Dissolution rate control of the release kinetics of water-soluble compounds from ethyl cellulose film-type microcapsules

Abstract

Microcapsules of sodium salicylate and potassium dichromate were prepared by the temperature reduction ethyl cellulose coacervation method using polyisobutylene as a protective colloid. Film-coated microcapsules were obtained. Minute empty wall polymer spheres were also formed in amounts increasing with protective colloid concentration or core particle size and decreasing with ethylcellulose concentration, in conformity with the mechanism previously proposed. Neither matrix diffusion nor first-order kinetics was observed by sodium salicylate experimental release data. Since small pores not initially present were formed, a dissolution model was applied; release conformed with the Hixson-Crowell cube-root law and was influenced by agitation rate in the sink solution. The effective diffusion coefficients from dissolution rate constants were near to the theoretical water value but several orders higher than the measured permeability constant through ethylcellulose cast films. In support of a hypothesis that pore-formation was caused by the high “internal osmotic pressure developed by water-soluble core material”, the release rate fell on increasing external osmotic pressure, which ultimately prevented pore-formation at 7M LiCl. This is the first case in which a dissolution model has been shown applicable to film-coated core materials.