Koh and Tucker (1988a) characterized the sodium carboxymethylcellulose-gelatin complex by adding the sodium carboxymethylcellulose solution to the gelatin solution at 40° C with stirring and allowing it to stand for lOmin before evaluation. Maximum coacervation as determined from maximum deviation from additive viscosity occurred at pH 3.5 and 30% sodium carboxymethylcellulose. Around this pH, carboxymethylcellulose is negatively charged and gelatin is positively charged, resulting in strong electrostatic behaviour and hence, complex coacervation. At a pH range of 5.0-7.0 positive deviation from additive viscosity behaviour occurred, but coacervation did not occur. The change in viscosity and complex coacervation is explained in terms of ionization and the folding of the colloid. For example, at pH 5, the isoelectric point of gelatin and the anionic carboxymethylcellulose, a negatively charged soluble carboxymethylcellulose-gelatin complex forms and, as a result, electrostatic repulsion leads to unfolding of the complex and the viscosity shows a large positive deviation. The pH range for complex coacervation was found to be 2.5-4.5, as observed by turbidity measurements. The authors suggest that coacervate wet weights and volumes cannot be used to predict optimal coacervate conditions due to a change in coacervate morphology with mixing ratio.
In a second paper, Koh and Tucker (1988b) determined the chemical composition of the coacervate and equilibrium fluid phases of the sodium carboxymethylcellulose-gelatin coacervation complex. The coacervate batches were prepared at 0.75 and 2% total colloid concentration at pH values of 3.0, 3.5 and 4.0 and a range of sodium carboxymethylcellulose compositions of 10-60%. The colloid mixing ratio at which the peak coacervate yield occurred varied with the pH. Low viscosity and high viscosity grades of sodium carboxymethylcellulose gave similar results. Phase diagrams of the three components, water, gelatin and sodium carboxymethylcellulose at different pH values were prepared. Changes in the colloid composition of the complex coacervate and equilibrium fluids of isohydric mixtures as a function of the sodium carboxymethylcellulose mixing ratio were determined. The authors concluded that the sodium carboxymethylcellulose-gelatin complex coacervation is fundamentally the same as the gelatin-acacia system.
Microencapsulation of hydrophobic oils employing gelatin, carboxy-
methylcellulose and a second anionic colloid was accomplished by first preparing an emulsion at which coacervation does not occur. The mixture is then acidified to promote coacervation and formation of microcapsules. After chilling, the solid walls are treated with a cross-linking agent (North, 1989).
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