Highly concentrated emulsions are mixtures of two immiscible liquids (water and oil generally) in which the volume fraction (Fv) of dispersed phase is higher than 0.90. These emulsions display a mechanical response that strongly depends on intensity of shear stress applied during process preparation. In this work, coupled effects between process and formulation variables on rheological characteristics of highly concentrated water-in-oil emulsions were determined.

Semi-batch process is mainly used for obtaining this kind of emulsions and it consists of two steps, the first one is aqueous phase incorporation into the surfactant-oil mixture phase under given mixing conditions, and the second one is the homogenization step in the same equipment (with possible modifications in the mixing conditions). The experimental set-up consists in a mixing vessel equipped with a 45° pitched blade turbine, whose diameter is 71% of the mixing vessel, and the turbine positioned at the oil phase free surface (before the aqueous phase incorporation). Agitation speed (N) was analyzed during the emulsification process.

The rheological behavior of highly concentrated water-in-oil emulsions was evaluated one hour after their preparation through mechanical spectrometry in the linear viscoelastic domain. The emulsions were prepared with different hydrocarbons oils as dispersed phase (each time only one type of oil was used), deionized water as aqueous phase and sorbitan monooleate (Span 80) as emulsifying agent. The studied formulation variables were volume fraction (Fv) of the dispersed phase, the surfactant/oil weight ratio, Hydrocarbon oil structure, and NaCl concentration in the dispersed phase.

We found a relationship between the rheological characteristics of highly concentrated water-in-oil emulsions (elastic modulus (G')) and energy consumption during emulsification process (E). This relationship can be described by G' is proportional to E^0.6 for all the formulations prepared with different hydrocarbons oils. Moreover, we established an influence of the hydrocarbon oil structure (carbons number of the chain) on the G' values. Finally we observed that the effect of NaCl on the G' value depends on process variables, specially the agitation speed (N). If N is higher, higher is the NaCl effect.