Formulation of microemulsions by the HLD method

Some water-oil-surfactant systems form microemulsions at equilibrium. Unlike macroemulsions, microemulsions are single-phase, thermodynamically stable, generally transparent or translucent, low-viscosity systems containing labile microdomains of water and oil separated by a flexible film of surfactant.

Depending on their formulation, water-oil-surfactant systems have one or more phases in equilibrium: either a microemulsion alone (Winsor IV), a microemulsion in equilibrium with an oil phase (Winsor I) or a water phase (Winsor II), or a microemulsion in equilibrium with both an oil and a water phase (Winsor III). In the latter case, the system formulation is said to be "optimal" when the median microemulsion contains equal amounts of water and oil. For this particular, experimentally and thermodynamically well-defined formulation, the simultaneous solubilization of water and oil is maximal for a given amount of surfactant, and both the microemulsion/water and microemulsion/oil interfacial tensions are ultra-low.

This formulation is chosen as the zero point for establishing an absolute classification scale for water-oil-surfactant systems, irrespective of their chemical composition. This scale is based on the notion of HLD (hydrophilic-lipophilic difference), which quantitatively expresses the gap between any formulation and the optimum formulation. Unlike conventional formulation concepts (HLB, PIT, Winsor's R), the HLD concept brings together, in the form of a numerical expression, the contribution of all formulation variables (nature of surfactant and oil, type and quantity of electrolyte and alcohol, temperature). This formulation tool enables :

• quantitatively compare the relative influence of each parameter;

• experimentally establish classification scales for oils and surfactants;

• formulate microemulsions with "tailor-made" properties.