Relationship between Phase Behavior and Emulsion Inversion for a Well-Defined Surfactant (C10E4)/n-Octane/Water Ternary System at Different Temperatures and Water/Oil Ratios

Abstract

The relationship between the phase behavior and the type of emulsion formed under stirring has been studied for a well-defined surfactant/oil/water (SOW) system (ultrapure C10E4/n-octane/water) at three surfactant concentrations (1%, 3%, and 7%). The phase behavior was determined from systems equilibrated at constant temperature. The type of emulsions formed when these pre-equilibrated systems were stirred was established from conductivity measurements, and the so-called “standard inversion frontier” between the two emulsion morphologies (O/W and W/O) was plotted on temperature–water/oil proportion and temperature–surfactant concentration maps (χ and γ cuts, respectively, of the SOW–T prism). Dynamic phase inversions, produced by imposing a temperature variation under continuous stirring, were also observed. In the χ and γ maps, an exact correspondence between phase behavior and emulsion type was not observed, under either standard or dynamic conditions, and some regions of the formulation–composition map clearly shed light on a violation of Bancroft’s rule. The strong impact of kinetics (mass transfer of the components), especially at low surfactant concentration, was demonstrated in experiments performed under dynamic inversion conditions. The transitional branch position was found to be significantly affected by the dynamic process, and its extension between the two catastrophic branches was directly associated with the range of three-phase behavior, which, in turn, was quite dependent on surfactant concentration.