Basic Principles of Microemulsion Formation and their Thermodynamic Stability

Basic Principles of Microemulsion Formation and their Thermodynamic Stability 

As discussed in Chapter 6, the formation of oil droplets from a bulk oil is accompanied by an increase in the interfacial area, AA, and hence an interfacial energy, A Ay. The entropy of dispersion of the droplets is equal to TAS. With macroemulsions (EWs), the interfacial energy term is much larger than the entropy term and, hence, emulsification is non-spontaneous. In other words, energy is needed to produce the emulsion, e.g. by the use of high-speed mixers. With microemulsions, the interfacial tension is made sufficiently low such that the interfacial energy becomes comparable to or even lower than the entropy of dispersion [101]. In this case, the 

Contributions to re are considered to be the crowding of surfactant and cosurfactant molecules and penetration of the oil phases into the hydrocarbon part of the molecules. If re (yo/w)a hen yj becomes negative, leading to the expansion of the interface until yj becomes zero or a small positive value. Since (yi-)a is of the order of 15-20 mN m surface pressures of that order have to be reached for y-j-to reach an ultralow value that is required for microemulsion formation. This is best achieved by the use of two surfactant molecules, as discussed above. 

The above simple theory can explain the nature of the microemulsion produced when using surfactants with different structures. For example, if the molecules have bulky hydrophobic groups such as Aerosol OT, a W/O microemulsion is produced. Conversely, if the molecule has bulky hydrophilic chains such as alcohol ethoxylates with high ethylene oxide units, an 0/W microemulsion is produced. These concepts will be rationalised using the packing ratio concept discussed in detail in Chapter 6. 

Microemulsions may also be considered as swollen micellar systems, as suggested by Shinoda and co-workers [105-107]. These authors considered the phase diagrams of the components of the microemulsion systems. As discussed in Chapter 10, the phase diagram of the three-component system water/surfactant/ cosurfactant (alcohol) shows two main regions at the water and alcohol corners, namely Li (normal micelles) and L2 (inverse micelles). These regions are separated 

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