Water sorption is greatly affected by the crystallinity of the material . For crystalline solids, water adsorbs on the surface as monolayers and multilayers (up to 3 -6 layers) on a crystal surface . In a study using Stöber silica spheres, it was estimated about 4 layers of water can adsorb on the surfaces at 25℃ and p/p0 = 0.89 . Water also penetrates pore structures and causes swelling, likely due to interactions with hydroxyl groups at the interface . The swelling, in turn, opens new sites for water sorption, including increasing the accessibility to micropores 2 nm or less in size .