Structure of fluorinated surfactants

Nuclear magnetic resonance is a very powerful tool for investigating micellar structures of fluorinated surfactants. NMR spectroscopy yields values of the free energy of micellization, AG,° and the corresponding enthalpy and entropy changes, AH° and AS° [26-32]. 

Fig. 10.12 Structure of fluorinated surfactants derived from xyletol (1) and maltose (2, 3). (From Ref. 127. Reproduced by permission of the American Oil Chemists Society.)...

The amphiphilic nature is the indispensable attribute of the structure of any surfactant. The nonpolar, "solvophobic" part of fluorosurfactants constitutes usually with perfluoroalkyl, co-hydroperfluoroalkyl or perfluoroether chain of normal or branched structure. Surfactants with semifluorinated carbon chains are not quite effective as the ones with completely fluorinated carbon chains. Usually the optimum perfluorocarbon chain length is from 6 to 10 carbon atoms. Three industrially important methods of fluorosurfactants synthesis are known [123-126] ... 

There is large body of data on the surface and interfacial tensions of aqueous surfactant solutions. This data show that the structure of the surfactant molecule has a pronounced effect on its ability to reduce these tensions. As the length of the alkyl or fluorinated alkyl chain increases, the CMC decreases and the surface excess concentration increases, causing a drop in the interfacial tension at a fixed surfactant concentration. At low surfactant concentrations the reduction in surface tension (or increase in surface pressure O = yo - y) is linear with the molar bulk solute concentration c (in the case of the dilute solution)... 

Fluorinated surfactants are both hydrophobic and lipophobic. For example, potassium per-fluorooctanesulfonate—an industrially important surfactant —forms a third phase with octanol and water, and it is impossible to determine its octanol-water partition coefficient. " Similar to fluorocarbon-hydrocarbon bulk solvent mixtures, mixed binary systems containing a perfluorocarbon surfactant and a structurally related hydrocarbon surfactant are known to behave nonideally, that is, exhibit phase separation in insoluble monolayers at the air-water interface or form two types of micelles simnltaneously in solution—one type is fluorocarbon-rich and the other is hydrocarbon-rich. This nonideal behavior of fluorocarbon-hydrocarbon surfactant mixtures is used in firefighting foams and powders—an important technical application of fluorinated surfactants. " ... 

The effect of surfactant structure on cmc is discussed in Section 6.6. The complex relationship between surface tension and cmc depends on the hydrophobe and the hydrophile, including the counterion, of the surfactant. An increase in the chain length of the hydrophobe decreases cmc branching of the carbon chain increases cmc. Fluorination of the hydrophobe lowers cmc considerably. In addition to the chemical structure of the surfactant, cmc depends on external factors, including electrolyte effects, temperature, and other dissolved or solubilized organic components. 

The effect of the hydrophilic group on surface tension depends on the structure of the hydrophile and, for ionic surfactants, also on the counterion. For a constant chain length of C7F15, the surface tension of 0.1% solutions of fluorinated surfactants varies between 17 and 47 mN/m, depending on the nature of the hydrophile [56] (Table 4.5). Nonionic fluorinated surfactants usually have lower surface tensions than their ionic counterparts. Nonionic surfactants derived from... 

The development of fluorinated surfactants for organic solvents is still ongoing. The structural design is limited by (1) the dependence of surface activity on the solubility of the surfactant in the solvent and (2) the initial surface tension of the solvent. Hence, a fluorinated surfactant cannot have the same optimum structure for all organic solvents [92]. For semifluorinated alkanes see Section 1.8. 

If this assumption is correct, the wettability of a solid should depend solely on the surface tension, 7lv, of a pure liquid or a surfactant solution, regardless of the structure of the surfactant used to lower the surface tension of water. However, this assumption was found to be incorrect. When Bernett and Zisman [32] used surfactant solutions instead of pure liquids in their wetting studies, fluorinated surfactants behaved differently than conventional surfactants with a hydrocarbon chain. Furthermore, the wetting characteristics of a fluorinated surfactant solution depend on the nature of the solid surface. 

The solubility of fluorinated surfactants depends on the hydrophile of the surfactant, in addition to the structure of the fluorinated group. The solubility of perfluoroalkanoic acids in water, like the solubility of nonfluorinated alkanoic acids, decreases with increasing chain length. At 25°C, perfluorohexanoic acid and shorter-chain perfluoroalkanoic acids are miscible with water in all proportions. However, perfluorooctanoic acid and perfluorodecanoic acid are only slightly soluble in water [4]. 

The amount of a solubilizate which can be solubilized depends on several factors. The dominant variables are the structures of the surfactant and the solubilizate. Both the structure of the hydrophobic chain and the nature of the counterion can affect solubilization. Although the relation between solubilization and surfactant structure is complex, it is clear that the interactions between a solubilizate molecule and the lipophobic hydrophobe of a fluorinated surfactant must be different from interactions between the solubilizate and the lipophilic hydrophobe of a hydrocarbon surfactant. Solubilization by fluorinated surfactants is therefore of great theoretical as well as practical interest. The published information on solubilization by fluorinated surfactants is, however, sparse. 

Unlike mixed hydrocarbon-chain surfactants of similar molecular structure, mixtures of fluorinated surfactants and hydrocarbon-chain surfactants do not behave ideally, even when the surfactants have a similar hydrophilic group. Mixtures of anionic fluorinated surfactants with anionic hydrocarbon surfactants exhibit a positive deviation from the ideal relation (Fig. 7.4). In contrast, surfactant mixtures containing a nonionic surfactant or oppositely charged ionic surfactants exhibit a negative deviation from ideal predictions. The formation of mixed micelles is governed by hydrophobic interactions between hydrocarbon and fluorocarbon chains and electrostatic effects [66]. Introduction of nonionic surfactants into micelles of anionic fluorinated surfactants reduces electrostatic repulsion between the ionic head groups. Apparently, the resulting electrostatic effect overcomes the hydrophobic interaction between the fluorocarbon and hydrocarbon chains. 

Research on the micelle structure and interactions of fluorinated surfactants is ongoing with the main focus on mixed-surfactant systems. Mixtures of fluorinated and nonfluorinated surfactants may consist of anionic, nonionic, or cationic components. Most of the systems investigated so far have contained a fluorinated anionic surfactant and an anionic hydrocarbon surfactant. Anionic fluorinated surfactant mixtures with nonionic or cationic hydrocarbon-type surfactants have been investigated as well. The nonionic fluorinated hydrocarbon surfactant mixtures and cationic fluorinated hydrocarbon surfactant mixtures have been the subject of only a few studies. 

The phase behavior of fluorinated surfactants and hydrocarbon surfactants is remarkably similar. Tiddy and co-workers [161-164] observed that ammonium perfluorooctanoate and lithium perfluorooctanoate, like hydrocarbon surfactants, form a hexagonal phase, a lamellar phase, and an intermediate phase. A reversed hexagonal structure originally postulated for the intermediate phase [163,164] was found to be inconsistent with F-NMR observations, and an alternative lamellar structure was proposed [165]. 

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