On the use of the quasi-equilibrium assumption for drug dissolution

The quasi-equilibrium assumption is often used to simplify the analysis of reaction diffusion problems, including those that arise in drug dissolution and ionization processes. This approximation often makes the governing equations tractable, and analytical solutions may then be obtained. However, the application of the quasi-equilibrium assumption may lead to simplified solutions that (1) are apparently inconsistent with stated boundary conditions and (2) have a physical interpretation that is different from those of the original problem statement. Herein we discuss these two issues as they arise in the modeling of drug dissolution processes. In spite of the different conceptualizations, the concentration profiles and dissolution fluxes obtained from the full and approximate solutions converge as the reaction response times exceed those of diffusion, thus supporting the applicability of the quasi-equilibrium assumption for ionized drug dissolution processes.