Micellization average hydrocarbon chain

The hydrophobic part of the aggregate molecules forms the core of the micelle while the polar head groups are located at the micelle-water interface in contact with the water molecules. Such micelles usually have average radii of 2... 4 nm and contain 50... 100 monomers in water. Their geometric structure is usually roughly spherical or ellipsoidal. In non-aqueous nonpolar solvents, the micellar structures are generally the inverse of those formed in water. In these solvents, the polar head groups form the interior of the micelle while the hydrocarbon chains of the ions are in contact with the nonpolar solvent. 

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. 

Funasaki and Hada [116] examined the mutual solubility of a fluorinated nonionic surfactant [(CF3)2CF]2C=C(CF3)0(CH2CH20) CH3 (NFE, average n — IS.4) and a nonionic hydrocarbon chain surfactant CH3(CH2)u-0(CH2CH20), H (DEm, w = 5, 7, 9, or 25). Curves of the surface tension plotted against the logarithm of total surfactant concentration for mixtures of NFE and DE7 are shown in Fig. 7.34. The constancy of surface tension beyond cmc (curve a) indicates that DE7 was highly pure, unlike NFE (curve f), which is difficult to purify and therefore contained impurities. The cmc values of NFE-DE7 mixtures exhibited a maximum at a mole fraction of 0.327 for DE7. The relation between the surface tension and the mole fraction of monomeric DE7 in the mixture of NFE and DE7 monomers is shown in Fig. 7.35 with open circles. The filled circles show the surface tension at about 10-fold concentrations above cmc as a function of DE7 mole fraction in the whole system. A plateau region in the micellar composition curve indicates the coexistence of two kinds of mutually saturated mixed micelles. 

The critical micelle concentrations of mixtures of POE nonionic surfactants are of particular interest, since the synthesis of such materials on a commercial basis will always produce a rather broad range of POE chain lengths. Because they contain no electrostatic contribution to the free energy of micelle formation, they can be treated theoretically with a simpler relationship between composition and cmc. In a mixture of nonionic surfactants in which the average POE chain lengths are approximately the same and the hydrocarbon chains different, there was a smooth decrease in the cmc of the mixture as the mole fraction of the more hydrophobic material (lower cmc) was increased, reminiscent of the surface tension-mole fraction curves found for miscible organic materials mixed with water. 

It may be expected that other, highly structured solvents with a tri-dimensional network of strong hydrogen bonds, would also permit micelle formation by surfactants, but little evidence of such occurrences has been reported. On the other hand, surfactants in non-polar solvents, aliphatic or aromatic hydrocarbons and halocarbons tend to form so-called inverted micelles, but these aggregate in a stepwise manner rather than all at once to a definite average size. In these inverted micelles, formed, e.g., by long-chain alkylammonium salts or dinonyl-naphthalene sulfonates, the hydrophilic heads are oriented towards the interior, the alkyl chains, tails, towards the exterior of the micelles (Shinoda 1978). Water and hydrophilic solutes may be solubilized in these inverted micelles in nonpolar solvents, such as hydrocarbons. 

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