Various dissolution theories provide insight into the factors that influence the dissolution rate. Danckwerts' Model suggests that turbulence, rather than a stagnant layer, characterizes the dissolution medium at the solid-liquid interface. In this model, the agitated solvent contains macroscopic packets that move to the interface via eddy currents, facilitating the absorption and delivery of the drug to the bulk solution. The regular replenishment of solvent packets maintains the concentration gradient, with the transport of the drug to the bulk solution serving as the rate-limiting step.
Another model, the Interfacial Barrier Model, describes a drug particle as a crystal with unique interfacial barriers on each face. Solvation at the interface, a process by which solvent molecules interact with and dissolve the drug particles, creates an intermediate concentration, which is a solubility function. The dissolution rate is then calculated per unit area based on the interfacial transport constant and the concentrations in the static layer and bulk solution. In this model, the rate-limiting step is the solubilization rate of the drug in the static layer.
These models, along with those described in previous lessons, underscore the complexity of the dissolution process and its impact on drug delivery.
