Perfluorooctanoates 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]. 

Fluorinated surfactants have been commercially available since the 1950s. The first available were perfluoroalkyl sulfonates (e.g., perfluorooctane sulfonate, CgFisSOs, PFOS) and perfluoroalkyl carboxylic acids (e.g., perfluorooctanoic acid, C7F15COOH, PFOA) manufactured using the electrochemical fluorination (ECF) process [10]. The unique properties (e.g., surface tension lowering in aqueous systems, high chemical and thermal stability) of these acids and their derivatives when used at low concentrations resulted in their widespread use in industrial processes and consumer uses [11-13]. The most common commercially produced perfluorinated surfactants are the perfluoroalkyl acids (PFAAs) ... 

Perfluorinated compounds have special physico-chemical properties, which make them valuable for use in various industries. They are chemically inert, highly thermally stable and reduce surface tension. They repel both water and oil, so they are both hydrophobic and oleophobic substances. Table 12.54 summarises some physico-chemical properties of perfluorooctane sulfonate (PFOS) potassium salt and perfluorooctanoic acid (PFOA). 

Shinoda and co-workers [33-35] varied the counterion concentration and found that usually only about half of the expected amount of the counterion was adsorbed at the surface. Recently, An et al. [27] examined the surface excess of perfluorooctanoate counterions with neutron reflection and surface tension measurements. The authors found that a prefactor less than 2 in the Gibbs equation [Eq. (9)1 is an artifact caused by the presence of a divalent cation impurity. Once the impurity, usually calcium, is removed, neutron reflection results are in agreement with surface tension results using a Gibbs prefactor of 2. 

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). 

Adsorption of ammonium perfluorooctanoate (APFO) and ammonium de-canoate (AmDec) at the air-liquid interface was studied by Simister et al. [24] using surface tension and neutron reflection measurements. At the cmc, the areas oc-... 

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]. 

Fig. 4.20 Surface tensions of aqueous solutions of perfluoroalkyl segmented alkanoic acids CF3(CF2)6CF(CF3)(CH2)ioCOONa (O), CF3(CF2)6CH=CH(CH2)8 COONa ( ), tetradecanoic acid (—), perfluorooctanoic acid ( ), and its ammonium salt ( ). (From Ref. 62. Reproduced by permission of the American Chemical Society.)...

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. 5.8 Cos e versus surface tension of aqueous surfactant solutions on paraffin at 25°C ( ) Aerosol OT in 0.1 MNaCI (O) perfluorooctanoic acid. (From Ref. 34. Reproduced by permission of Academic Press.)...

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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