Emulsification rheokinetics of nonionic surfactant-stabilized oil-in-wateremulsions

The emulsification process of o/w emulsions containing poly(ethylene glycol ) nonylphenyl ether has been investigated by following the evolution of tor que, droplet size distribution (DSD), and linear viscoelastic proper-ties w ith emulsification time. The emulsification process was carried out in a co ntrolled-rotational speed mixing rheometer using an anchor or a helical rib bon impeller under different processing conditions. The kinetics of emulsif ication has been discussed in terms of two stages: breakup of droplets (cou nterbalanced by coalescence) and transport (and adsorption) of surfactant m olecules to the o/w interface to prevent coalescence. An increase in the em ulsification time yields lower values of the mean droplet size and, subsequ ently, 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 l ong emulsification time. A further enhancement of the emulsion droplet netw ork takes place in a few hours after emulsification. This rearrangement may be explained in terms of the development of a depletion-flocculation proce ss, so that the surfactant molecules act both as emulsifying and depleting agents.