Influence of Ostwald Ripening on Rheology of Oil-in-Water Emulsions Containing Electrostatically Stabilized Droplets

The influence of droplet size and Ostwald ripening on the dynamic shear rheology of 25 wt % hydrocarbon oil-in-water emulsions containing small droplets (75 to 100 nm) stabilized by sodium dodecyl sulfate (SDS) was measured. Emulsions underwent a liquid-to-solid transition when the mean droplet radius was decreased below ∼85 nm because of overlap of the electrical double layers surrounding the droplets. Below this critical radius the emulsions had a yield stress and shear modulus that increased with decreasing droplet radius. Ostwald ripening studies were carried out using emulsions that initially contained small n -hexadecane ( r initial = 72 nm) or n -octadecane droplets ( r initial = 65 nm) and therefore had solidlike characteristics. There was appreciable growth in droplet radius of the n- hexadecane emulsions due to Ostwald ripening, which led to a solid-to-liquid phase transition when the double layer thickness became less than the surface-to-surface droplet separation. On the other hand, the n -octadecane emulsions remained solidlike during the same period because n -octadecane has a much lower water solubility than n- hexadecane and therefore Ostwald ripening was much slower. Our data has important implications for the formulation of emulsion-based products that must have specific textural properties.