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.

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.

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

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.