Particle size analysis of gelatin-acacia coacervate and ethylcellulose walled microcapsules

The Coulter counter has become one of the methods of choice for the measurement of small particle sizes. However, many colloidal walled microcapsules are prone to hydration when dispersed in the electrolyte solution used in Coulter measurements. This hydration causes the walls to swell, producing different size analyses dependent upon the time of measurement. In the present work the change in size with time was studied for microcapsules with gelatin-acacia coacervate, or ethylcellulose, walls. The former, although rendered insoluble with formalin, still hydrated; the latter were almost unaffected by water. A sample size of 30 mg was required to prevent blocking of the orifice with larger samples, or too few particles to permit measurement in smaller samples. The coacervate coated microcapsules showed a bimodal distribution, in part due to aggregate of smaller microcapsules, and this distribution changed significantly over the time of measurement. Although the peak due to larger microcapsules did not disappear, the number of large microcapsules fell significantly, most probably because of the disintegration of aggregated microcapsules. Microcapsules with ethyl cellulose walls did not show a bimodal distribution and the particle size analysis did not alter significantly with time.