Interfacial surfactant film flexibility

Most single-chain surfactants do not lower the oil-water interfacial tension sufficiently to form microemulsions nor are they of the correct molecular structure, and short- to medium-chain length alcohols are necessary as cosurfactants. The cosurfactant also ensures that the interfacial film is flexible enough to deform readily around each droplet as their intercalation between the primary surfactant molecules decreases both the polar head group interactions and the hydrocarbon chain interactions. Medium-chain alcohols such as pentanol and hexanol have been used by many investigators as they are particularly effective... 

In most of the emulsions, surfactants alone are not able to sufficiently reduce the interfacial tension between oil and water. Cosurfactants further reduce the interfacial tension and increase the fluidity of the interfacial film. The use of cosurfactants imparts sufficient flexibility to the interfacial film to take up different curvatures, which may be required to form microemulsion over a wide range of proportions of the components. The main role of cosurfactant is to destroy liquid crystalline or gel structures that form in place of a microemulsion phase. Typically used cosurfactants are short chain alcohols (C3-C8), glycols such as propylene glycol, medium chain alcohols, amines, or acids. - Cosurfactants are mainly used in microemulsion formulation for the following reasons ... 

By varying several parameters such as the W/O ratio, one can induce an inversion from an O/W to a W/O microemulsion and vice versa. The type of structure in the inversion domain depends essentially on the bending constant a characteristic of the elasticity of the surfactant layer [7]. If Ke is on the order of kT (where k is the Boltzmann constant and T absolute temperature), the persistence length of the film (i.e., the distance over which the film is locally flat) is microscopically small. The interfacial film is flexible and is easily deformed under thermal fluctuations. The phase inversion occurs through a bicontinuous structure formed of water and oil domains randomly interconnected [8,9]. The system is characterized by an average curvature around zero, and the solubilization capacity is maximum. When K kT, is large and the layers are flat over macroscopic distances. The transition occurs through a lamellar phase. 

All these results should convince us that the monomer plays a crucial role in the phase diagrams of these systems. In fact, this role, more or less ignored by formulators until recently, is two-fold. As a cosurfactant, the monomer increases flexibility of the interfacial film. It can therefore deform more easily to produce a sponge structure. And as an electrolyte, the monomer reduces aqueous solubility of ethoxylated surfactants. It thereby favours their gradual transfer into the organic phase and the formation of a bicontinuous structure. This effect of salt in the formation of bicontinuous microemulsions is well known to the users of such systems. It is significant in this respect that the same systems without monomer or electrolyte (if the monomer is neutral) do not lead to bicontinuous structures. 

Cosurfactants are molecules with weak surface-active properties that are combined with the surfactants to enhance their ability to reduce the interfacial tension and promote the formation of an ME [3]. On the other hand cosolvents are weak amphiphiles that tend to distribute between the aqueous phase, the oil phase and the interfacial layer. They promote ME formation by rendering the oily phase less hydrophobic, the aqueous phase less hydrophilic, and the interfacial film more flexible and less condensed [9,10]. 

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