Enrichment Factors of Perfluoroalkyl Oxoanions at the Air/Water Interface

Abstract

The refractory, water-bound perfluoro-n-alkyl carboxylate F(CF_2)_nCO_2^− and sulfonate F(CF_2)_nSO_3^− 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(CF_2)_nSO_3^−] ~2f[F(CF_2)_nCO_2^−] ~200 f(Br^−) values above n ~ 8. The larger f values for F(CF_2)_nSO_3^− over their F(CF_2)_nCO_2^− congeners are consistent with a closer approach of the bulkier, less hydrated −SO_3^− headgroup to the air/water interface. A hyperbolic, rather than the predicted linear log f[F(CF_2)_nCO_2^−] 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(CF_2)_nCO_2^−/F(CF_2)_nSO_3^− species.