Hygroscopy

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's molecules, adsorbing substances can become physically changed, e.g. changing in volume, boiling point, viscosity or some other physical characteristic or property of the substance. For example, a finely dispersed hygroscopic powder, such as a salt, may become clumpy over time due to collection of moisture from the surrounding environment.

''Deliquescent'' materials are sufficiently hygroscopic that they dissolve in the water they absorb, forming an aqueous solution.

Hygroscopy is essential for many plant and animal species' attainment of hydration, nutrition, reproduction and/or seed dispersal. Biological evolution created hygroscopic solutions for water harvesting, filament tensile strength, bonding and passive motion – natural solutions being considered in future biomimetics.

Etymology and pronunciation

The word ''hygroscopy'' () uses combining forms of '' hygro-'' and '' -scopy''. Unlike any other ''-scopy'' word, it no longer refers to a viewing or imaging mode. It did begin that way, with the word ''hygroscope'' referring in the 1790s to measuring devices for humidity level. These hygroscopes used materials, such as certain animal hairs, that appreciably changed shape and size when they became damp. Such materials were then said to be ''hygroscopic'' because they were suitable for making a hygroscope. Eventually, the word ''hygroscope'' ceased to be used for any such instrument in modern usage, but the word ''hygroscopic'' (tending to retain moisture) lived on, and thus also ''hygroscopy'' (the ability to do so). Nowadays an instrument for measuring humidity is called a hygrometer (''hygro-'' + ''-meter'').

History

Early hygroscopy literature began circa 1880. Studies by Victor Jodin (''Annales Agronomiques'', October 1897) focused on the biological properties of hygroscopicity. He noted pea seeds, both living and dead (without germinative capacity), responded similarly to atmospheric humidity, their weight increasing or decreasing in relation to hygrometric variation.

Marcellin Berthelot viewed hygroscopicity from the physical side, a physico-chemical process. Berthelot's principle of reversibility, briefly- that water dried from plant tissue could be restored hygroscopically, was published in "Recherches sur la desiccation des plantes et des tissues végétaux; conditions d'équilibre et de réversibilité," (''Annales de Chimie et de Physique'', April 1903).

Léo Errera viewed hygroscopicity from perspectives of the physicist and the chemist. His memoir "Sur l'Hygroscopicité comme cause de l'action physiologique à distance" (''Recueil de l'lnstitut Botanique Léo Errera, Université de Bruxelles'', tome vi., 1906) provided a hygroscopy definition that remains valid to this day. Hygroscopy is "exhibited in the most comprehensive sense, as displayed

Overview

Hygroscopic substances include cellulose fibers (such as cotton and paper), sugar, caramel, honey, glycerol, ethanol, wood, methanol, sulfuric acid, many fertilizer chemicals, many salts (like calcium chloride, bases like sodium hydroxide etc.), and a wide variety of other substances.

If a compound dissolves in water, then it is considered to be hydrophilic.

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% v/v or below. A hygroscopic material will tend to become damp and cakey when exposed to moist air (such as the salt inside salt shakers during humid weather).

Because of their affinity for atmospheric moisture, desirable hygroscopic materials might require storage in sealed containers. Some hygroscopic materials, e.g., sea salt and sulfates, occur naturally in the atmosphere and serve as cloud seeds, cloud condensation nuclei (CCNs). Being hygroscopic, their microscopic particles provide an attractive surface for moisture vapour to condense and form droplets. Modern-day human cloud seeding efforts began in 1946.

When added to foods or other materials for the express purpose of maintaining moisture content, hygroscopic materials 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 volume of a particular material or compound is affected by ambient moisture and may be considered its coefficient of hygroscopic expansion (CHE) (also referred to as CME, or coefficient of moisture expansion) or the coefficient of hygroscopic contraction (CHC)—the difference between the two terms being a difference in sign convention.

