Perfluoroalkanoic acids surface tension

Increasing the carbon chain length increases the efficiency of the surfactant in surface tension reduction [50,51,55] (Fig. 4.16). Surface tension of perfluo-roalkanoic acid solutions plotted against their log molar concentration is shown in Fig. 4.17 [56]. The surface tension of perfluoroalkanoic acids obeys Traube s rule ... 

Fig. 4.16 Surface tensions of perfluoroalkanoic acids in aqueous solutions versus log(weight percent). (From Ref. 51. Reproduced by permission of the American Chemical Society.)...

The stability of perfluorinated anionic surfactants is also remarkable in alkali. Glockner et al. [79] found the surface tensions of perfluorinated anionic surfactants in saturated aqueous sodium hydroxide to be constant for at least 100 days (Table 4.11). Salts of perfluoroalkanoic, perfluoroalkanesulfonic, or perfluo-roalkenesulfonic acids survived an alkaline fusion [5 g KN03/LiN03 (35-65 mol%), 0.5 g NaOH, 0.05 g surfactant] for 2 h at 200°C, as indicated by the surface tensions of the aqueous solutions of the fusion mass. 

Solutions of fluorinated surfactants have been investigated and their micellar nature has been confirmed [58,59]. The substitution of the larger and highly electronegative fluorine atom for the smaller hydrogen increases the amphiphilic nature of the surfactant and lowers the surface tension and cmc. The alkali and ammonium salts of perfluoroalkanoic acids exhibit surfactant properties and form micelles for a chain length of four carbon atoms, whereas eight carbon atoms are needed for the nonfluorinated alkanoates. 

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