AN INTERPRETATION OF CONCENTRATION GRADIENTS DEVELOPED DURING DISSOLUTION (META-ACETOTOLUDIDE)
Abstract
The dissolution of meta-acetotoludide from fused discs was studied using a column dissolution model. This model allowed the study of dissolution under both ascending and descending conditions. Two significant conclusions, based on the rate of mass transfer, were obvious from the experimental results obtained. The dissolution of a solid has long been proposed to be a diffusion controlled process, yet in no case have the observed results been compared with theoretical values predicted by Fick's second law of diffusion. The excellent correlation of the theoretical and experimental values for dissolution and diffusion in the ascending model are proof of the diffusion control of this dissolution model. The descending model produced mass transfer rates far in excess of any that could be predicted by diffusion alone and which must result from gravitational convection. A solute front moving from the solid-liquid interface was clearly indicated in the descending dissolution studies. In addition, an effective interfacial concentration was developed during the descending process. This interfacial concentration appeared to be in equilibrium with the solid-liquid interface and was interpreted as representing the effective concentration available to the bulk solution at this interface. This observed interfacial concentration was considerably less than saturation. Thus, it was predicted that the rate of convective mass transfer was fast and that dissolution was controlled by the rate of solvation at this interface. It was concluded that diffusion control of the dissolution of a solid can be obtained only under specific conditions in which external foces, such as convection, are not present.
Degree
Ph.D.
Subject Area
Pharmaceuticals
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