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Effect of Micelles on Ostwald Ripening In Emulsions of Various Size Distributions

Suwimon Ariyaprakai, Food Science and Technology, University of California, Davis, Davis, CA 95616 and Stephanie R. Dungan, Food Science & Technology, Chemical Engineering and Materials Science, University of California, Davis, One Shields Ave., Davis, CA 95616.

In Ostwald ripening, sparingly soluble oils migrate from smaller droplets into larger, driven by the difference in chemical potential created by the Laplace pressures at drops of different sizes. In this study, we experimentally monitored Ostwald ripening in decane-in-water and dodecane-in-water emulsions with various size distributions: polydisperse, nearly monodisperse unimodal, and bimodal. Sodium dodecyl sulfate (SDS) or nonionic ethylene oxide surfactants of varying structure were used to stabilize the emulsion. The unimodal and bimodal emulsions were formed using the method of Bibette (1991), in which a polydisperse emulsion is fractionated to form more narrowly distributed droplet sizes. We observed Ostwald ripening in the monodisperse emulsions of small droplet sizes, but no ripening in the larger-sized unimodal system. Ostwald ripening in bimodal emulsions, formed from two nearly monodisperse emulsions of different mean sizes, clearly showed that oil transferred from the small mode to the large. We also examined the influence of micelles within the continuous phase on ripening in the emulsions of different distribution types. We found that the presence of micelles generally had a weak though perceptible influence on ripening rates, except for the large nonionic surfactant Brij 35 (C12E23), which dramatically accelerated droplet coarsening.