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Enrichment Factors of Perfluoroalkyl Oxoanions at the Air/Water Interface

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journal contribution
posted on 06.08.2009, 00:00 by Elefteria Psillakis, Jie Cheng, M. R. Hoffmann, A. J. Colussi
The refractory, water-bound perfluoro-n-alkyl carboxylate F(CF2)nCO2 and sulfonate F(CF2)nSO3 surfactant anions reach remote locations by mechanisms that are not well understood. Here we report experiments in which the relative concentrations of these anions on the surface of microdroplets produced by nebulizing their aqueous solutions are measured via electrospray ionization mass spectrometry. Enrichment factors f (relative to Br: f(Br) ≡ 1) increase with n, asymptotically reaching f[F(CF2)nSO3] ∼ 2 f[F(CF2)nCO2] ∼ 200 f(Br) values above n ∼ 8. The larger f values for F(CF2)nSO3 over their F(CF2)nCO2 congeners are consistent with a closer approach of the bulkier, less hydrated −SO3 headgroup to the air/water interface. A hyperbolic, rather than the predicted linear log f[F(CF2)nCO2] vs n dependence suggests the onset of conformational restrictions to interfacial enrichment above n ∼ 4. Marine aerosols produced from contaminated ocean surface waters are therefore expected to be highly enriched in F(CF2)nCO2/F(CF2)nSO3 species.