Theories of Dissolution: Diffusion Layer Model

Dissolution, the process by which drug particles dissolve in a solvent, is explained by the diffusion layer model, a theoretical framework that simulates the absorption of oral drugs and allows us to analyze experimental data.

This process starts with a thin layer, saturated with the drug, forming at the interface between the solid and liquid. The solute then diffuses from this layer into the main solution. The Noyes-Whitney equation suggests that the rate of dissolution relies on the diffusion rate, surface area, concentration, and thickness of the stagnant layer. The concentration gradient between the stagnant layer and the bulk solution fuels the dissolution, with a steeper gradient typically resulting in a higher diffusion rate..

To create a strong correlation between in vitro (in the lab) and in vivo (in the body) dissolution rates, tests are conducted at 37^oC with a consistent stirring rate. These conditions replicate the body temperature and peristaltic movements in the gastrointestinal tract. Sink conditions ensure that the concentration of the drug remains low enough to drive the dissolution process. This can be done by adsorbing the dissolved drugs, increasing the solution volume, regularly using fresh solvent, or separating the drug from the water-based medium with an organic phase.