Theories of Dissolution: The Danckwerts' Model and Interfacial Barrier Model

Danckwerts' model proposes that the dissolution medium exhibits turbulence at the solid-liquid interface rather than a stagnant layer. Macroscopic packets of agitated solvent move to the interface via eddy currents, absorbing and delivering the drug to the bulk solution. Regular replenishment of solvent packets maintains the concentration gradient, with drug transport to the bulk solution serving as the rate-limiting step.

The interfacial barrier model describes a drug particle as a crystal with distinct interfacial barriers on each face. Solvation at the interface creates an intermediate concentration, a solubility function. The dissolution rate is calculated per unit area based on the interfacial transport constant and concentrations in the static layer and bulk solution, with solubilization rate in the static layer as the rate-limiting step.

In Danckwerts' model, the rate-limiting step is the transport of the drug to the bulk solution. The model emphasizes that turbulent eddy currents continuously replenish solvent packets at the solid-liquid interface, maintaining the concentration gradient and controlling how quickly dissolved drug reaches the bulk solution.

In the interfacial barrier model, the rate-limiting step is the solubilization rate of the drug in the static layer. This differs from Danckwerts' model because the interfacial barrier model focuses on the solvation process at the crystal surface and the intermediate concentration established at the interface.

Eddy currents in Danckwerts' model transport macroscopic packets of agitated solvent to the solid-liquid interface. These currents facilitate the absorption and delivery of dissolved drug to the bulk solution while continuously replenishing the solvent at the interface, maintaining a dynamic concentration gradient essential for sustained dissolution.

The interfacial transport constant in the interfacial barrier model determines the dissolution rate per unit area. Combined with the concentrations in the static layer and bulk solution, this constant quantifies how efficiently the drug crosses the interfacial barrier and enters the static layer during the dissolution process.

Danckwerts' model emphasizes turbulent solvent transport and bulk solution delivery as the rate-limiting step, while the interfacial barrier model focuses on solvation at the crystal surface and solubilization in the static layer as the rate-limiting step. Both models explain dissolution through different mechanisms of drug transport and concentration gradients.