Critical packing parameter microemulsions

The thermodynamic modeling of microemulsions has taken various lines and gave conflicting results in the period before the thermodynamic stability and microstructure were established. It was early realized that a maximal solubilization of oil and water simultaneously could be discussed in terms of a balance between hydrophilic and lipophilic interactions the surfactant (surfactant mixture) must be balanced. This can be expressed in terms of the HLB balance of Shinoda,Winsor s R value, and a critical packing parameter (or surfactant number), as introduced to microemulsions by Israelachvili et al. [37], Mitchell and Ninham [38], and others. The last has become very popular and useful for an understanding of surfactant aggregate structures in general. 

The selection of surfactants for producing OAV or W/O microemulsions is not simple and one needs to establish the phase diagram of the ternary system oil-water-surfactant-cosurfactant to arrive at the regions of formation of OAV or W/O microemulsions. A useful concept for selection of surfactants is based on the critical packing parameter (CPP) of the molecules. 

Phospholipids are perhaps the common zwitterionic surfactants used in microemulsions. However, they are too lipophihc to be used alone and have a fairly high critical packing parameter, making their incorporation into microemulsions difficult (Moreno et al, 2003). If used, co-surfactants are also required to increase the fluidity of the interface (Gaonkar and Bagwe, 2002 Patel et al, 2006). 

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