Specific surface area measurement of ethyl cellulose-walled microcapsules containing theophylline

The choice of microencapsulation system was limited by drug solubility and the possibility of its thermal decomposition at elevated temperatures. On the basis of the solubility study the toluene-petroleum ether system was found to be suitable. An accurate determination of specific surface area was obtained by gas adsorption. By use of the BET equation, the monolayer capacity of the microcapsules could be calculated. The results showed variations due to microcapsule size, petroleum ether fraction used in the preparation and the core to wall ratio. Dissolution from the microcapsules appeared to depend on a number of factors including the wall thickness, the amount of core material enclosed and the surface area available for diffusion.