A desiccant is a

hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance' ...

substance that is used to induce or sustain a state of dryness (

desiccation Desiccation () is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic (attracts and holds water) substance that induces or sustains such a state in its local vicinity in a moderately sealed container. ...

) in its vicinity; it is the opposite of a

humectant A humectant is a hygroscopic (water-absorbing) substance used to keep things moist. They are used in many products, including food, cosmetics, medicines and pesticides. When used as a food additive, a humectant has the effect of keeping moistur ...

. Commonly encountered pre-packaged desiccants are solids that absorb

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

. 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.

Types of desiccants

Although some desiccants are

chemically inert In chemistry, the term chemically inert is used to describe a substance that is not chemically reactive. From a thermodynamic perspective, a substance is inert, or nonlabile, if it is thermodynamically unstable (positive standard Gibbs free en ...

, others are extremely reactive and require specialized handling techniques. The most common desiccant is silica gel, an otherwise inert, nontoxic, water-insoluble white solid. Tens of thousands of tons are produced annually for this purpose. Other common desiccants include

activated charcoal "Activated" is a song by English singer Cher Lloyd. It was released on 22 July 2016 through Vixen Records. The song was made available to stream exclusively on ''Rolling Stone'' a day before to release (on 21 July 2016). Background In an inter ...

, calcium sulfate, calcium chloride, and

molecular sieve A molecular sieve is a material with pores (very small holes) of uniform size. These pore diameters are similar in size to small molecules, and thus large molecules cannot enter or be adsorbed, while smaller molecules can. As a mixture of molecu ...

s (typically,

zeolite Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ･y where is either a metal ion or H+. These p ...

s). Desiccants may also be categorized by their type, either I,II,III,IV, or V. These types are a function of the shape of the desiccant's moisture sorption isotherm. Alcohols and acetones are also dehydrating agents.

Performance efficiency

One measure of desiccant efficiency is the ratio (or

percentage In mathematics, a percentage (from la, per centum, "by a hundred") is a number or ratio expressed as a fraction of 100. It is often denoted using the percent sign, "%", although the abbreviations "pct.", "pct" and sometimes "pc" are also use ...

) of water storable in the desiccant relative to the

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different ele ...

of desiccant. Another measure is the residual

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, dew, or fog to be present. Humidity dep ...

of the air or other fluid being dried. The performance of any desiccant varies with

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

and both relative humidity and absolute humidity. To some extent, desiccant performance can be precisely described, but most commonly, the final choice of which desiccant best suits a given situation, how much of it to use, and in what form, is made based on testing and practical experience.

Colored saturation indicators

Sometimes a humidity indicator is included in the desiccant to show, by color changes, the degree of water-saturation of the desiccant. One commonly used indicator is cobalt chloride (CoCl₂).

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 achi ...

cobalt chloride is blue. When it bonds with two water molecules, (CoCl₂•2H₂O), it turns purple. Further hydration results in the pink hexaaquacobalt(II) chloride complex o(H₂O)₆l₂.

Applications

One example of desiccant usage is in the manufacture of insulated windows where zeolite spheroids fill a rectangular spacer tube at the perimeter of the panes of glass. The desiccant helps to prevent the condensation of moisture between the panes. Another use of zeolites is in the "dryer" component of refrigeration systems to absorb water carried by the refrigerant, whether residual water left over from the construction of the system, or water released by the degradation of other materials over time. Bagged desiccants are also commonly used to protect goods in barrier-sealed shipping containers against moisture damage: rust, corrosion, etc. Hygroscopic cargo, such as cocoa, coffee, various nuts and grains, and other foods can be particularly susceptible to mold and rot when exposed to condensation and humidity. Because of this, shippers often take measures by deploying desiccants to protect against loss. Pharmaceutical packaging often includes small packets of desiccant to keep the atmosphere inside the package below critical levels of water vapor. Desiccants induce dryness in any environment and reduce the amount of moisture present in air. Desiccants come in various forms and have found widespread use in the food, pharmaceuticals, packing, electronics and many manufacturing industries. Air conditioning systems can be based on desiccants, as drier air feels more comfortable and absorbing water itself removes heat. Desiccants are used in livestock farming, where, for example, new-born piglets are highly susceptible to hypothermia owing to their wetness.

Drying of solvents

Desiccants are also used to remove water from solvents, typically required by chemical reactions that do not tolerate water, e.g., the Grignard reaction. The method generally, though not always, involves mixing the solvent with the solid desiccant. Studies show that molecular sieves are superior as desiccants relative to chemical drying reagents such as sodium-benzophenone. Sieves offer the advantages of being safe in air and recyclable.

Types of desiccants

Performance efficiency

Colored saturation indicators

Applications

Drying of solvents

See also

References

Further reading