Diffusion Layer Effects on Permeation of Phenylbutazone through Polydimethylsiloxane

Abstract

The permeation of phenylbutazone through polydi-methylsiloxane was studied under conditions of varying membrane thickness, stirrer speed, and pH. The results are described in terms of the two-phase model of general transport theory and provide experimental evidence of its usefulness. It was found that permeation rates are controlled by the aqueous diffusion layer at the membrane surface, especially when permeation cells equipped with thin membranes are weakly stirred. Gentle agitation and thin membranes exist in the intestinal tract, so that drug absorption rates in vivo may be limited by a diffusion layer. Diffusion layer control may lead to a breakdown of the pH-partition theory because the ionic and nonionic forms of the drug diffuse through the aqueous diffusion layer. The diffusion of phenylbutazone in polydimethylsiloxane did not obey Fick's law of diffusion.