3. Concept of hydrophilic-lipophilic difference (HLD)

3.1 Equivalences between formulation variables

In the 1970s, numerous studies focused on enhanced oil recovery using surfactants. An unprecedented theoretical and experimental effort was devoted to the formulation of water-oil-surfactant systems. What emerged was the notion of optimal formulation, for which the interfacial tension between the microemulsion and the oil is lowered to such an extent that the capillary forces trapping the oil in the pores of the deposit disappear or become negligible. The term "optimal" was used in this case, as it enabled maximum oil recovery.

Subsequently, the notion of optimal formulation was retained as it corresponds to a very specific formulation for which :

• the Winsor R ratio...