(92e) Effect of Droplet Distribution, Concentration, and Surfactant Self-Assembly On Ostwald Ripening in Emulsions

In Ostwald ripening, sparingly soluble oils diffuse from smaller droplets into larger, driven by the difference in chemical potential created by the Laplace pressures at drops of different sizes. As researchers improve their ability to create emulsions with smaller droplet sizes (so-called nanoemulsions) and controlled distribution shapes, an understanding of the kinetics of Ostwald ripening becomes ever more important. In this study, we monitored Ostwald ripening experimentally in alkane-in-water emulsions with various size distributions: polydisperse lognormal, nearly monodisperse unimodal, and bimodal. We also examined the influence of the droplet volume fraction, including dilute emulsions of volume fractions of 2-10%, to highly concentrated systems with 70% droplet phase. The effects of droplet distribution shape and droplet concentration that we observe experimentally highlight limitations in the current theoretical understanding of Ostwald ripening kinetics, since the effects of these emulsion properties fall outside of the classic asymptotic theory by Lifshitz -Slyozov and Wagner (LSW theory). 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. Ostwald ripening rates were approximately a factor of ten more rapid in the highly concentrated emulsions compared to the dilute case. Straightforward extension of LSW theory also does not capture effects of excess surfactant that self-assembles in the continuous phase. We examined the influence of micelles within the continuous phase on ripening in the emulsions of different distribution types and concentrations. 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. We propose that the observed effects are attributable to the ability of micelles to become supersaturated with oils, which in turn depends on the bending energies of their surfactant monolayers.