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Visualization of the release behaviour of microcapsules

This paper describes an experimental technique that visualizes the release behaviour that microcapsules loaded initially with a halide salt experience when immersed in aqueous media. The technique, based on the principle of silver halide photography, involves observing the effect that contact with a dilute aqueous silver nitrate solution has on individual microcapsules. Rapid precipitation of an insoluble silver halide salt on a capsule surface provides a permanent record of the location(s) on a capsule surface at which halide ion release occurs. Results obtained by examining the behaviour of individual capsules selected from one KCl and three NaCl capsule samples produced by fluidized bed coating illustrate the wide variety of halide ion release behaviour exhibited by capsules from different capsule samples as well as capsules from the same sample. Scanning electron microscopy showed that halide ion release from all capsules studied does not occur uniformly over the capsule surface, but is limited to specific points on the capsule surface. Whereas most capsules from two NaCl samples broke open spontaneously after a short immersion time, thereby producing a large plume of AgCl precipitate, capsules from one NaCl and the KCl sample remained intact even after prolonged immersion. In these latter cases, a AgCl bolus typically grows on the surface of a capsule as immersion time increased.