Hygroscopicity

Hygroscopicity is the tendency of a substance to attract and hold water molecules from the surrounding environment, through adsorption on surfaces or absorption into the bulk. The degree of hygroscopicity depends on chemical composition, molecular structure, and surface properties, and materials with strong hygroscopicity can take up moisture even at relatively low relative humidity.

Moisture uptake occurs by adsorption, where water molecules bind to surfaces via hydrogen bonding or ionic interactions, and by absorption, where water penetrates into the material. Factors that influence hygroscopicity include relative humidity, temperature, porosity, surface area, and the presence of soluble salts or highly polar groups. Deliquescent materials absorb moisture to the point that they dissolve, forming an aqueous solution, while non-deliquescent hygroscopic substances may simply swell, plasticize, or cake.

Measurement and modeling commonly use moisture sorption isotherms, which describe how moisture content at a fixed temperature varies with relative humidity. Techniques such as dynamic vapor sorption (DVS) provide experimental data. Equilibrium moisture content (EMC) is the moisture level at equilibrium. Isotherm models (for example BET or GAB) are used to fit data and predict behavior under different humidity conditions.

Applications and implications span pharmaceuticals, foods, and construction materials. In pharmaceuticals, hygroscopicity affects powder flow, stability, and storage requirements, often necessitating desiccants and moisture-controlled packaging. In foods, moisture uptake influences texture, caking, and shelf life. In materials science, moisture absorption can alter mechanical properties and dimensional stability, underscoring the importance of humidity control in product design and storage.

Related concepts include deliquescence, efflorescence, sorption, and water activity.