Ostwald ripening in emulsions

Abstract

The rates of Ostwald ripening in decane O/W emulsions were measured as a function of sodium dodecylsulphate concentration and of droplet volume fraction. It was found that the presence of micelles had only a small but systematic effect upon the rate of ripening, contrary to expectation, where the presence of micelles might be expected to aid the process of ripening through solubilization of oil. This small effect was discussed in terms of Kabal'nov's treatment of the process. It was concluded that the micelles were not in true equilibrium with the material diffusing from the droplets undergoing Ostwald ripening, and that only a small fraction of the micelles could, at any time, solubilize material from the droplets. This would lead to the reduced effect of micellar concentration on the rate of ripening. No systematic variation in rate was found with the volume fraction of the oil phase in the range 0–0.3. Theoretical treatments due to a number of authors suggest that the rate should increase by a factor of 2.5 in this range of volume fractions. The lack of volume fraction dependence was attributed to the presence of micelles which would appear to mask the effect of interactions between neighbouring diffusion fields surrounding the droplets.