Modelling glass/water reactivity

The two over-arching models for glass/water reactivity... 

With continuous development of systems for controlled drug release, new materials are being used whose influence on peptide stability must be carefully examined. Thus, the model hexapeptide Val-Tyr-Pro-Asn-Gly-Ala (Fig. 6.30) embedded in poly (vinyl alcohol) and poly(vinyl pyrrolidone) matrices had rates of deamidation that increased with increasing water content or water activity, and, hence, with decreasing glass transition temperature (Tg). However, the degradation behavior in the two polymers differed so that chemical reactivity could not be predicted from water content, water activity, or T% alone. Furthermore, the hexapeptide was less stable in such hydrated polymeric matrices than in aqueous buffer or lyophilized polymer-free powders [132],... 

Michalske and Freiman introduced a model for subcritical crack growth in silicate glasses in which an originally unreactive Si-O bond becomes more reactive as it is deformed in the stress field at the crack tip. At some level of stress intensity, a water molecule adjacent to the bond is able to react with it and produce two silanols that is, it breaks the bond. At low K, the reactivity of the bond is low but it increases approximately logarithmically as K increases. The higher the reactivity, the more rapidly bonds break and the faster the crack grows. 

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