Formulating emulsions using the HLD method

According to IUPAC, an emulsion is a dispersion of droplets of a liquid or liquid crystal in a continuous phase of another liquid with which it is practically immiscible. This system is thermodynamically unstable, but can exhibit kinetic stability, sometimes considerable, in the presence of amphiphilic compounds or particles located at the interface of the two phases.

The microscopic (particle size distribution, morphology) and macroscopic (stability, viscosity, conductivity) properties of emulsions depend on three types of variable:

• formulation variables (nature of surfactant and oil, salinity, more or less hydrophilic additives, temperature, pressure, etc.) which can be combined into a single generalized formulation variable, the HLD ;

• composition variables corresponding to the relative percentages of the three main constituents of the system: the aqueous phase, the oily phase and the surfactant(s);

• process variables (order of incorporation of constituents, geometry of stirring device, stirring speed, temperature profile during emulsion preparation, etc.).

In this dossier, we tackle the problem of emulsion formulation, starting with the case of simple ternary systems (polyethoxylated alcohol/n-alkane/water), which show the similarities between W/O/W systems at equilibrium (micelles and microemulsions) or under agitation (macroemulsions).

Generalization to pseudoternary surfactant/oil phase/water phase (S/H/E) systems has led to the introduction of an important conceptual tool, the formulation-composition map, which graphically visualizes the influence of formulation (HLD) and composition (% water) parameters on emulsion morphology. In this two-dimensional representation, emulsions with continuous aqueous and oily phases respectively, obtained from pre-equilibrated W/O/W systems, are separated by the standard inversion boundary. The same concept is then broken down into different property maps on which the evolution of final emulsion properties (stability, viscosity, conductivity, particle size) is highlighted as a function of the same parameters.

Finally, the influence of other parameters, including process parameters, is detailed, showing how the vertical branches of the standard inversion boundary shift, creating zones of hysteresis within which W/O or O/W emulsions can be obtained at will for systems with identical formulation and composition. All these concepts lead us to classify emulsification methods into two categories: those that do not involve phase inversion, and those that require crossing the inversion boundary. Finally, some common emulsification methods (PIT, self-emulsification,...