Hygroskopicity

Hygroskopicity, also spelled hygroscopicity, is the tendency of a material to attract and absorb moisture from the surrounding environment. This property depends on the chemical nature of the material and its physical form. Substances with high hygroskopicity may gain noticeable amounts of water at ordinary ambient humidity, causing changes in mass, volume, texture, and other physical or chemical properties.

Mechanisms and factors: Water uptake is driven by interactions between water molecules and the material, such as hydrogen bonding with polar groups or capillary condensation in porous structures. Factors that influence hygroskopicity include chemical composition (presence of hydroxyl, carboxyl, or amine groups), porosity and surface area, particle size, temperature, and ambient relative humidity. Some materials can reach a point where they dissolve in the absorbed moisture, a process related to deliquescence.

Measurement and implications: Hygroscopicity is typically quantified with moisture sorption isotherms or gravimetric methods, such as dynamic vapor sorption. Understanding hygroskopicity is important for pharmaceuticals, foods, polymers, cement, and electronics, since water uptake can cause clumping, dissolution, hydrolysis, swelling, or degradation, and can alter mechanical, thermal, or optical properties. Storage and packaging decisions often rely on desiccants and humidity control to manage moisture exposure.

Examples and applications: Common hygroscopic substances include certain salts like calcium chloride, organic compounds such as glucose and glycerol, and many polymers. In materials science and product design, controlling hygroskopicity helps improve shelf life, reliability, and performance in humid environments.