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Hygroscopy
Hygroscopy
Hygroscopy
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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.[1][2]

Etymology and pronunciation

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The word hygroscopy (/haɪˈɡrɒskəpi/) uses combining forms of hygro- (for moisture or humidity) and -scopy (observation). Originally, the word hygroscope referred to devices for measuring humidity level by visual observation. Early hygroscopes (circa 1790s) used materials such as certain animal hairs that visibly 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 visual instrument, but the word hygroscopic lived on, referring to the property of retaining moisture, and thus also hygroscopy (the ability to do so). In modern usage, an instrument for measuring humidity is called a hygrometer.

History

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Early hygroscopy literature began circa 1880.[3] Studies by Victor Jodin (Annales Agronomiques, October 1897) focused on the biological properties of hygroscopicity.[4] 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).[4]

Léo Errera viewed hygroscopicity from perspectives of the physicist and the chemist.[4] 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

  1. in the condensation of the water-vapour of the air on the cold surface of a glass;
  2. in the capillarity of hair, wool, cotton, wood shavings, etc.;
  3. in the imbibition of water from the air by gelatine;
  4. in the deliquescence of common salt;
  5. in the absorption of water from the air by concentrated sulphuric acid;
  6. in the behaviour of quicklime".[4]

Overview

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Apparatus for the determination of the hygroscopicity of fertilizer, Fixed Nitrogen Research Laboratory, c. 1930

Hygroscopic substances include cellulose fibers (such as cotton and paper), sugar, caramel, honey, glycerol, ethanol, wood, methanol, sulfuric acid, many fertilizer chemicals, many salts and a wide variety of other substances.[5]

If a compound dissolves in water, then it is considered to be hydrophilic.[6]

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.[7]

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

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"Deliquescence" redirects here. For the album by Swans, see Deliquescence (album).

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).[8]

Biology

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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[1] and reproduction – demonstrated by convergent evolution examples.[2] 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).[9] When movement becomes larger scale, affected plant tissues are colloquially termed hygromorphs.[10] Hygromorphy is a common mechanism of seed dispersal as the movement of dead tissues respond to hygrometric variation,[11] e.g. spore release from the fertile margins of Onoclea sensibilis. Movement occurs when plant tissue matures, dies and desiccates, cell walls drying, shrinking;[12] and also when humidity re-hydrates plant tissue, cell walls enlarging, expanding.[11] 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

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  • Air plant (Tillandsia bulbosa)
    Air plant (Tillandsia bulbosa)
  • The aquatic file snake (A. granulatus) with hygroscopic skin, shown out of water
    The aquatic file snake (A. granulatus) with hygroscopic skin, shown out of water
  • An orb-weaver spider (Larinioides cornutus) with hygroscopic coated capture threads
    An orb-weaver spider (Larinioides cornutus) with hygroscopic coated capture threads
  • Waxy monkey tree frog (Phyllomedusa sauvagii)
    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.[1]
  • 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.[13]
  • 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.[14] 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[15] and reduced evaporative water loss[16] due to the condensed water film barrier covering their skin. Condensation volume is enhanced by the hygroscopic secretions they wipe across their granular skin.[15]
  • 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.[16]
    Seeds of Trifolium pratense (red clover) next to a U.S. dime for scale.
Saguaro (Carnegiea gigantea) fruit bearing hygroscopic, humidity absorbing seed
  • 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.[17] The saguaro (Carnegiea gigantea), another eudicots species, also has hygroscopic seeds shown to imbibe up to 20% atmospheric moisture, by weight.[18] 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.[17] Physiologically, the inner and outer epidermis 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).[19] 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.[17]

Hygroscopic-assisted propagation examples

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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)".[10] 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.[2] 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.),[20] described below.

