Mass transport phenomena in oil-in-water emulsions containing surfactant micelles: Ostwald ripening

The influence of nonionic surfactant type and concentration on the kinetics of Ostwald ripening (OR) of hydrocarbon emulsion droplets suspended in aqueous micellar solutions was investigated. OR rates were determined by measuring changes in the droplet size distribution of 5 wt % n-tetradecane oil-in-water emulsions containing different surfactant types (Tween 20, Tween 40, Tween 60, Tween 80, Triton SP 190 and Triton SP 175) and concentrations (0.5-10 wt %). At low surfactant concentrations, the OR rate increased with surfactant concentration probably because micelles incorporated oil molecules and transported them across the aqueous phase. At high surfactant concentrations, the OR rate decreased with surfactant concentration for some of the surfactants. The OR rate also depended on the molecular structure of the surfactants: Tween 80 > Tween 20 > Tween 40 > Tween 60 > Triton SP 190 > Triton SP 175 (at 2 wt % surfactant). No clear correlation was found between the OR rate and the solubilization kinetics or solubilization capacity of the micelles.