Water Adsorption on Hydrophilic Fibers and Porous and Deliquescent Materials: Cellulose, Polysaccharide, Silica, Inorganic Salt, Sugar Alcohol, and Amino Acid.

Water adsorption isotherms are systematically summarized by using celluloses and polysaccharides as hydrophilic crystal/amorphous materials with functional groups, silicas as hydrophilic porous materials, and inorganic salts, sugar alcohols, and amino acids as hygroscopic deliquescent materials. For hydrophilic fibers such as celluloses and polysaccharides, water was adsorbed on amorphous solids, and water clusters were formed around functional groups. For porous materials such as silicas, capillary condensation occurred in the micropores of silicas. For deliquescent materials such as inorganic salts, sugar alcohols, and amino acids, water adsorption rapidly increased stepwise at a specific threshold relative humidity, accompanied with a structure transformation to a liquid state. In addition, the water activity (Aw) of materials used in packed products was able to be estimated from the water adsorption isotherms of the pure component. This indicated that the deliquescent materials have a great effect on the depression of Aw for the suppression of microbial growth at an extremely high water content. The deliquescent materials could be useful to develop new environmentally and sustainable products and technologies with the mediation of water vapor and/or hydration.