THE FORMATION OF SUBMICRON ORGANIC PARTICLES BY PRECIPITATION IN AN EMULSION

Submicron organic particles are produced by precipitation in an emulsi on. The poorly water soluble organic substance is dissolved in a non-p olar solvent. This solution is dispersed in an aqueous phase in the pr esence of emulsifier. When the non-polar solvent is removed by evapora tion, the organic substance precipitates and one particle is formed in each emulsion droplet. Experimental work are here reported on the rat e and the influence of the evaporation rate on the formation of choles teryl acetate particles. An introductory theoretical analysis of the c onditions experienced by each droplet during the evaporation is given. The experimental results suggest that the evaporation of the solvent is a rapid process mainly determined by the transport of the droplets to the vicinity of the air water interface. However, the results also show that the size of the particles formed by precipitation in the emu lsion, is insensitive to the evaporation conditions, for instance if t he evaporation is artificially slow. The particle size is mainly gover ned by the size of the emulsion droplets. The analysis discuss the pos sibility that during the evaporation of the emulsion, each individual droplet is emptied of its content of the non-polar solvent very rapidl y. The hypothesis that each solid particle is formed during a very sho rt period of time without obtaining a concentration equilibrium within the droplet is tested. However, the order-of-magnitude estimations do not support a kinetic explanation for the fact that the particles obt ain the size of the emulsion droplets.