Sheorey and Dorle (1990) described a method for encapsulating sulfadiazine with rosin using benzene as the solvent and this was emulsified with stirring into an aqueous bentonite suspension. After evaporation, the microcapsules were filtered, washed and dried. It was found that as the core to coat ratio increased from 1:1 to 4:1 , the drug in the microencapsulated product increased from 50 to 91% and the mean diameter decreased from 924 ^m to 238 pim and the time for 50% of the drug to dissolve decreased from more than 180 min to 80 min in gastric fluid pH 1.2 and 50 min in intestinal fluid pH 7.6. It was found that bentonite, rather than ionic and non-ionic emulsifiers and other protective colloids such as gelatin or acacia, minimizes aggregration of microcapsules.
Sheorey and Dorle (1991a) again used the solvent evaporation method to encapsulate sulfadiazine with rosin. Ten different organic solvents with different rates of evaporation were used to dissolve the rosin, for example ether (fast), chloroform (medium) and petroleum ether (slow). Solvents with fast relative evaporation rates gave large microcapsules and lower drug content, and an increase in wall thickness and surfaces with many pores and fissures compared with solvents with slow relative evaporation rates. The authors suggest that quick evaporation of the polymer solvent causes rapid agglomeration of the visco-elastic polymer droplets and subsequent drying and rigidization occurs rapidly before a uniform coating of the drug occurs; this leaves a loosely deposited wall structure. Dissolution studies showed that while ether produced thick-walled microcapsules, the rate of release was rapid because of the surface characteristics. The influence of solid dispersing agents on the formation of rosin microcapsules was also investigated by Sheorey and Dorle (1991b).
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