Aqueous Erosion Behavior

More interestingly, the results suggest also that for these new matrices a limit of solubility is attained stopping the material alteration, which is not the case for the classical nuclear waste glass for which the process of erosion never stops in water [42, 43]. The dissolution rate of amorphous silica and loaded glass-ceramics is still lower than that of nuclear glass in stagnant, silica-saturated solutions. 

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When a hydrophobic polymer with a physically dispersed acidic excipient is placed into an aqueous environment, water will diffuse into the polymer, dissolving the acidic excipient, and consequently the lowered pH will accelerate hydrolysis of the ortho ester bonds. The process is shown schematically in Fig. 6 (18). It is clear that the erosional behavior of the device will be determined by the relative movements of the hydration front Vj and that of the erosion front V2- If Vj > V2, the thickness of the reaction zone will gradually increase and at some point the matrix will be completely permeated with water, thus leading to an eventual bulk erosion process. On the other hand, if V2 = Vj, a surface erosion process wiU take place, and the rate of polymer erosion will be completely determined by the rate at which water intrudes into the matrix. 

When added to an aqueous solution simulating the environment of the small intestine, SDDs rapidly dissolve and/or disperse to produce a wide variety of species that facilitate absorption. To enhance the bioavailability of poorly soluble drugs, fundamental understanding of the drug species formed and the mechanism of action of SDDs is essential. Two general routes of HPMCAS SDD dissolution and drug speciation have been observed, which seem to bracket the behavior for most SDDs that have been studied. The two mechanisms of action— referred to as nanoparticle formation and erosion, respectively—are illustrated in Fig. 9.6a and 9.6b, respectively, and... 

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