Oleophobic-oleophilic surfactants

Oleophilic/oleophobic fluorinated surfactants without a hydrophile, designed for use in hydrocarbon systems, are in a structural sense also nonionic fluorinated surfactants (see Section 1.8) for example, the semifluorinated alkanes [266-270], block polyethylene-polypropylene glycol ethers prepared with perfluoroalkene trimers [271], surfactants featuring an oligo(hexafluoropropene oxide) chain [272], and carboxamides and sulfonamides derived from A -(perfluorooctanesul-fonyl)piperazine [273]. 

The reason for this behaviour is that the surfactant molecule contains two structurally distinct parts, one of which is hydrophilic while the other is hydrophobic. Oil-soluble surfactants have an oleophilic and an oleophobic part. In the great majority of surfactants, the hydrophobic part is a hydrocarbon chain, which usually has an average length of 12 to 18 carbon atoms and may include an aromatic ring. A single molecule in aqueous solution seeks the surface, because its hydrocarbon tail is repelled by the water, and it tends to remain there, with the hydrophobic part above the surface and the hydrophilic part below, i.e. in the water phase. Further molecules seek the surface or the walls of the vessel until both are full, at which point further additions result in the formation of micelles, which are clusters of molecules arranged with the hydrophobic parts towards the centre and the hydrophilic parts on the outside. 

Nonionic surfactants—do not dissociate into ions, for example, C7Fi5CH2CH20(CH2CH20) H. a special class of nonionic fluorinated surfactants are compounds which do not have a hydrophile but consist of an oleophobic (fluorinated) segment and a oleophilic segment (see Section 1.8). 

The method selected for the preparation of a nonionic fluorinated surfactant depends on the hydrophile, which can be either an polyalkyl ether chain or a polyhydroxy group. Because oxyethylation results in a mixture of oligomers, special methods have been devised for the preparation of monodisperse surfactants. A unique group of nonionic surfactants are amphiphiles without a hydrophile. Semi-fluorinated alkanes with an oleophilic and an oleophobic segment function as nonionic surfactants in oleophilic solvents (see Section 1.8). 

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