Perfiuorinated Sulfonates

All the organic compounds discussed previously in this chapter act either as hydrophobic (water-repelling) or as oleophobic (oil-repelling) substances, but not as both. There exists a small class of organic compounds that will dissolve in neither of these classes, Fluorinated surfactants are compounds that consist of molecules and ions having a long perfiuorinated carbon tail; i.e., a hydrocarbon chain in which each hydrogen atom has been replaced by a fluorine atom. The best-known example of such a molecule is peifluorooctane sulfonate (PFOS):

Notice the similarity in its structure to that of sulfuric acid: One —OH group in the latter has been replaced by an unbranched, perfiuorinated eight-carbon octane chain. This substance was used to make the 3M product Scotchgard, a fabric protector that, because of the characteristics of the perfluoro chain, repelled both water and oily spills and potential stains. Other compounds based upon PFOS were used in fire-fighting foams, pesticide formulations, cosmetics, lubricants, grease-resistant coatings for paper products, adhesives, and paints and polishes.

The 3M company has voluntarily phased out the production of PFOS because it persists long enough in the environment to eventually be detected in human blood samples. Although it is not very toxic, its concentration in some wildlife had reached levels of concern to some scientists. Since 2003, 3M has used the corresponding perfluorosulfonate having a chain of only four, rather than eight, carbon atoms, since such chains do not seem to either bioaccumulate or be toxic.

The fully fluorinated eight-carbon carboxylic acid compound per/luorooc-tanoic acid, PFOA (CF3(CF2)6COOH), and its associated carboxylate salts have also become of environmental concern since they have no environmental or metabolic degradation pathways. Indeed, the very lack of reactivity that makes perfluorinated compounds so appealing in practical uses also results in their persistence in the environment. Although its lifetime in rats is only several hours, PFOA is only slowly eliminated in humans, resulting in an average lifetime of about four years. Due to its slow elimination, the acid is now found at the parts-per-billion level in the blood of most humans and wild animals worldwide. It is potentially acutely toxic, potentially carcinogenic, and may cause developmental problems. Human exposure to PFOA results from its use in producing polymers used to coat surfaces of nonstick cookware, including frying pans, as well as for the membranes of breathable outdoor garments. Fully fluorinated carbon chains are added by covalent bonds to polymer chains in order to make the materials stain-resistant. PFOA has been detected in samples of drinking water in several U.S. states; there is no federal standard for it. In 2006, the U.S. EPA announced a voluntary program, requesting companies using PFOA in consumer products to stop using the compound.

PFOA is the most prominent member of the family of perfluoroalkyl acids, PFAAs. In general, the longer the carbon chain in such molecules, the more persistent is the acid in the human body. 3M is formulating products that use PFAAs with relatively short chains in order to overcome the persistence problems of eight-carbon-chain substances.

PFOA and similar compounds are now found even in remote regions such as the Arctic, being transported there by LRTAP. Apparently such PFOA results from the atmospheric reaction of fluorotelomer alcohols, CF3 (CF2 )nCH2CH2OH, industrial compounds that are used to make stain repellants. Unfortunately, a small fraction of the alcohols are released inadvertently into the atmosphere during the manufacturing process. In addition, the very small concentration of the reactant that was not converted into a repellant but was instead weakly incorporated into the material slowly degasses from it, adding to the atmospheric load of the alcohol.

In air, the fluorotelomer alcohols are converted to the carboxylic acids by a chain reaction that begins when a hydroxyl radical, OH, in air abstracts a hydrogen atom from the —CI I,— group bonded to OH. This process initiates a sequence of free-radical reactions, the net result of which is the oxidation of the terminal —CH2CH2OH group to COOH, producing the final perfluorocarboxylic acid, CF3(CF2)nCOOH. It is also thought that microbial action and animal metabolism play a role in converting the alcohols to acids. Fluorotelomer alcohols have become the main remaining source of PFOA in the environment. In 2006, the Canadian government proposed to ban fluorotelomer polymers that can decompose into long-chain perfluorinated carboxylic acids.

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