Basic steps in the drug dissolution mechanism

Although the diffusion layer model is the most commonly used, various alterations have been proposed. The current views of the diffusion layer model are based on the so-called effective diffusion boundary layer, the structure of which is heavily dependent on the hydrodynamic conditions, fn this context, Levich [102] developed the convection-diffusion theory and showed that the transfer of the solid to the solution is controlled by a combination of liquid flow and diffusion. In other words, both diffusion and convection contribute to the transfer of drug from the solid surface into the bulk solution, ft should be emphasized that this observation applies even under moderate conditions of stirring. 

Noyes and Whitney published [103] in 1897 the first quantitative study of a dissolution process. Using water as a dissolution medium, they rotated cylinders of benzoic acid and lead chloride and analyzed the resulting solutions at various time points. They found that the rate c(t) of change of concentration c (t) of dissolved species was proportional to the difference between the saturation solubility cs of the species and the concentration existing at any time t. Using 

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Figure 5.1 The basic steps in the drug dissolution mechanism. (1) The molecules (o) of solvent and/or the components of the dissolution medium are moving toward the interface (2) adsorption-reaction takes place at the liquid-solid interface (3) the dissolved drug molecules ( ) move toward the bulk solution.

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