3.21 : 溶解理论：扩散层模型

Dissolution, the process of drug particles dissolving in a solvent, is explained by the diffusion layer model.

According to this model, initially, a thin layer saturated with the drug forms at the solid-liquid interface. Next, the soluble solute diffuses from the stagnant layer into the bulk solution.

According to Noyes-Whitney's equation, the dissolution rate depends upon the stagnant layer's diffusion rate, surface area, concentration, and thickness. The driving force for dissolution is the concentration gradient between the stagnant layer and the bulk solution.

Experimentally, to establish a robust in vitro to in vivo dissolution rate correlation, in vitro testing is performed at 37 degrees Celsius at a constant stirring rate to mimic body temperature and peristaltic movements in the GI tract.

In addition, sink conditions are maintained to avoid drug build-up in the bulk solution by adsorbing the dissolved drugs, increasing the volume of the solution, using fresh solvent at regular intervals, or partitioning the drug from the aqueous medium using an organic phase.