Water-solid interactions

Maintenance of constant relative humidity environments is essential for studying water-solid interactions. There are primarily four techniques that are frequently employed to maintain constant relative humidity ... 

Solids that form specific crystal hydrates sorb small amounts of water to their external surface below a characteristic relative humidity, when initially dried to an anhydrous state. Below this characteristic relative humidity, these materials behave similarly to nonhydrates. Once the characteristic relative humidity is attained, addition of more water to the system will not result in a further increase in relative humidity. Rather, this water will be sorbed so that the anhydrate crystal will be converted to the hydrate. The strength of the water-solid interaction depends on the level of hydrogen bonding possible within the lattice [21,38]. In some hydrates (e.g., caffeine and theophylline) where hydrogen bonding is relatively weak, water molecules can aid in hydrate stabilization primarily due to their space-filling role [21,38]. 

Other supportive evidence for a specific water-solid interaction is available from thermal studies showing the amount of nonfreezable water [57-59], nuclear magnetic resonance [29,60-66], and diffusion studies [67,68]. The evidence is less clear, however, concerning whether there is distinct binding of water to... 

In summary, it is clear that water absorbs into amorphous polymers to a significant extent. Interaction of water molecules with available sorption sites likely occurs via hydrogen bonding such that the mobility of the sorbed water is reduced and the thermodynamic state of this water is significantly altered relative to bulk water. Yet accessibility of the water to all potential sorption sites appears to be dependent on the previous history and physical-chemical properties of the solid. In this regard, the water-solid interaction in amorphous polymer systems is a dynamic relationship depending quite strongly on water activity and temperature. 

M. Bergren, Water-Solid Interactions, AAPS Short Course, Orlando, Fla. (1993). 

Zografi, G. Hancock, B.C. Water-solid interactions in pharmaceutical systems. Proceedings of the International Pharmaceutical Federation Conference, Tokyo, Japan, 1994, 405 19. 

A USP Advisory Panel on Moisture Specifications initiated a revision process with the inclusion of background material in the general chapter (1241) Water-Solid Interactions in Pharmaceutical Systems. However, this occurred over 10 years ago and it has not been followed by the inclusion of standard analytical tests that can be used to characterize the state of water. 

Bergren, M. Water-Solid Interactions AAPS Shortcourse Orlando, FL, 1993. 

Stubberud L, Arwidsson HG, Hjortsberg V, Graffner C. Water-solid interactions. Part 3. Effect of glass transition temperature, Tg and processing on tensile strength of compacts of lactose and lactose/polyvinyl pyrrolidone. Pharm Dev Technol 1996 1(2) 195-204. 

Probing Water-Solid Interactions in Crystalline and Amorphous Systems Using Vibrational Spectroscopy... 

In this presentation, two examples of the use of vibrational spectroscopy to probe water-solid interactions in materials of interest to the food and pharmaceutical sciences are described. First, the interaction of water vapor with hydrophilic amorphous polymers has been investigated. Second, water accessibility in hydrated crystalline versus amorphous sugars has been probed using deuterium exchange. In both of these studies, Raman spectroscopy was used as the method of choice. Raman spectroscopy is especially useful of these types of studies as it is possible to control the environment of the sample more easily than with infrared spectroscopy. 

Stubberrud L, Eriksson M, Kordnejad K, Graffner C. Water-solid interactions. IV. Influence of moisture sorption on the compaction of film-coated particles. Pharm Dev Technol 1998 3 141-151. 

---