Reason for Combining Two Surfactants

For a multicomponent system i, each with an adsorption (molm , referred to as the surface excess), the reduction in y (i.e., dy) is given by the following 

The reason for the lowering of y when using two surfactant molecules can be understood from consideration of the Gibbs adsorption equation for multicomponent systems [9]. For two components, sa (surfactant) and co (cosurfactant). Equation (15.4) becomes. 

The two surfactant molecules should adsorb simultaneously and they should not interact with each other, otherwise they lower their respective activities. Thus, the surfactant and cosurfactant molecules should vary in nature, one predominantly water-soluble (e.g., an anionic surfactant) and the other predominantly oil-soluble (e.g., a medium-chain alcohol). 

In some cases a single surfactant may be sufficient for lowering y far enough for microemulsion formation to become possible examples include Aerosol OT (sodium diethyl hexyl sulphosuccinate) and many nonionic surfactants. 

The duplex film theory predicts that the nature of the microemulsion formed depends on the relative packing of the hydrophobic and hydrophilic portions of the surfactant molecule, which determined the bending of the interface. For example, a surfactant molecule such as Aerosol OT, 

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