Nanoemulsion formation by phase inversion emulsification: on the nature of inversion.

Emulsification processes are usually characterized by the way they allow the surfactants, as well as the dispersed phase, to be incorporated into emulsions. A model cyclohexane-in-water emulsion using a pair of polyoxyethylene nonylphenyl ether surfactants, one oil-soluble and one water-soluble, was considered. Two surfactant mixing approaches consisting of mixed surfactants (agent-in-oil and agent-in-water) and segregated surfactants (agent in corresponding oil and water phases) were used to produce the model emulsion. Formation of oil-in-water nanodroplets could be only achieved if emulsification was associated with the formation of a three-phase microemulsion structure (transitional phase inversion) across the path. This occurred only if segregated surfactants were used in a process in which water was added to oil. With decreasing surfactant concentration, a point was reached below which the inversion mechanism transformed from transitional to catastrophic, leading to the formation of large droplets. The transformation was also accompanied by a shift in the evolution of the drop size. Drop size variations showed a minimum at the inversion point for the transitional phase inversion, whereas they showed a maximum for the catastrophic phase inversion. The agent-in-oil technique followed a catastrophic phase inversion mechanism and ranked second in terms of drop size.