Ostwald ripening of hydrocarbon emulsion droplets in surfactant solutions

Ultrasonic attenuation spectra (1-150 MHz) of a series of 5 wt % oil-in-wat er emulsions containing hydrocarbon droplets (n-decane, n-dodecane, n-tetra decane, n-hexadecane, n-octadecane) stabilized by various surfactants (Brij 35, Tween 20, SDS, Triton X-100) were measured as a function of time. Drop let size distributions were calculated from attenuation spectra using ultra sonic scattering theory. Changes in droplet size distribution were also mea sured by static light scattering on the same emulsions after dilution (<0.0 1 wt %). Ostwald ripening rates were determined from the time-dependence of the mean droplet size using the Lifshitz-Slyozov-Wagner theory. Ultrasonic spectroscopy was more sensitive to small changes in droplet size than ligh t scattering and could be used to analyze emulsions containing smaller drop lets (d < 100 nm). Ripening rates decreased exponentially as the molar volu me of hydrocarbon increased. They also depended on the type of surfactant u sed to stabilize the emulsions, decreasing in the following order: Brij 35 > Tween 20, Triton X-100 > SDS. The addition of excess surfactant (0.5-5 wt % Tween 20) to n-tetradecane emulsions stabilized by Tween 20 had little i nfluence on ripening rates.