Illustration botanique, Xerochrysum (Helichrysum) bracteatum; No.1- Capitulum [bracts, florets, stamens]
  • Hygroscopic bi-layered cell arrays act as a 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[21] and assists dispersion by protecting delicate pappi filaments from entanglement or destruction by precipitation,[22] 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.[21] Physiologically, the bract's lower section is source to the hinge-like function, consisting of sclerenchyma-like abaxial (inner petal) tissue, parenchyma and adaxial epidermis (outer petal tissue).[21] Bract cell wall composition is rather uniform but its cells gradually change in orientation. The bract's hygroscopic bending is due to the differing cell orientations of its inner and outer epidermides, causing adaxial–abaxial force gradients between opposing sides that change with moisture; thus, the aggregate hygrometric force, in whorl unison, controls the capitulum's repetitive opening and closing.
  • Some trees and shrubs in fire-prone regions evolved a dual-stage hygroscopic dispersal; an initial thermo-sensitive enabling (extreme heat or fire), then a serotinous hygroresponsive seed release. Examples are the woody fruits of Myrtaceae (e.g. Eucalyptus species plurimae, Melaleuca spp.) and Proteaceae (e.g. Hakea spp., Banksia spp., Xylomelum spp.) and the woody cones of Pinaceae (e.g. Pinus spp.) and the cypress family (Cupressaceae), e.g. the giant sequoia (Sequoiadendron giganteum)).[2][23] Typical in lodgepole pine (Pinus contorta), Eucalyptus, and Banksia are resin-sealed seed encapsulations that require the heat of fire to physically melt the resin, enabling serotinous seed release.[24] Such seed encapsulations may "reduce seed loss or damage from granivores, desiccation, and fire (Moya et al., 2008; Talluto & Benkman, 2014; Lamont et al., 2016, 2020)."[2] The similarity of dual-stage dispersal techniques between different clades, angiosperms and gymnosperms, can be interpreted as a result of convergent evolution (e.g. Clarke et al., 2013).[2]
    Banksia Attenuata cone with open follicles
    • Banksia attenuata, typical of Banksia spp., has a seed bearing follicle composed of a bi-layer hygroscopic cell network. The woody follicle is thermo-sensitive, then hygroresponsive; serotinous humidity opening the ventral suture and exposing seed when germination conditions are favorable.[23] Physiologically, the heat-sensitive follicle valves of Banksia spp. are sealed by a wax (resin) layer, released by high ambient temperatures (fire), "thereby facilitating opening (e.g. Huss et al., 2018)."[2] The follicle mesocarp consists of high density branched fiber bundles; the endocarp, low density parallel fibers. A suture is caused by differential hygroscopic movements between layers, their microfibril structures having a large angle disparity (microfibril angle (MFA) γ = 75–90°).[2]
    • Pine cone scales (pinaceae spp.) employ a hygromorphic hinge for their seed release. Physiology involves a bi-layered structure of closely packed long parallel thick-walled cells. Fiber alignments within layers are non-uniform, varying longitudinally, producing different microfibril angles (MFAs) of 30° and 74° between layers over the span of the scale.[23] The region of greatest MFA, the hinge knuckle, is a small region near the scale and midrib (central stem) union.[10] In mature pine cones the outer scale layer is the controlling tissue, its long thick-walled cells responding longitudinally to environmental humidity. Distortion occurs in the knuckle region as movement of the outer layer overtakes that of the more passive inner scale layer, forcing the scale to bend or flex. The remainder of the scale is hygroscopically passive, though amplifies apex displacement via length and geometrically;[10] e.g. bending the scale closed with hydration or flexing it open with dehydration- releasing seed.
      Taraxacum officinale capitulum and achene [seed-beak-apical plate-pappus]
  • Flowering plants of the Asteraceae family have hygroscopically-influenced dispersion, coordinating anemochory (wind dispersal) with favorable environmental conditions,[25] common in A. genera Erigeron, Leontodon, Senecio, Sonchus and Taraxacum.[26] As example, the flight-enabling pappus of the common dandelion achene undergoes binary morphing (opened or closed) of its whisker-like filaments, in unison with chorused responses of the remaining achenes. Pappus movement is controlled via a hygroscopic actuator in the apical plate, at the beak's top, the locus for all the achene's filaments. High humidity causes each pappus to close, contracting its radially patterned structure, reducing its area and the likelihood of wind current dispersal.[25] For any achene that become released, flight dynamics of the reduced pappus dramatically limit dispersal range.[25] The hygroscopic actuator's responsiveness to changes in relative humidity (RH) is predictable, repeatable; e.g. the pappi of centaurea imperialis remain closed at ≥ 78% RH and open completely at ≤ 75% RH.[22] During more favorable lower humidity conditions, pappi fully expand and wind current allochory is re-enabled.
    Orchid tree (Bauhinia variegata) seed pods
  • The orchid tree (Bauhinia variegata) depends upon hygro-responsive twisting for its dispersal. Its seed pod contains two hygroscopic sclerenchyma fibre layers, nearly orthogonal, joining at the valves. During dehiscence the large 90° microfibril angle between endocarp layers,[23] combined with dual sided shrinkage, results in opposing helical torques[2] that force a suture at the weakest point, the seed case valves; their opening releases seed.[21]
    Ruschia sp. (Aizoaceae) flowers and multi-stage seed capsules
  • Some plants synchronize the opening of their mature seed capsule with active rainfall- hygrochasy. This dispersal technique is frequently observed in the arid regions of southern and eastern Africa, the Israeli desert, parts of North America and Somalia, and believed evolved to offer higher survival rates in arid environs.[27] Hygrochasy is commonly associated with family Aizoaceae spp., the ice plant, as > 98% of its species utilize post-wetting dehiscence; such dispersal is also observed in family Plantaginaceae with the alpine Veronica of New Zealand, evolving in the last 9Myr.[27] Common to all seed capsules are triangular circumferentially-arranged hygroscopic keels (valves) covering its seeds. These protective valves mechanically open only when hydrated with liquid water.[28] Each keel (five for Delosperma nakurense (Engl.) Herre) is composed of cellulosic lattice tissue that swells with hydration, opening within minutes. The enlarged cells force straightening of an inherent desiccated fold in the keel, the hygroscopic hinge, near the keel's union with the capsule perimeter. Fully opened, the keel pivots over 150°,[28] upward then backward, exposing seed compartments, one beneath each valve, separated by septa, all resting upon the capsule floor. Seeds are visible, but restrained by the cup-like ring created by the encircling keels. The final requirement for dispersal is rainfall, or sufficient moisture, to flush seed from this barrier, colloquially termed the splash cup.[27] Seed that overflows or splashes from the cup is dispersed to the nearby ground. Any remaining seed will be preserved when keels desiccate, hygroscopically shrink, and restore to their natural folded, closed state. The hygromorphic process is reversible, repeatable; neglected seed having subsequent dispersal opportunity via future rainfalls.
    Common stork's-bill (Erodium cicutarium) achenes with coiled awns
    Needle-and Thread (Hesperostipa comata) seedbuds
  • The seeds of some flowering herbs and grasses have hygroscopic appendages (awns) that bend with changes in humidity, enabling them to disperse over the ground, termed herpochory. The awn will thrust (or twist) when the seed is released, its motion dependent upon plant physiology. Subsequent hygrometric changes cause movements to repeat, thrusting (or twisting), pushing the seed into the ground.[20]

Two angiospermae families have similar methods of dispersal, though method of implementation varies within family: Geraniaceae family examples are the common stork's-bill (Erodium cicutarium) and geraniums (Pelargonium sp.); Poaceae family, Needle-and-Thread (Hesperostipa comata) and wheat (Triticum spp.). All rely upon a bi-layered parallel fiber hygroscopic cell physiology to control the awn's movement for dispersal and self-burial of seeds.[2] Alignment of cellulose fibrils in the awn's controlling cell wall determines direction of movement. If fiber alignments are tilted, non-parallel venation, a helix develops and awn movement becomes twisting (coiling) instead of bending;[21] e.g. coiling occurs in awns of Erodium,[2] and Hesperostipa.[29]

  • Some plants use hygroscopic movements for Ballochory (self-dispersal), active ballists forcibly ejecting their seeds; e.g. species of geranium, violet, wood sorrel, witch hazel, touch-me-not (Impatiens), and acanthus. Rupturing of the Bauhinia purpurea seed pod reportedly propels its seed up to 15 metres distance.[30]

Engineering properties

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Hygroscopic qualities of various materials illustrated in graph form; relative humidity on the X-axis and moisture content on the Y-axis.

