Release kinetics of sparingly soluble drugs from ethyl cellulose-walled microcapsules: salicylamide microcapsules

Release rates of salicylamide from single-core ethyl cellulose (EC) coated microcapsules were measured as a function of wall thickness and core particle size. Whereas up to ca 50% release zero order kinetics were observed, the overall reaction fitted the first order and Higuchi matrix treatment. These were distinguished by the differential rate treatment, which showed that the overall release in fact followed the first order pattern. For investigating whether the process was membrane-controlled, the experimental rate constants were transformed into effective permeability constants (P0 and P1) with the aid of the microcapsule dimensional parameters needed in the relevant equations and compared with the salicylamide permeability constant for planar ethyl cellulose membranes (P), measured experimentally. P0 and P1 values obtained for a given microcapsule preparation were not identical: P0 was of the same order as P, P1 being much lower. While membrane-controlled release is evident, it is apparently accompanied by a first order concentration gradient change inside the microcapsule.