hygroscopy

Hygroscopy is the ability of a substance to attract and retain water molecules from the surrounding air. The term derives from the Greek hygros, damp, and skopein, to observe. Materials with hygroscopic properties interact with atmospheric moisture by both adsorption on surfaces and absorption into their structure.

The amount of water a material absorbs depends on relative humidity, temperature, and the material’s chemical composition and porosity. Hygroscopic materials may reach an equilibrium moisture content at a given humidity, described by sorption isotherms. Some hygroscopic substances are deliquescent, meaning they absorb so much moisture that they dissolve.

Common hygroscopic substances include silica gel and other desiccants, calcium chloride, salts, sugars, starches, and many biological materials such as wood, paper, cotton, wool, and other textiles. Non-deliquescent hygroscopic materials absorb moisture without dissolving, while deliquescent materials dissolve in the absorbed water.

Applications rely on moisture control and sensing. Desiccants used in packaging and electronics protect against humidity; humidity-control devices and sensors monitor ambient moisture levels; in art conservation and archaeology, controlling environmental moisture helps prevent damage from mould and salt crystallization.

In nature, moisture-related changes can drive seed dispersal and other plant processes. Studying hygroscopicity informs materials science, polymer design, and food and pharmaceutical storage.