Hygroscopicity is a general term used to describe a material's ability to absorb moisture from the environment.[31] There is no standard quantitative definition of hygroscopicity, so generally the qualification of hygroscopic and non-hygroscopic is determined on a case-by-case basis. For example, pharmaceuticals that pick up more than 5% by mass, between 40 and 90% relative humidity at 25 °C, are described as hygroscopic, while materials that pick up less than 1%, under the same conditions are regarded as non-hygroscopic.[32]

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.

Hygroscopy also plays an important role in the engineering of plastic materials. Some plastics, e. g. nylon, are hygroscopic while others are not.

Polymers

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Many engineering polymers are hygroscopic, including nylon, ABS, polycarbonate, cellulose, carboxymethyl cellulose, and poly(methyl methacrylate) (PMMA, plexiglas, perspex).

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.[33]

Type-6 nylon (a polyamide) can absorb up to 9.5% of its weight in moisture.[34]

Applications in baking

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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 cookie (British English: biscuit) versus a soft, chewy cake. Sugars such as honey, brown sugar, and molasses are examples of sweeteners used to create moister and chewier cakes.[35]

Research

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Several hygroscopic approaches to harvest atmospheric moisture have been demonstrated and require further development to assess their potentials as a viable water source.

  • Experiments with fog collection, in select environs, duplicated the hydrophilic surfaces and hygroscopic surface wetting observed in tree frog hydration (biomimicry). Subsequent material optimizations developed artificial hydrophilic surfaces with collection rates of 25 mg H2O/(cm2 h), more than twice the collection rate of tree frogs under comparable conditions, i.e. 100% RH (relative humidity).[16]
  • Another approach performs at lower 15–30% RHs but also has environs limitations; a sustainable biomass source is necessary. Super hygroscopic polymer films composed of biomass and hygroscopic salts are able to condense moisture from atmospheric humidity.[16] By implementing rapid sorption-desorption kinetics and operating 14–24 cycles per day, this technique produced an equivalent water yield of 5.8–13.3 L kg−1 of sustainable raw materials, demonstrating the potential for low-cost, scalable atmospheric water harvesting.[36]

Hygroscopic glues are candidates for commercial development. The most common cause of synthetic glue failure at high humidity is attributed to water lubricating the contact area, impacting bond quality. Hygroscopic glues may allow more durable adhesive bonds by absorbing (pulling) inter-facial environmental moisture away from the glue-substrate boundary.[14]

Integrating hygroscopic movement into smart building designs and systems is frequently mentioned, e.g. self-opening windows.[20] Such movement is appealing, an adaptive, self-shaping response that requires no external force or energy. However, capabilities of current material choices are limited. Biomimetic design of hygromorphic wood composites and hygro-actuated building systems have been modeled and evaluated.[37]

  • Hygrometric response time, precise shape changes and durability are lacking. Most currently available hygro-actuated composites are inferior and exhibit fatigue failure well before that seen in nature, e.g. in pine cone scales, indicating that a better understanding of the plants' biological structures is needed.[37] Materials composed of fluid-responsive active bilayer systems that can direct planned conformational hygromorphing are necessary.[20]
  • Current composites require undesirable trade-offs between hygromorphic response time and mechanical stability that must also be balanced with changing environmental stimuli.[37]

See also

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References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Hygroscopy

View on Grokipedia
from Grokipedia
Hygroscopy is the property of certain substances, termed hygroscopic materials, to attract and retain molecules from the surrounding environment through processes of absorption or adsorption, typically reaching an equilibrium content based on ambient relative . This phenomenon occurs at or near ordinary temperatures and involves molecules bonding intimately within the material, often via hydrogen bonds in polar structures, making the absorbed difficult to remove without specific conditions. Key mechanisms of hygroscopy include physical adsorption on surfaces, bulk absorption into the material matrix, and within porous structures, with the extent of uptake influenced by the substance's polarity, surface area, and environmental . Hygroscopy differs from deliquescence, a more extreme form where sufficient is absorbed to dissolve the substance entirely into a solution; hygroscopic materials, by contrast, retain their or semi-solid form while holding moisture. Hygroscopic properties have significant implications across fields, including pharmaceuticals where water uptake can induce hydrate formation or stability issues, materials engineering for controlling moisture in building components, and atmospheric science where hygroscopic growth of aerosol particles—defined as the increase in particle diameter due to water uptake—affects , formation, and . Common hygroscopic substances, such as and , are employed as desiccants to absorb excess in storage and transport applications.

Introduction

Definition and Fundamentals

Hygroscopy refers to the physical phenomenon in which certain substances attract and retain molecules from the surrounding environment, typically through either absorption into the bulk of the material or adsorption onto its surface. This property is characteristic of materials with a high affinity for , allowing them to interact with atmospheric under ambient conditions. The extent of hygroscopic behavior is fundamentally governed by environmental factors, including relative humidity (RH), which measures the ratio of the current water vapor pressure to the saturation vapor pressure at a given , and the equilibrium vapor pressure of . At equilibrium, the substance reaches its equilibrium moisture content (EMC), the stable moisture level where the rate of water uptake equals the rate of release, directly proportional to RH and inversely influenced by , as higher temperatures reduce water vapor affinity. Hygroscopicity thus varies with these parameters, enabling substances to adjust their moisture content dynamically in response to ambient conditions. Measurement of hygroscopicity often involves moisture sorption isotherms, which graphically represent the relationship between EMC and RH at a constant , providing insights into water-binding capacity. A key model for describing surface adsorption in these isotherms is the Brunauer-Emmett-Teller () equation, which extends the Langmuir to multilayer adsorption: VVm=cx(1−x)(1−x+cx)\frac{V}{V_m} = \frac{c x}{(1 - x)(1 - x + c x)} where VV is the volume of adsorbed gas, VmV_m is the adsorption capacity, xx is the relative (P/P0P/P_0), and cc is a constant reflecting the net of adsorption. Hygroscopic substances differ from non-hygroscopic ones, such as hydrophobic materials like waxes or certain plastics, which exhibit negligible uptake from the air due to low and lack of polar groups. While all hygroscopic materials are hydrophilic—possessing an affinity for —hygroscopicity specifically denotes the ability to absorb atmospheric , often leading to measurable or structural changes, whereas broader hydrophilic properties may involve interactions with liquid without vapor-phase uptake.

