Emulsification rheokinetics of nonionic surfactant-stabilized oil-in-water emulsions

The emulsification process of o/w emulsions containing poly(ethylene glycol) nonylphenyl ether has been investigated by following the evolution of torque, droplet size distribution (DSD), and linear viscoelastic proper-ties with emulsification time. The emulsification process was carried out in a controlled-rotational speed mixing rheometer using an anchor or a helical ribbon impeller under different processing conditions. The kinetics of emulsification has been discussed in terms of two stages: breakup of droplets (counterbalanced by coalescence) and transport (and adsorption) of surfactant molecules to the o/w interface to prevent coalescence. An increase in the emulsification time yields lower values of the mean droplet size and, subsequently, an increase in the linear viscoelastic properties of the emulsion at emulsification temperatures up to 25 degreesC. Above this temperature, the evolution with emulsification time is different, since a higher degree of coalescence takes place. As a result, emulsion DSD tends to be bimodal at long emulsification time. A further enhancement of the emulsion droplet network takes place in a few hours after emulsification. This rearrangement may be explained in terms of the development of a depletion-flocculation process, so that the surfactant molecules act both as emulsifying and depleting agents.