Water vapor diffusion in model tablet systems: I. Design of a diffusion apparatus

Abstract

A diffusion apparatus was constructed to investigate the diffusion of water vapor through model tablet systems. The apparatus consisted of three main components; a humidification system; a flow-through diffusion cell; and a modified gas Chromatographie system which served as the detector. The system was qualified using compressed high density polyethylene powder as an ideal non-reactive porous barrier. Results from preliminary investigations suggested that the donor and receptor compartments of the diffusion cell were well mixed. Results from studies examining the effect of surface area, compact thickness, water vapor concentration in the donor compartment and particle size of the HDPE powder were consistent with a model of Fickian Diffusion through a porous barrier. Subsequently, the pore structure of the HDPE samples was determined using mercury intrusion porosimetry. The pore size distribution of the samples was in the range where normal gaseous diffusion was expected. Thus, the apparent diffusivity of water vapor through the HDPE samples was controlled exclusively by sample porosity and was independent of pore size.