WATER-VAPOR DIFFUSION IN MODEL TABLET SYSTEMS .1. DESIGN OF A DIFFUSION APPARATUS

A diffusion apparatus was constructed to investigate the diffusion of water vapor through model tablet systems. The apparatus consisted of t hree main components; a humidification system; a flow-through diffusio n cell; and a modified gas chromatographic system which served as the detector. The system was qualified using compressed high density polye thylene powder as an ideal non-reactive porous barrier. Results from p reliminary investigations suggested that the donor and receptor compar tments of the diffusion cell were well mixed. Results from studies exa mining the effect of surface area, compact thickness, water vapor conc entration in the donor compartment and particle size of the HDPE powde r were consistent with a model of Fickian Diffusion through a porous b arrier. Subsequently, the pore structure of the HDPE samples was deter mined using mercury intrusion porosimetry. The pore size distribution of the samples was in the range where normal gaseous diffusion was exp ected. Thus, the apparent diffusivity of water vapor through the HDPE samples was controlled exclusively by sample porosity and was independ ent of pore size.