Differential dissolution techniques

Many techniques have been used to characterize the physical nature of solid dispersions. These include thermal analysis (e.g., cooling-curve, thaw-melt, differential scanning calorimetry and X-ray diffraction, microscopic, spectroscopic, dissolution rate, and thermodynamic methods) Usually, a combination of two or more methods is required to obtain a complete picture of the solid dispersion system. 

When organic (drug) molecules crystallize from a solvent, the crystal structure is dependent upon the speed of crystallization, temperature, polarity of the solvent, concentration of the material, etc. Since the energy of the crystal affects the (physiological) rate of dissolution and thus the potency and activity of the drug, polymorphism is an important pharmaceutical concern [39]. The most common tool to determine crystal form is differential scanning calorimetry (DSC). Unfortunately, DSC uses small samples and may not represent the bulk of the material. X-ray diffraction is another excellent technique, but quite slow and sometimes difficult to interpret. 

Trace levels of free carbon, produced by pyrolysis of organic matter in the limestone, is of interest in a few applications. The technique described in [18.6] differentiates between combined carbon in carbonates and free carbon. Acid dissolution is used to decompose carbonates and the residue is heated in oxygen to oxidise free carbon. The carbon dioxide produced is determined using its absorption in the infra-red spectrum. 

Toi et al. applied the dual sorption mechanism to analysis of the time-lag diffusion (permeation) under the constraint that the penetrant fraction attributed as the Langmuir component is completely immobilized, but in local equilibrium with the Henry s law dissolution component [1]. They yielded a mathematical description of transient permeation, consisting of a nonlinear partial differential equation. This equation was then solved by a finite-difference technique for the case of permeation... 

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