Anomalous stability, relationship

The RH dependency of the first-order rate constant of aspirin decomposition in the CPG 170 mixture at 50°C is illustrated in Fig. 10. An anomalous linear relationship was obtained between the RH and the rate constants as the RH was raised, the rate constant decreased. Leeson and Mattocks [21] proposed the mechanism of aspirin decomposition in the solid state as follows. After the formation of a water layer on aspirin particles, aspirin was dissolved into the water layer. The aspirin then decomposed in the solution. In the crystalline state, the amount of adsorbed water increased at a high RH therefore, faster aspirin decomposition was observed. On the other hand, the stability of aspirin in the solid dispersed system was affected by many factors such as hygroscopicity of additives, pH on the surface, specific surface area, and dispersed state. El-Banna et al. [22] reported that the aspirin in the coprecipitated samples with povidone or urea had slightly higher degradation rates due to its increased water sorption ability. In the case of a urea solid dispersion. 

Ionic radius. The wide variation of metal-oxygen distances within individual coordination sites and between different sites in crystal structures of silicate minerals warns against too literal use of the radius of a cation, derived from interatomic distances in simple structures. Relationships between cation radius and phenocryst/glass distribution coefficients for trace elements are often anomalous for transition metal ions (Cr3+, V3+, Ni2+), which may be attributed to the influence of crystal field stabilization energies. 

As pointed out by Mirrless et al. 78), a major limitation of the HPLC technique as compared to the shaking-flask method has to do with the behavior of sparingly soluble compounds (high A ow) which tend to give anomalously low log A ow values. This phenomenon results presumably from a slow equilibration of these compounds in the colunm phases so that a true equilibrium is not reached at elution breakthrough. To overcome part of this difficulty, Veith et al. 104) used a solvent mixture of water/methanol (15/85, v/v) as the mobile phase with octadecylsilane as the stationary phase, which was permanently bonded to a support (LiChrosorb). They found a reasonably good linear relationship between log RT (retention time) and log K over 6 orders of magnitude in K. This technique does not involve tedious column preparation and the column has better stability. With proper choice of standard compounds, the accuracy of the predicted log K values was estimated to be within 22.8 20.0%. 

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