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 thermostat's bimetallic strip. Inexpensive dial-type hygrometers make use of this principle using a coiled strip. Deliquescence is the process by which a substance absorbs moisture from the atmosphere until it dissolves in the absorbed water and forms a solution. Deliquescence occurs when the vapour pressure of the solution that is formed is less than the partial pressure of water vapour in the air.

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 (e.g., water molecules do not become suspended between the glass molecules).

Deliquescence

Deliquescence, like hygroscopy, is also characterized by a strong affinity for water and tendency to absorb moisture from the atmosphere if exposed to it. Unlike hygroscopy, however, deliquescence involves absorbing sufficient water to form an aqueous solution. Most deliquescent materials are salts, including calcium chloride, magnesium chloride, zinc chloride, ferric chloride, carnallite, potassium carbonate, potassium phosphate, ferric ammonium citrate, ammonium nitrate, 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. Some deliquescent compounds are used in the chemical industry to remove water produced by chemical reactions (see drying tube).

Biology

Hygroscopy appears in both plant and animal kingdoms, the latter benefiting via hydration and nutrition. Some amphibian species secrete a hygroscopic mucus that harvests moisture from the air. Orb web building spiders produce hygroscopic secretions that preserve the stickiness and adhesion force of their webs. One aquatic reptile species is able to travel beyond aquatic limitations, onto land, due to its hygroscopic integument.

Plants benefit from hygroscopy via hydration and reproduction – demonstrated by convergent evolution examples. Hygroscopic movement (hygrometrically activated movement) is integral in fertilization, seed/spore release, dispersal and germination. The phrase "hygroscopic movement" originated in 1904's "''Vorlesungen Über Pflanzenphysiologie''", translated in 1907 as "Lectures on Plant Physiology" (Ludwig Jost and R.J. Harvey Gibson, Oxford, 1907). When movement becomes larger scale, affected plant tissues are colloquially termed hygromorphs. Hygromorphy is a common mechanism of seed dispersal as the movement of dead tissues respond to hygrometric variation, e.g. spore release from the fertile margins of ''Onoclea sensibilis''. Movement occurs when plant tissue matures, dies and desiccates, cell walls drying, shrinking; and also when humidity re-hydrates plant tissue, cell walls enlarging, expanding. The direction of the resulting force depends upon the architecture of the tissue and is capable of producing bending, twisting or coiling movements.

Hygroscopic hydration examples

File:Air Plant (Tillandsia bulbosa) (8307461501).jpg, Air plant (''Tillandsia bulbosa'')
File:File snake (Acrochordus granulatus).jpg, The aquatic file snake (''A. granulatus'') with hygroscopic skin, shown out of water
File:Herbstspinne445.JPG, An orb-weaver spider (''Larinioides cornutus'') with hygroscopic coated capture threads
File:Makifrosch-59.jpg, Waxy monkey tree frog (''Phyllomedusa sauvagii'')

* Air plants, a ''Tillandsia'' species, are epiphytes that use their degenerated, non-nutritive roots to anchor upon rocks or other plants. Hygroscopic leaves absorb their necessary moisture from humidity in the air. The collected water molecules are transported from leaf surfaces to an internal storage network via osmotic pressure with capacity sufficient for the plant's growing requirements.
* The file snake (''Acrochordus granulatus''), from a family known as completely aquatic, has hygroscopic skin that serves as a water reservoir, retarding desiccation, allowing it to travel out of water.
* Another example is the sticky capture silk found in spider webs, e.g. from the orb-weaver spider (''Larinioides cornutus''). This spider, as typical, coats its threads with a self-made hydrogel, an aggregate blend of glycoproteins, low molecular mass organic and inorganic compounds (LMMCs), and water. The LMMCs are hygroscopic, thus is the glue, its moisture absorbing properties using environmental humidity to keep the capture silk soft and tacky.
* The waxy monkey tree frog (''Phyllomedusa sauvagii'') and the Australian green tree frog (''Litoria caerulea'') benefit from two hygroscopically-enabled hydration processes; transcutaneous uptake of condensation on their skin and reduced evaporative water loss due to the condensed water film barrier covering their skin. Condensation volume is enhanced by the hygroscopic secretions they wipe across their granular skin.
* Some toads use hygroscopic secretions to reduce evaporative water loss, ''Anaxyrus sp.'' being an example. The venomous secretion from its parotoid gland also includes hygroscopic glycosaminoglycans. When the toad wipes this protective secretion on its body its skin becomes moistened by the surrounding environmental humidity, considered an aid in water balance.

