Veis (1970b) studied the complex formation of gelatins with different isoelectric points of pH 9 and 5 as a function of initial mixing concentration. At a temperature of 20°C, which is below the conformational transition temperature of approximately 25°C, the fraction of gelatin in the coacervation phase increases with increasing mixing concentration but at 30°C the fraction decreases with increasing initial mixing conditions.
A complex coacervate of two oppositely charged gelatins has been prepared by Burgess and Carless (1985). They noted the previous work on this coacervate by Veis and coworkers from 1960 to 1967. The optimal concentration occurred when equal volumes of 1 % deionized solutes of Types A and B were mixed together at 45°C, with stirring for 1 h. Subsequently, the temperature was reduced to 25°C for 4h, then a 16% formaldehyde solution was added to harden the walls, followed by cooling to 4-5 °C. After centrifugation and decanting the product was washed with water and isopropanol. The predicted optimum pH was 5.4, the electrical equivalence point, where the two gelatins have an equal and opposite charge. At this pH the electrophoretic mobility of the gelatins was low and was probably insufficient to effect coacervation. If the ionic strength is lowered, the electrophoretic mobility increases appreciably, promoting gelatin-gelatin coacervation. However, coacervation was not evident at 40°C and it was necessary to decrease the temperature to obtain complex flocculation. By controlled slow cooling, a more ordered gelation occurred, promoting coacervates with liquid, rather than flocculated properties. Concentrations of gelatin higher than 2.5% caused self suppression of the coacervation phase separation, likely due to the neutralization of charges, to form a large stable gel network. It was found that the shape of the droplets depended upon the final temperature. Higher temperatures (30°C) produced ellipsoid droplets, while at 15°C aggregation occurred. The authors suggest that the morphology of the droplets at 25°C is a result of the viscosity of the coacervate phase. The stirring forces may or may not be balanced by stabilizing forces within the droplet. Slower stirring speeds resulted in an increase in the droplet size and the fraction of amorphous droplets. The drug naproxen was encapsulated at a drug to colloid ratio of 1:5 at temperatures ranging from 5 to 30°C. The per cent drug encapsulated was highest at 25°C and the drug content of the microcapsule was higher when the microcapsules were produced at 30°C; however, the microcapsule yield was highest at 10°C.
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