Schematic representation of the dissolution mechanisms

Although it was not stated in the original article of Noyes and Whitney, it should be pointed out that the validity of the previous equation relies on the assumption that the amount used, qo, is greater than or equal to the amount required to saturate the dissolution medium, qs. Later on, (5.1) was modified [102,104] and expressed in terms of the dissolved amount of drug q(t) at time t while the effective surface area A of the solid was taken into account  

The integrated form of (5.2) gives the cumulative mass dissolved at time t  

The limit t — oo defines the total drug amount, qs = csV, that could be eventually dissolved in the volume V assuming that the amount used qo is greater than qs. Thus, we can define the accumulated fraction of the drug 

Equation (5.5) is the classical equation quoted in textbooks indicating the exponential increase of concentration c (t) approaching asymptotically the saturation solubility cs. 

If this applies then we consider that sink conditions exist. Under sink conditions the concentration c (t) increases linearly with time, 

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Figure 5.2 Schematic representation of the dissolution mechanisms according to (A) the diffusion layer model, and (B) the interfacial barrier model.

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