Etymology and Pronunciation

The term hygroscopy originates from roots: ὑγρός (hygrós), meaning "wet" or "moist," combined with the suffix derived from σκοπεῖν (skopeîn), meaning "to look at" or "examine," reflecting the observation of moisture attraction. This noun form entered English usage in the mid-19th century, with the earliest recorded instance in by Mayne. In standard , hygroscopy is rendered in International Phonetic Alphabet (IPA) as /ˌhaɪ.ɡrəˈskɒp.i/ in and /ˌhaɪ.ɡroʊˈskɑː.pi/ in , with stress on the third syllable. A related term, hygroscope, denoting an instrument that indicates changes through the deformation of hygroscopic materials, was coined in the 1660s from hygro- + -scope (instrument for viewing).

Historical Development

Early Observations

Ancient civilizations empirically observed the hygroscopic properties of certain salts, particularly , through their practical applications in . In , records indicate that salt was employed to extract from meats and as early as around 2000 BCE, inhibiting and enabling long-term storage in a hot climate. This technique, which relied on the salt's ability to absorb atmospheric and dehydrate surrounding materials, was essential for sustaining populations and was documented in early Egyptian practices for mummification and dietary needs as well. Similar observations emerged in ancient , where hygroscopic salts were integral to preservation methods dating back to the (approximately 1500–500 BCE). High-salt and (known as achaar) were used to protect , fruits, and meats from spoilage by drawing out moisture, a rooted in coastal and agricultural traditions that highlighted salt's role in extending food viability without . These pre-modern uses demonstrated an intuitive understanding of hygroscopy, as communities noted how salts like altered in humid environments, becoming damp or clumping, which informed their selective application in humid tropical conditions. In the , European natural philosophers began systematic empirical studies of interactions with materials, building on these ancient insights. , a prominent figure in the Royal Society, documented how various substances expanded or contracted with changes, observing effects on materials like and fabrics in his meteorological experiments around 1664–1665. These reports, detailed in his philosophical discourses, prompted early controlled tests using balances and sealed environments to quantify absorption, laying groundwork for recognizing hygroscopic behavior beyond preservation. Hooke's work emphasized the variable nature of air's content and its impact on everyday materials, influencing subsequent empiricists to explore related phenomena. Instrumental precursors to modern humidity detection emerged in the mid-18th century, with the development of the hygroscope as a device exploiting hygroscopic materials. In 1755, Swiss polymath invented a more refined that indicated relative changes through the expansion of hygroscopic substances, improving upon earlier designs. This was followed by Horace Bénédict de Saussure's 1783 innovation of a , which utilized the length changes in human —a highly hygroscopic material—to visibly detect atmospheric variations, providing a simple, mechanical precursor to quantitative instruments. These early devices, often constructed with everyday hygroscopic elements like or vegetable fibers, marked a shift from qualitative observations to rudimentary technological applications of hygroscopic principles.

Key Scientific Milestones

In the early , foundational work on gas absorption laid the groundwork for understanding hygroscopic phenomena. William Henry published his seminal paper in 1803, detailing experiments on the quantity of gases absorbed by water under varying temperatures and pressures, which established proportionality principles later extended to the uptake of by solids. Advancing this foundation, provided a thermodynamic framework in the 1870s through his comprehensive treatise On the Equilibrium of Heterogeneous Substances (1876–1878), where he derived equations for adsorption at interfaces, including the Gibbs adsorption isotherm that describes equilibria essential for modeling hygroscopic behavior in heterogeneous systems. The brought experimental breakthroughs in quantifying multilayer water adsorption on solids. In 1938, Stephen Brunauer, Paul Hugh Emmett, and introduced the in their paper on gas adsorption in multimolecular layers, offering a model that accurately predicts sorption isotherms for hygroscopic materials by accounting for successive layers beyond saturation. This theory became a cornerstone for analyzing interactions with porous and non-porous solids. Key conceptual distinctions emerged in the , with the chemical literature formalizing deliquescence as a where hygroscopic salts dissolve into aqueous solutions at a critical relative defined by the of their saturated solutions. Studies like those by Randall and Failey on gas activity coefficients in aqueous salt solutions quantified these thresholds, distinguishing deliquescence from mere adsorption. Following , hygroscopicity research expanded into applied fields. In , post-1940s investigations into utilized hygroscopic seeding agents, such as salts, to promote droplet coalescence and enhancement, as explored in early experiments. Concurrently, studies addressed moisture control in polymers and composites, improving stability in humid environments through targeted hygroscopic property assessments.

