Ostwald ripening of water-in-hydrocarbon emulsions

The purposes of this study are to determine Ostwald ripening rates in water-in-oil (W/O) emulsions and evaluate the potential application of the LSW theory to W/O emulsions. Water-in-oil emulsions were prepared by mixing water and hydrocarbon phases containing Span 83 and homogenizing. n-Heptane, n-decane, n-dodecane, and n-tetradecane were used to obtain a range of solubilities. A linear increase in the cube of the droplet size with time was observed (within the initial period of 1-2 h after emulsion preparation), supporting the LSW theory. Based on this linear relationship, Ostwald ripening rates were determined to be 3.0 x 10(-24), 2.3 x 10(-24), 1.8 x 10(-24), and 5.8 x 10(-25) m3 s(-1) for water-in-heptane, water-in-decane, water-in-dodecane, and water-in-tetradecane emulsions, respectively. These values are in agreement with theoretical predictions calculated using the LSW equation. It was observed that the ripening process gradually slowed, resulting in deviations from the LSW theory. This was attributed to the effect of the interfacial surfactant film through which the dispersed material has to diffuse during Ostwald ripening. This effect is not taken into consideration by the LSW theory. The results showed that Ostwald ripening of W/O emulsions was less sensitive to the nature of oil used and slower compared to O/W emulsions consisting of the same hydrocarbons.