying info
Silica Gel Dehumidifier FAQs Helpful Information on Rechargeable Silica Packs for Home Usage
Silica-bound reagents Information on development and Tables illustrating reactivity
Silica-bound scavengers General notes and Tables that summarize scavenging capacity
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