Evolutionary behaviour of miniemulsion phases: II. Growth mechanism of miniemulsion droplets

Abstract

The growth mechanism of droplets of hexadecane-in-water emulsions stabilized by heptaethylene glycol mono-n-dodecyl ether was studied by means of photon correlation spectroscopy. When the water-to-oil ratio (WOR) was varied from 4 to 10, a translucent oil in water emulsion (a so-called `miniemulsion') was obtained, while an opaque oil in water emulsion (O/W emulsion) was prepared when WOR was equal to 3. The z-average hydrodynamic radius of droplets, rz, was measured over the 24 days after preparation. Plotting the cube of the z-average radius as a function of time, we found that r 3z varies linearly with time in the system of the O/W emulsion (WOR = 3). This suggests that the process by which the droplets grow is that of Ostwald ripening. On the other hand, the same plot for the system of miniemulsions (4 < WOR < 10) shows two regimes in the process. In both regimes, r 3z varies linearly with time. The rate obtained for the earlier stage is smaller than that of the latter stage. The droplet size distribution became too broad compared with the prediction of Lifshitz-Slezov-Wagner theory for Ostwald ripening ~10 days after preparation. Then it approached the predicted distribution again ~20 days after preparation.