Physical and Chemical Principles

Mechanisms of Hygroscopicity

Hygroscopicity arises from the interaction of a substance with in the atmosphere, primarily through two distinct mechanisms: absorption and adsorption. Absorption involves the bulk incorporation of molecules into the interior structure of the material, such as the formation of hydration shells around ions in salts, where integrates into the lattice or molecular framework. In contrast, adsorption occurs at the surface, encompassing , which relies on weak van der Waals forces, and , involving stronger chemical bonds between molecules and the surface sites. These processes determine the extent to which a substance can take up , with absorption often leading to volumetric changes and adsorption dominating in porous or high-surface-area materials. The thermodynamic drivers of hygroscopic behavior are governed by the change for water incorporation, expressed as ΔG=ΔH−TΔS\Delta G = \Delta H - T \Delta S, where ΔH\Delta H is the change, TT is the temperature, and ΔS\Delta S is the change. For spontaneous , ΔG<0\Delta G < 0, typically favored by exothermic (ΔH<0\Delta H < 0) from hydrogen bonding or ion-dipole interactions, though decreases due to ordering of water molecules can oppose the process. In polar substances like sugars, the high polarity and ability to form hydrogen bonds with water enhance the negative ΔH\Delta H, making more favorable at lower temperatures. Studies show that is often -driven and non-spontaneous at higher temperatures, with isokinetic temperatures around 325 K marking the transition where effects dominate. Kinetic factors influence the rate of water uptake, primarily through diffusion rates and the activation energy required for water entry into the material. Water molecules diffuse from the surface to the bulk, with the rate limited by the material's porosity and the activation energy barrier, typically 40–60 kJ/mol for diffusion in polymers or rubbers. In ionic compounds, crystal lattice defects, such as vacancies or dislocations, lower this activation energy by providing pathways for water ingress, accelerating sorption kinetics. The overall process follows Fickian diffusion in many cases, where the diffusion coefficient decreases with increasing moisture content due to swelling. Quantitative models, such as the Langmuir isotherm, describe monolayer sorption on surfaces and provide insight into hygroscopic equilibrium. The Langmuir model assumes adsorption on a fixed number of independent sites, with no interaction between adsorbed molecules, leading to the coverage fraction θ\theta as a function of water vapor pressure pp. To derive it, consider the adsorption rate proportional to the empty sites and pressure: rate of adsorption = kap(1−θ)k_a p (1 - \theta), where kak_a is the adsorption rate constant. The desorption rate is kdθk_d \theta, with kdk_d the desorption constant. At equilibrium, kap(1−θ)=kdθk_a p (1 - \theta) = k_d \theta, rearranging gives θ=kap/kd1+kap/kd=Kp1+Kp\theta = \frac{k_a p / k_d}{1 + k_a p / k_d} = \frac{K p}{1 + K p}, where K=ka/kdK = k_a / k_d is the equilibrium constant. This model applies to hygroscopic sorption at low relative humidities, where water forms a monolayer before multilayer buildup or capillary condensation occurs.

Types of Hygroscopic Behavior

Hygroscopic behavior varies across a spectrum depending on the interaction between a substance and atmospheric water vapor. Hygroscopic materials absorb moisture from the air at relative humidities (RH) below 100%, resulting in a net mass gain as water molecules are incorporated into the solid structure via adsorption or absorption. In contrast, efflorescent materials, typically hydrated salts, exhibit the opposite behavior by releasing bound water as vapor when the ambient RH drops below their efflorescence relative humidity (ERH), leading to mass loss and potential crystallization. This dichotomy illustrates the reversible phase equilibria influenced by environmental RH, with hysteresis often observed between the DRH and ERH due to kinetic barriers in nucleation. Deliquescence is a pronounced type of hygroscopic behavior exhibited by certain soluble salts, where the solid transitions to a liquid solution upon absorbing atmospheric moisture. This process begins with the formation of a thin aqueous film on the crystal surface at RH approaching the deliquescence relative humidity (DRH), followed by continued absorption until the solid fully dissolves in the resulting saturated solution. The DRH marks the critical threshold and is determined by the equilibrium vapor pressure over the saturated solution, calculated as RHd=100×awRH_d = 100 \times a_w, where awa_w is the water activity of that solution. For instance, calcium chloride (CaCl₂) deliquesces at approximately 29% RH at 25°C, forming a highly concentrated aqueous phase that can further absorb water at higher RH. Beyond inorganic salts, hygroscopic behavior in polymers often involves swelling, where absorbed water molecules intercalate into the polymer matrix, causing volumetric expansion. This phenomenon arises from hydrogen bonding between water and polar groups in the polymer chains, leading to increased intermolecular spacing. Swelling is generally reversible, with the polymer contracting upon desorption in dry conditions; however, excessive or prolonged exposure can induce irreversible sorption effects, such as chain scission or cross-linking alterations, permanently modifying the material's dimensions and mechanical properties. Hygroscopic materials are commonly classified by the degree of moisture uptake, typically assessed by percentage weight gain after exposure to 80% RH at 25°C for 24 hours. Non-hygroscopic substances gain less than 0.2% by weight, slightly hygroscopic ones gain 0.2–2%, moderately hygroscopic gain 2–15%, and strongly (or very) hygroscopic exceed 15%. Representative examples include starch as weakly hygroscopic, with limited uptake suitable for controlled moisture environments; sodium hydroxide (NaOH) as moderately hygroscopic, absorbing significant moisture but requiring specific storage; and phosphorus pentoxide (P₂O₅) as strongly hygroscopic, rapidly reacting with water to form phosphoric acid.

