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 rhe ology of 25 wt% hydrocarbon oil-in-water emulsions containing small droplet s (75 to 100 nm) stabilized by sodium dodecyl sulfate (SDS) was measured. E mulsions underwent a liquid-to-solid transition when the mean droplet radiu s was decreased below similar to 85 nm because of overlap of the electrical double layers surrounding the droplets. Below this critical radius the emu lsions had a yield stress and shear modulus that increased with decreasing droplet radius. Ostwald ripening studies were carried out using emulsions t hat initially contained small n-hexadecane (r(initial) 72 nm) or n-octadeca ne droplets (r(initial) = 65 nm) and therefore had solidlike characteristic s. There was appreciable growth in droplet radius of the n-hexadecane emuls ions due to Ostwald ripening, which led to a solid-to-liquid phase transiti on 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 wate r solubility than n-hexadecane and therefore Ostwald ripening was much slow er. Our data has important implications for the formulation of emulsion-bas ed products that must have specific textural properties.