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Tablets & Capsules September 2020 33 RH greater than 50 percent (at 25°C). This structural change reduces the material's hygroscopicity, modifying the sorption isotherm while it is being measured. The initial material (partial amorphous lactose) and the crys- tallized material will have different sorption behavior as well as material properties. Measuring DVS sorption iso- therms can therefore be very useful to reveal if a product is changing upon exposure to high RH. If the original structure is important for functionality, you can identify a critical humidity value. Predicting the water activity of mixtures Water activity will always try to reach equilibrium within a system. Water will migrate from regions with high A w to regions of low A w for all system com- ponents, including the ambient atmo- sphere. However, you cannot predict the amount of water migration by looking at water activities alone. It also depends on each component's ability to bind water in its structure. Understanding moisture migration in mixtures requires an understanding of the components' sorption isotherms at the temperature of interest. Sorption isotherms are unique for each material Reversibility of moisture sorption isotherms A material's sorption behavior depends on its chemical composition, physical-chemical state, and physical struc- ture and is influenced by many factors, including capil- lary forces, surface, and colligative effects. Some materials have different adsorption and desorp- tion profiles, which is called hysteresis. A hysteresis often exists with desorption giving higher equilibrium moisture content than sorption. A hysteresis can be the result of different interactions and forces during sorption or desorption, as with capillary condensation [17]. Cellulosic material, for example, often shows a hysteresis that may be related to the mechanics of shrinkage and swelling. Cellulose swells to form water-cellulose bonds upon adsorption, but these bonds do not break upon desorption at the same chemical potential [18]. Another change in sorption behav- ior occurs when structural modification happens because of exposure to a rela- tive humidity. In this case, the materi- al's structure changes during a dynamic vapor sorption measurement. Amorphous lactose, for example, is crystallized to lactose monohydrate upon exposure to YOU ASK WE DELIVER! MiniPress ST The solution to your SOLID DOSAGE NEEDS 2420 Emrick Blvd. | Bethlehem, PA 18020 | 908-534-1500 www.smitmc.com P The New MiniPress ST P Configured for TSM/EU B&D Tooling P Advanced Data Acquisition & Analysis System P Excellence in Quality & Service Understanding moisture migration in mixtures requires an understanding of the components' sorption isotherms at the temperature of interest.