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absorbing moisture (as from the air)

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湿りやすい;吸湿性の

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/08/22 10:57:18」(JST)

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This article is about a chemical property. For the underwater optical device, see Hydroscope.

Hygroscopy is the ability of a substance to attract and hold water molecules from the surrounding environment. This is achieved through either absorption or adsorption with the absorbing or adsorbing material becoming physically "changed" somewhat, by an increase in volume, stickiness, or other physical characteristic of the material, as water molecules become "suspended" between the material's molecules in the process. While some similar forces are at work here, it is different from capillary attraction, a process where glass or other "solid" substances attract water, but are not changed in the process (for example, water molecules becoming suspended between the glass molecules).

Hygroscopic substances include cellulose fibers such as cotton and paper, sugar, caramel, honey, glycerol, ethanol, methanol, diesel fuel, sulfuric acid, methamphetamine, many fertilizer chemicals, many salts (including table salt), and a wide variety of other substances.

Zinc chloride and calcium chloride, as well as potassium hydroxide and sodium hydroxide (and many different salts), are so hygroscopic that they readily dissolve in the water they absorb: this property is called deliquescence. Not only is sulfuric acid hygroscopic in concentrated form but its solutions are hygroscopic down to concentrations of 10 Vol-% or below. A hygroscopic material will tend to become damp and "cake" when exposed to moist air (such as the salt inside salt shakers during humid weather).

Because of their affinity for atmospheric moisture, hygroscopic materials might necessitate their being stored in sealed containers. When added to foods or other materials for the express purpose of maintaining moisture content, such substances are known as humectants.

Materials and compounds exhibit different hygroscopic properties, and this difference can lead to detrimental effects, such as stress concentration in composite materials. The amount a particular material or compound is affected by ambient moisture may be considered its coefficient of hygroscopic expansion (CHE) (also referred to as CME, or coefficient of moisture expansion) or coefficient of hygroscopic contraction (CHC)—the difference between the two terms being a difference in sign convention and a difference in point of view as to whether the difference in moisture leads to contraction or expansion.

Differences in hygroscopy can be observed in plastic-laminated paperback book covers—often, in a suddenly moist environment, the book cover will curl away from the rest of the book. The unlaminated side of the cover absorbs more moisture than the laminated side and increases in area, causing a stress that curls the cover toward the laminated side. This is similar to the function of a thermometer's bi-metallic strip. Inexpensive dial-type hygrometers make use of this principle using a coiled strip.

Engineering properties[edit source | edit]

The amount of moisture held by hygroscopic materials is usually proportional to the relative humidity. Tables containing this information can be found in many engineering handbooks and is also available from suppliers of various materials and chemicals.

Biology[edit source | edit]

The thorny devil features hygroscopic grooves between the spines of its skin to capture water in its desert habitat.

The seeds of some grasses have hygroscopic extensions that bend with changes in humidity, enabling them to disperse over the ground. An example is Needle-and-Thread, Hesperostipa comata.^[1] Each seed has an awn that twists several turns when the seed is released. Increased moisture causes it to untwist, and, upon drying, to twist again, thereby drilling the seed into the ground.

Thorny devils collect moisture in the dry desert via nighttime condensation of dew that forms on their skin and is channeled to their mouths in hygroscopic grooves between the spines of their skin. Water also collects in these grooves when it rains. Capillary action allows the lizard to suck in water from all over its body.

Deliquescence[edit source | edit]

Deliquescent materials are substances (mostly salts) that have a strong affinity for moisture and will absorb relatively large amounts of water from the atmosphere if exposed to it, forming a liquid solution. Deliquescent salts include calcium chloride, magnesium chloride, zinc chloride, potassium carbonate, potassium phosphate, carnallite, ferric ammonium citrate, potassium hydroxide, and sodium hydroxide. Owing to their very high affinity for water, these substances are often used as desiccants, which is also an application for concentrated sulfuric and phosphoric acids. These compounds are used in the chemical industry to remove the water produced by chemical reactions.^{[citation needed]}

Polymers[edit source | edit]

Many engineering polymers are hygroscopic, including nylon, ABS, polycarbonate, cellulose, and Poly(methyl methacrylate).

Other polymers, such as polyethylene and polystyrene, do not normally absorb much moisture, but are able to carry significant moisture on their surface when exposed to liquid water.^[2]

Type-6 nylon can absorb up to 9.5% of its weight in moisture.^[3]

Applications in baking[edit source | edit]

The use of different substances' hygroscopic properties in baking are often used to achieve differences in moisture content and, hence, crispiness. Different varieties of sugars are used in different quantities to produce a crunchy, crisp biscuit (US: cracker) versus a soft, chewy cake. Sugars such as honey, brown sugar, and molasses are examples of sweeteners used to create more moist, chewy cakes.

References[edit source | edit]

^ Fire Effects Information System, Species: Hesperostipa comata U.S. Department of Agriculture Forest Service.
^ Schwartz, S., Goodman, S. (1982). Plastics Materials and Processes, Van Nostrand Reinhold Company Inc. ISBN 0-442-22777-9, p.547
^ http://www.sdplastics.com/nylon.html

External links[edit source | edit]

Look up hygroscopy or hygroscopic in Wiktionary, the free dictionary.

Video of Deliquescense of Calcium Chloride

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Japanese Journal

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橋本明弘,久芳奈遠美,村上正隆
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… The present article reviews the current status of numerical models and simulations in precipitation enhancement studies, including the latest achievements in terms of optimal seeding method, snowfall enhancement in a dam catchment by glaciogenic seeding, idealized simulations of hygroscopic seeding, and rainfall enhancement in a dam catchment by hygroscopic seeding. …
NAID 130005065038

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折笠成宏,村上正隆,斎藤篤思
エアロゾル研究 30(1), 24-31, 2015
… As one of the physical evaluation techniques of seeding effects, cloud seeding experiments were conducted utilizing an instrumented aircraft, with the use of dry-ice pellets in cold clouds and hygroscopic flares and salt micro-powders in warm clouds as a seeding agent. … The observational results we obtained so far showed that hygroscopic seeding of warm clouds might be effective under limited conditions. …
NAID 130005065037

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田尻拓也,山下克也,斎藤篤思 [他],村上正隆
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… In this report, the laboratory studies on the assessments of hygroscopic seeding and glaciogenic seeding materials at MRI are introduced. … We also investigated a widely-used delivery technique of silver iodide particles from the ground-based smoke generator into supercooled clouds, changing dissolving agent and hygroscopic substances in AgI acetone solutions to improve CCN and IN abilities. …
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