Biological Roles

Hygroscopic Processes in Organisms

Hygroscopic processes play a crucial role in cellular hydration within living organisms, where hygroscopic proteins and salts act as osmolytes to regulate water uptake and maintain turgor pressure through osmotic balance. In plant cells, for instance, inorganic salts such as potassium ions and organic osmolytes contribute to the generation of osmotic pressure, which drives water influx across the plasma membrane and sustains the hydrostatic pressure against the cell wall, preventing wilting and supporting structural integrity. Similarly, in fungal and bacterial cells, hygroscopic compatible solutes like polyols and ions balance external osmotic stress, ensuring cellular hydration and turgor even in fluctuating environmental conditions. These mechanisms allow cells to achieve equilibrium water potentials essential for metabolic functions and growth. Evolutionary adaptations have led to the development of specialized hygroscopic tissues in organisms inhabiting arid environments, enhancing survival by facilitating water absorption and retention from low-humidity air. In plants, seed mucilage—a gel-like polysaccharide layer—evolved as a key adaptation in species from dry habitats, where it imbibes atmospheric moisture upon hydration, delaying desiccation and promoting germination timing aligned with favorable conditions. This trait is particularly prevalent in families like and , where mucilage production is phylogenetically conserved, allowing seeds to persist in soil seed banks under prolonged drought by forming a hydrated envelope that reduces evaporative loss. Such adaptations underscore hygroscopy's role in reproductive success, as evidenced by higher establishment rates in mucilage-producing species in semi-arid ecosystems. At the biochemical level, hygroscopic molecules interact directly with cellular components to avert desiccation damage. In fungi, glycerol serves as a primary osmolyte that accumulates intracellularly, forming hydrogen bonds with cell membranes and stabilizing lipid bilayers against dehydration-induced phase transitions. This interaction preserves membrane fluidity and prevents leakage, enabling fungal spores and hyphae to tolerate water activities as low as 0.64 while retaining viability. Likewise, hydrophilic proteins known as hydrophilins or late embryogenesis abundant (LEA) proteins in various organisms bind water molecules via charged residues, acting as molecular sponges that slow water loss during stress. These processes yield quantifiable effects on water retention in biological tissues, with hygroscopic components enabling capacities up to 90% moisture content relative to fresh weight in hydrated states, far exceeding non-hygroscopic counterparts. For example, in plant leaves, osmolytes and hydrophilic proteins sustain this high hydration level, buffering against rapid drying and supporting resilience in variable humidity. In arid-adapted seeds, mucilage can further amplify retention by absorbing 30-40 times its dry weight in water, translating to tissue moisture levels approaching 90% during imbibition. Such efficiency is critical for physiological recovery post-desiccation, as demonstrated by survival rates exceeding 80% in osmolyte-enriched cells.

Examples in Plant and Animal Biology

In plants, hygroscopic swelling of seed coats plays a crucial role in breaking dormancy and enabling germination, particularly in desert species adapted to arid environments. The mucilage in seed coats absorbs atmospheric moisture, leading to swelling that ruptures impermeable layers and facilitates water imbibition for embryo activation; this mechanism is evident in various species where the hygroscopic properties of the mucilage promote timely germination during brief wet periods. For instance, in model desert-adapted plants like , seed dormancy release involves moisture-dependent swelling, though the process is modulated by environmental cues such as rainfall to synchronize with favorable conditions. Hygroscopic moisture absorption also drives pollen tube elongation during plant reproduction. Pollen grains exhibit significant water uptake in response to humidity, swelling hygroscopically to initiate hydration and turgor pressure buildup necessary for tube growth toward the ovule; this process ensures fertilization efficiency in fluctuating environmental moisture levels. The osmotic influx of water, facilitated by the pollen's hygroscopic components, maintains the rapid tip extension required for successful pollination. In ferns, hygroscopic movements in fertile fronds assist spore dispersal by responding to humidity fluctuations. Dead fronds of species like the sensitive fern () open leaflets in dry conditions to release spores and close them in wet weather, driven by differential hygroscopic expansion and contraction of cell walls; this timing maximizes dispersal distance via wind while protecting spores from excess moisture. Arthropods employ hygroscopic cuticles to maintain water balance in low-humidity environments. The chitin-based exoskeleton absorbs atmospheric water vapor through hygroscopic salts and proteins in the epicuticle, allowing species like ticks and insects to absorb atmospheric water vapor and regain moisture without drinking; this mechanism is vital for survival in deserts, where cuticular uptake prevents desiccation. Hygroscopic cellulose fibers enable adaptive propagation in structures like pine cone scales. In dry conditions, the sclerenchyma fibers contract, causing scales to open and release seeds for dispersal, while hydration leads to swelling that closes the scales to shield undeveloped seeds from rain; this reversible movement, powered by anisotropic cellulose alignment, enhances reproductive success in variable climates. Fungal spore release often involves hygroscopic responses to humidity for passive dispersal. In many basidiomycetes, spore capsules or appendages swell or shrink with moisture changes, ejecting spores when relative humidity drops below 90%, as the hygroscopic matrix dehydrates and builds tension for launch; this ensures spores are released during optimal dry conditions for airborne travel. Resurrection plants like Selaginella lepidophylla revive from extreme desiccation through hygroscopic rehydration of leaf tissues. Upon water availability, leaf proteins such as late embryogenesis abundant (LEA) types facilitate rapid moisture absorption, restoring cellular turgor and metabolic activity within hours; the hygroscopic cell wall gradients enable the unrolling of curled fronds, a process that protects against dehydration damage during dormancy.