* Red and white clover (''Trifolium pratense'') and (''Trifolium repens''), yellow bush lupine (''Lupinus arboreus'') and several members of the legume family have a hygroscopic hilar valve (hilum) that controls seed embryo moisture levels. The saguaro (''Carnegiea gigantea''), another ''eudicots'' species, also has hygroscopic seeds shown to imbibe up to 20% atmospheric moisture, by weight. Functionally, the hilar valve allows water vapor to enter or exit to ensure viability, while blocking liquid water. If however, humidity levels gradually rise to a high enough level, the hilar valve remains open, allowing liquid water passage for germination. Physiologically, the inner and outer epidermides have independent hilar valve control. The outer epidermis has columnar-shaped cells, annularly arranged about the hilum. These counter palisade cells, being hygroscopic, respond to external humidity by swelling and closing the hilar valve during high humidity, preventing water absorption into the seed. Reversibly, they shrivel, opening the valve during low humidity, allowing the seed to expel excess moisture. The inner epidermis, inside the seed's impermeable integument, has palisade epidermis cells, a second annularly arranged hygroscopic layer attuned to the embryo's moisture level. There exists a moisture tension between inner and outer palisade cells. For the hilum to close, this moisture needs to exceed some minimum level (14–25% for these species). While the hilar valve is open (i.e., low outer humidity) if the humidity suddenly increases, the moisture tension reaches that protective threshold and the hilum closes, preventing moisture (liquid water) from entering. If, however, the outer humidity rises gradually, implying suitable growing conditions, the moisture tension level doesn't immediately exceed the threshold, keeping the hilum open and enabling the gradual moisture entry necessary for imbibition.

Hygroscopic-assisted propagation examples

Typical of hygroscopic movement are plant tissues with "closely packed long (columnar) parallel thick-walled cells (that) respond by expanding longitudinally when exposed to humidity and shrinking when dried (Reyssat et al., 2009)". Cell orientation, pattern structure (annular, planar, bi-layered or tri-layered) and the effects of the opposite-surface's cell orientation control the hygroscopic reaction. Moisture responsive seed encapsulations rely on valves opening when exposed to wetting or drying; discontinuous tissue structures provide such predetermined breaking points (sutures), often implemented via reduced cell wall thickness or seams within bi- or tri-layered structures. Graded distributions varying in density and/or cell orientation focus hygroscopic movement, frequently observed as biological actuators (a hinge function); e.g. pinecones (''Pinus spp.''), the ice plant (''Aizoaceae spp.'') and the wheat awn (''Triticum spp.''), described below.

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* Hygroscopic bi-layered cell arrays act as a Pseudanthium, capitulum hinge in some plants, ''Xerochrysum bracteatum'' and '' Syngonanthus elegans'' being examples. The hygroscopic bending of involucral bracts surrounding a capitulum contributes to flower protection and pollination and assists dispersion by protecting delicate pappi filaments from entanglement or destruction by precipitation, e.g. ''Taraxacum'' (dandelions). In nature these involucral bracts have a diurnal rhythm. The whorl of hygroscopic bracts bend outward exposing the capitulum (see illustration) during the day, then inward, closing it at night, as the relative humidity shifts in response to the daily temperature change. Bracts are scarious, the hinge and blade composed exclusively of dead cells (Nishikawa et al., 2008), allowing the hygroscopically activated bracts to function from flowering through achene dispersal. Physiologically, the bract's lower section is source to the hinge-like function, consisting of sclerenchyma-like abaxial