Sodium perfluorooctanoate surface tension

The application of the activity of the surfactant has been examined also for the surface tension and adsorption of disodlum alkyl phosphate(6,7), sodium dodecyl sulfate(37), alkyl trimethylammonium bromide(35 ), and sodium perfluorooctanoate(13) solutions. These studies show that the surface tension and theadsorption amount are controlled by the activity of surfactant, irrespective of the added electrolyte concentration. 

Another interesting feature of fluorinated surfactants is their ability to greatly reduce the surface tension of water, even at very small concentrations. Some surfactants, such as sodium perfluorooctanoate, can lower the surface tension at 25 °C down to 15 mN m" . It should be pointed out that such low tensions can be achieved with a rather short pefluoalkyl chain, because the perfluoalkyl chains are significantly more hydrophobic than the hydrocarbon alkyl chain with the same number of carbon atoms [46]. A remarkable example is sodium fluorobu-tanoate, with a very short chain length, but still able to form self-aggregates in water [47]. 

The effect of added ammonium chloride and sodium chloride on the area occupied by an adsorbed perfluorooctanoate molecule has been estimated using the Gibbs equation [Eq. (13)] and by neutron reflection [26]. The results indicated that sodium and ammonium ions have a different specific effect on the formation of perfluorooctanoate micelles and on the surface tension above the cmc (see Section 6.6). 

Fluorination of the hydrophobe decreases markedly the surfactant concentration needed for a substantial surface tension reduction. At 1 g/L concentration, sodium octanesulfonate, C8H 7S03Na, lowers the surface tension of water at 20°C to 65 nM/m, but sodium perfluorooctanoate, CgHivSOsNa, at the same concentration lowers surface tension to 32 mN/m [49]. Conventional surfactants with a hydrocarbon-type hydrophobe can lower surface tension to 30-35 mN/m at 0.1% concentration. With fluorinated surfactants, surface tensions below 30 mN/m have been achieved at concentrations as low as 10-100 ppm. With some fluorinated surfactants, only 100-200 ppm of the surfactant is needed to lower surface tension below 20 mN/m [48,50]. 

The surface tension data for the mixtures containing sodium dodecyl sulfate and 1.0, 0.75, and 0.20 mole fractions of perfluorooctanoic acid are plotted as a function of perfluorooctanoic acid concentration in Fig. 4.23. All three systems attain the same low and constant surface tension value after a sharp change in the slope corresponding to the formation of perfluorooctanoic acid micelles. The cmc values for the 1.0 and 0.75 molar systems are indistinguishable, indicating that the solubility of sodium dodecyl sulfate is insignificant in those micelles. The curve... 

Fig. 4.23 Surface tension of perfluorooctanoic acid and its mixtures with sodium dodecyl sulfate. Mole fractions of perfluorooctanoic acid 1.00, 0.75, 0.20. (From Ref. 66. Reproduced by permission of the American Chemical Society.)...

Guo et al. [80] have studied mixtures of sodium perfluorooctanoate (SPFO) with a nonionic, an amphoteric, and a cationic hydrocarbon-type surfactant by NMR and surface tension meaurements ... 

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