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 similar to 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.