Silica gel

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Silica gel is a granular, porous form of silica made synthetically from sodium silicate. Despite the name, silica gel is a solid.

Silica gel is most commonly encountered in everyday life as beads packed in a semi-permeable plastic. In this form, it is used as a desiccant to control local humidity in order to avoid spoilage. Because of poisonous dopants, silica gel packets usually bear warnings for the user not to eat the contents, but to throw them away instead.

1 History
2 Properties
3 Applications
4 Hazards
5 References
6 External links

[edit] History

Silica gel was patented by chemistry professor Walter A. Patrick at Johns Hopkins University, Baltimore, Maryland in 1919. Prior to that it was already used in World War I for the adsorption of vapors and gases in gas mask canisters. Prior to that, the substance was in existence as early as the 1640s as a scientific curiosity <ref>Maryann Feldman and Pierre Desrochers (March 2003). "Research Universities and Local Economic Development: Lessons from the History of the Johns Hopkins University". Industry and Innovation 10 (1, 5–24). </ref>

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.

[edit] Properties

Silica gel's high surface area (around 800 m²/g) allows it to adsorb water readily, making it useful as a desiccant (drying agent). It can reduce the relative humidity in a closed space to around 40%^{[citation needed]}. Once saturated with water, the gel can be regenerated by heating to 150°C (300°F) for 1.5 hours per liter of gel. Some types of silica gel will "pop" when exposed to sufficient amounts of water.

[edit] Applications

[edit] Desiccant

See also: Desiccant

In many items such as leather and pepperoni, 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. By adding sachets of silica gel, these items can be preserved longer.

Silica gel may also be used to keep the relative humidity inside a waveguide as low as possible. Excessive moisture buildup within a waveguide can cause decreased TX/RX gain by increasing Voltage Standing Wave Ratio (VSWR). The beads of water that form and condense inside the waveguide change the characteristic impedance and frequency.

[edit] 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. 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 aluminum or plastic sheets for thin layer chromatography.

Chelating groups have also been covalently bound to silica gel. These materials have the ability to remove metal ions selectively from aqueous media. Chelating goups 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.

[edit] 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, albeit expensive.

[edit] Hazards

Alone, silica gel is non-toxic, non-flammable and chemically unreactive. However, some of the beads may be doped with a moisture indicator, such as cobalt (II) chloride, which is toxic and may be carcinogenic. Cobalt (II) chloride is deep blue when dry (anhydrous) and pink when moist (hydrated). This is the reason most silica gel packets are labeled as dangerous or poisonous when eaten.

Crystalline silica dust can cause silicosis but synthetic amorphous silica, which is what silica gel is, does not cause silicosis. A chemically similar substance with far greater porosity is aerogel.