A Study of the Adsorption of Ammonium Perfluorononanoate at the Air-Liquid Interface by Vibrational Sum-Frequency Spectroscopy

Vibrational sum frequency spectroscopy (VSFS) has been used to study the adsorption of ammonium perfluorononanoate (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 similar to 1665 cm(-1) were detected under the polarization combinations ssp, ppp, and sps. The first of these peaks was assigned to CF3 stretching vibrations after a detailed comparison with other surfactant molecules with different degrees of fluorination, while the last two resonant features were assigned to symmetric and antisymmetric carboxylate stretching vibrations. From the combined analysis of the VSFS, ellipsometry, and previous FTIR measurements, a consistent physical model for the adsorption of APFN to the liquid surface is proposed. At low surface coverages the APFN molecules lie essentially flat on the surface. At concentrations from 1 mM to the critical micellar concentration (molecular areas < 70 angstrom(2)) the mean orientation of both the surfactant head group and terminal CF3 group remained constant. The formation of surface micelles at higher concentrations is consistent with the experimental data.