Materials Science and Engineering

Hygroscopic Properties of Materials

Hygroscopic properties in non-biological materials refer to the capacity of these substances to attract and retain water molecules from the surrounding environment, often leading to changes in physical structure, stability, or functionality. This behavior arises primarily from intermolecular forces such as hydrogen bonding, ionic interactions, and polar attractions, which enable materials to achieve equilibrium with ambient humidity levels. In materials science, understanding these properties is crucial for predicting performance under varying environmental conditions, as moisture uptake can alter mechanical strength, thermal characteristics, and phase states without involving biological processes. Ionic compounds exhibit pronounced hygroscopicity due to the strong electrostatic interactions between their ions and water dipoles, driven by high hydration energies. For instance, lithium chloride (LiCl) is extremely hygroscopic, readily absorbing water vapor to form hydrates, which is attributed to the large negative hydration enthalpy of the Li⁺ ion, approximately -520 kJ/mol. This energetic favorability allows LiCl to deliquesce in humid air, dissolving into a liquid phase as water coordinates around the small, highly polarizing Li⁺ cation. Similar behavior is observed in other alkali metal halides, where smaller cations like Li⁺ and Na⁺ promote greater water affinity compared to larger ones, influencing applications in desiccants and chemical processing. Organic materials such as sugars and starches demonstrate hygroscopicity through their polar hydroxyl groups, which facilitate hydrogen bonding with water, often resulting in transitions to amorphous states. Glucose, for example, in its amorphous form, absorbs moisture rapidly, leading to increased stickiness as water plasticizes the structure and reduces intermolecular forces between sugar molecules. This uptake can cause the material to form a viscous, cohesive matrix at relative humidities above 60%, where water molecules disrupt the glassy state and promote aggregation. Starches similarly exhibit this property, with moisture ingress into their amorphous regions causing swelling and a shift from a rigid, glassy phase to a rubbery one, enhancing flexibility but risking structural instability. These changes are particularly evident in processed starches, where hydration levels exceeding 10-15% lead to gel-like behaviors due to the disruption of crystalline domains. In polymers, hygroscopicity manifests as dimensional swelling and alterations in thermal transitions, primarily because water acts as a plasticizer by forming hydrogen bonds with polar chain segments. Nylon, a polyamide, absorbs up to 8% water by weight at equilibrium in humid conditions, resulting in volumetric expansion of 1-2% and reduced tensile modulus. This moisture-induced plasticization lowers the glass transition temperature (Tg), with reports indicating a decrease of approximately 20°C or more for saturated conditions, or roughly 1-2°C per 1% water content, shifting the polymer from a brittle to a more ductile state below ambient temperatures. Such shifts affect the material's viscoelastic properties, making nylons sensitive to environmental humidity in structural uses. Hygroscopic properties are quantitatively assessed using techniques like dynamic vapor sorption (DVS), which measures mass changes as a function of relative humidity to generate sorption isotherms. DVS exposes samples to controlled vapor environments, revealing uptake kinetics and equilibrium capacities with microgram sensitivity, essential for characterizing isotherm types such as Type II for non-porous materials. For natural polymers like wood, equilibrium moisture content (EMC) curves derived from such methods show values of 9-10% at 50% relative humidity and 20°C, reflecting the balance between bound and free water in cell walls. These isotherms guide material selection by quantifying hygroscopic limits without delving into behavioral classifications.

Engineering Applications

In structural engineering, hygroscopic materials such as are widely employed as desiccants to manage moisture levels in buildings, preventing issues like mold growth and material degradation by adsorbing water vapor from the air. These desiccants are integrated into systems like desiccant wheels or packets placed in HVAC units and storage areas to maintain indoor relative humidity below 50%, thereby enhancing occupant comfort and extending the lifespan of building components. However, hygroscopic expansion in , particularly through alkali-silica reaction (ASR), poses significant challenges, where reactive silica in aggregates forms a hygroscopic gel that absorbs moisture and swells, generating internal pressures up to 8 MPa and leading to cracking and structural deterioration. This expansion can reduce concrete's compressive strength by 20-50% over time, necessitating design strategies like low-alkali cement or moisture barriers to mitigate risks in humid environments. In electronics engineering, hygroscopic polymers are utilized in capacitive humidity sensors, where water vapor absorption alters the polymer's dielectric constant, causing measurable changes in capacitance—typically increasing by 2-10 pF per 10% relative humidity rise—to enable precise monitoring in devices like smartphones and industrial controls. These sensors operate reliably across 0-100% relative humidity with response times under 1 minute, supporting applications in automated climate control systems. Conversely, moisture ingress remains a critical challenge in microelectronics packaging, where absorbed water can accelerate corrosion of interconnects and delamination, potentially reducing device reliability by factors of 10 or more under high-humidity exposure; to counter this, JEDEC standards (J-STD-033D) mandate storage of moisture-sensitive devices at <5% relative humidity using desiccants or dry cabinets to limit absorption to <0.1% by weight. Compliance with these guidelines ensures failure rates below 1% in reflow soldering processes for advanced chips. Environmental engineering leverages hygroscopic seeding in cloud physics to enhance rainfall, dispersing highly hygroscopic salts like or into warm clouds to promote droplet coalescence and precipitation efficiency, often increasing rainfall by 10-30% in targeted areas. While (AgI) is more commonly associated with glaciogenic seeding in cold clouds due to its ice-nucleating properties, certain hygroscopic formulations incorporating AgI particles have been tested to augment convective rain enhancement by attracting water vapor and accelerating hydrometeor growth. These techniques, deployed via aircraft or ground generators, require precise timing to seed clouds at -5°C to 20°C, optimizing water resource management in arid regions without significant ecological disruption when dosages are limited to 100-500 g per operation. Performance metrics for hygroscopic applications in dehumidifiers highlight the efficiency of desiccant wheels, which achieve dehumidification capacities of 2-5 g/kg of air at regeneration temperatures around 80°C, with moisture removal effectiveness reaching 80-90% under optimal airflow rates of 200-500 m³/h. At this temperature, energy consumption for regeneration is minimized to 1.5-2.5 kWh per kg of water removed, outperforming mechanical dehumidifiers by 20-40% in humid climates, as the process relies on low-grade heat sources like solar or waste heat for sustainable operation.

Industrial and Practical Applications

Applications in Food Science and Baking

In food preservation, hygroscopic salts such as (NaCl) play a critical role in curing meats by absorbing moisture and reducing water activity (a_w) to levels below 0.85, which inhibits microbial growth and extends shelf life. This process creates an osmotic environment that dehydrates microbial cells, preventing pathogens like Staphylococcus aureus and spoilage bacteria from proliferating, as a_w values under 0.85 are generally insufficient for their survival. For instance, in dry-cured sausages, NaCl regulates a_w as the primary hurdle against microbial contamination during storage. In baking, the hygroscopic nature of sugars influences dough handling and final product texture; high sugar content increases dough stickiness by enhancing water absorption during mixing, which can complicate processing but contributes to tenderness. Sugars also promote crispiness in baked goods like cookies and crusts by facilitating dough spreading, Maillard reactions for browning, and controlled dehydration to achieve crunch. Post-baking, hygroscopic components drive moisture migration within the product, leading to staling through amylopectin recrystallization in starch, where water redistributes from the crumb to the crust, firming the interior over time. Humectants like glycerol are employed in food science to control texture by leveraging their strong hygroscopic properties, which bind water and maintain softness in low-moisture products such as candies and confections. In candies, glycerol prevents hardening by retaining equilibrium moisture, ensuring a pliable chew even at low humidity levels. Similarly, in baked goods like bread, the hygroscopicity of starch leads to moisture redistribution and gradual loss, accelerating staling and firmness unless mitigated by humectants. A key challenge in powdered foods, such as spices or milk powders, arises from hygroscopicity causing caking at high relative humidity (RH >60%), where absorbed moisture forms liquid bridges between particles, leading to clumping and reduced flowability. This phenomenon is particularly problematic in hygroscopic ingredients like sugars or salts, resulting in product spoilage and processing inefficiencies if storage conditions exceed critical RH thresholds.

