| Abstract
| - Vibrational sum frequency spectroscopy (VSFS) has been used to study the adsorption of ammoniumperfluorononanoate (APFN) at the air−liquid interface, specifically targeting the C−F and carboxylate stretches.In the spectral region extending from 1050 to 1850 cm-1, three major peaks situated at 1369, 1408, and∼1665 cm-1 were detected under the polarization combinations ssp, ppp, and sps. The first of these peakswas assigned to CF3 stretching vibrations after a detailed comparison with other surfactant molecules withdifferent degrees of fluorination, while the last two resonant features were assigned to symmetric andantisymmetric carboxylate stretching vibrations. From the combined analysis of the VSFS, ellipsometry, andprevious FTIR measurements, a consistent physical model for the adsorption of APFN to the liquid surfaceis proposed. At low surface coverages the APFN molecules lie essentially flat on the surface. At concentrationsfrom 1 mM to the critical micellar concentration (molecular areas <70 Å2) the mean orientation of both thesurfactant head group and terminal CF3 group remained constant. The formation of surface micelles at higherconcentrations is consistent with the experimental data.
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