Hygroscopy

The word hygroscopy () 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==

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 {{ordered list|type=lower-alpha ==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 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 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 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 • 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. 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. 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)". described below. ] • 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 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. 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)). 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)." 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. 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. combined with dual sided shrinkage, results in opposing helical torques 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. 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°, • 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. ==Engineering properties==

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. 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. 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==

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. Type-6 nylon (a polyamide) can absorb up to 9.5% of its weight in moisture. ==Applications in baking==

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. ==Research==

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). 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. • 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. Materials composed of fluid-responsive active bilayer systems that can direct planned conformational hygromorphing are necessary. • Current composites require undesirable trade-offs between hygromorphic response time and mechanical stability that must also be balanced with changing environmental stimuli. ==See also==