Uses in Pharmaceuticals and Other Industries

In pharmaceuticals, hygroscopicity plays a critical role in drug stability, as moisture absorption can induce degradation in solid dosage forms such as tablets. For instance, aspirin undergoes hydrolysis in the presence of atmospheric moisture, converting to salicylic acid and acetic acid, which compromises its efficacy and leads to altered physicochemical properties. This degradation is exacerbated in hygroscopic active pharmaceutical ingredients (APIs), where exposure to ambient humidity can lead to significant potency loss due to increased water content and altered assay results. To mitigate these issues, low-hygroscopic excipients like are incorporated into formulations to enable controlled drug release while managing moisture interactions. , a , serves as a in direct compression tablets and coatings, such as mannitol-coated hydroxypropyl methylcellulose (HPMC), which protects moisture-sensitive drugs and sustains release profiles over time. Formulation strategies further address hygroscopic challenges, including the application of film coatings to prevent deliquescence and moisture ingress into tablet cores. These barrier coatings, often polymer-based, reduce rates and maintain product integrity during storage. Stability testing follows International Council for Harmonisation (ICH) guidelines, evaluating drug products under accelerated conditions of 40°C and 75% relative humidity (RH) for six months to predict and ensure compliance with degradation limits. Beyond pharmaceuticals, hygroscopicity benefits other industries, particularly textiles, where fibers like absorb 8-10% moisture under normal atmospheric conditions, enhancing wearer comfort by wicking away and regulating against the skin. In agriculture, hygroscopic fertilizers such as those containing provide readily available magnesium and , with controlled-release formulations reducing leaching and improving efficiency in water-retentive environments.

Recent Advances and Research

Innovations in Hygroscopic Materials

Recent innovations in hygroscopic materials have focused on sustainable, high-performance composites derived from renewable resources, particularly post-2020 developments aimed at enhancing moisture sorption capacities for environmental and industrial needs. One notable advancement involves wood-derived integrated with hygroscopic salts to create lightweight aerogels capable of serving as high-capacity moisture sorbents. These materials leverage the porous structure of nanocellulose to immobilize salts like , enabling high water uptake under ambient conditions, which outperforms traditional desiccants while maintaining structural integrity during repeated adsorption-desorption cycles. In the realm of metal-organic frameworks (MOFs), the incorporation of hygroscopic salts into structures like UiO-66 has enabled selective adsorption with improved efficiency at moderate levels. For instance, salt-modified UiO-66 composites demonstrate water uptake of over 200% at high relative , attributed to the synergistic effect of the MOF's microporous framework confining deliquescent salts to prevent leakage while facilitating rapid vapor and uptake. This design allows for tunable adsorption isotherms, making these materials suitable for targeted humidity control without excessive input for regeneration. Advancements in polymeric systems include smart hygroscopic hydrogels designed for sensing applications, where responsive swelling enables real-time humidity detection. These hydrogels exhibit high responsiveness to moisture changes, allowing precise volumetric expansions that transduce environmental stimuli into measurable signals for sensors. This high responsiveness, combined with and flexibility, positions them as key enablers for next-generation wearable and devices.

Emerging Technologies and Future Directions

Recent advancements in atmospheric water harvesting leverage hygroscopic agents such as metal-organic frameworks (MOFs) and salts to extract potable from air without , particularly in arid environments with relative below 30%. A comprehensive review highlights that MOF-801 and similar structures enable water production of approximately 2.8 L/kg under solar-driven cycles in low- conditions, demonstrating scalability through modular designs. These systems operate via adsorption-desorption mechanisms, where hygroscopic materials capture at night and release it during daytime heating, offering a sustainable alternative to traditional sources in water-scarce regions. In cooling technologies, 2025 developments introduce hygroscopic fabrics that enhance through accelerated sweat evaporation, improving in hot climates. Advanced textiles incorporating hygroscopic polymers and absorb rapidly, promoting evaporative cooling that can lower skin temperature by 2-5°C without external power. Complementing this, sorption-based systems utilize hygroscopic sorbents like silica gels or desiccants to dehumidify air, achieving up to 50% savings compared to conventional vapor-compression units by minimizing loads in humid conditions. These innovations reduce reliance on electricity-intensive compressors, aligning with global efforts to lower building . Hygroscopic are transforming sensors and , with 2025 prototypes enabling ultrafast detection for (IoT) applications in smart agriculture and . Graphene oxide-based sensors, for instance, exhibit response times under 1 second to changes, offering high sensitivity (up to 100% impedance variation per 10% RH) and low power needs suitable for wireless networks. This rapid detection supports real-time data collection, enhancing precision in climate-controlled systems. Looking ahead, future directions emphasize bio-inspired hygroscopic designs. Research explores materials that swell or contract with , potentially revolutionizing adaptive structures in and wearables. However, challenges persist in , where large-scale production of MOFs and gels faces cost barriers exceeding $10/kg, and toxicity concerns from metal ions in some sorbents require eco-friendly alternatives like bio-based desiccants. By 2030, experts predict widespread adoption in climate strategies, including decentralized and cooling solutions that could mitigate impacts for over 2 billion people in arid zones.

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