Abstract:

Surfactants perfluoroheptanoic acid (PFHA) and perfluorooctanoic acid (PFOA) were studied in solvents 1,1,2-trifluorotrichloroethane (TFTCE) and n-pentanol (C₅-OH) by ¹⁹F NMR. The spectra of both compounds in both solvents displayed only one set of time-averaged spectral peaks, indicating that molecular exchange rate between micelle and monomer is faster than NMR timescale. All ¹⁹F resonance observed were assigned. Within the concentration range studied, all 19F nuclei in PFHA had lower chemical shifts (upfield shift) in TFTCE than in C₅-OH, indicating the existence of larger deshielding effects in C₅-OH. When PFHA dissolved in C₅-OH, the ¹⁹F nuclei in the α-CF₂ group had the largest chemical shift (δ), those in the ε-CF₂ and ω-CF₃ groups had
 the smallest δ; and the ¹⁹F nuclei in the β- and δ-CF₂ groups had chemical shifts in between. When dissolved in different solvents, the chemical shift differences (in TFTCE vs. in C₅-OH) of the ¹⁹F nuclei in the α-CF₂ and ω-CF₃ groups of PFHA, which locate at the ends of the fluorocarbon chain, were larger than those of the other ¹⁹F nuclei. By plotting chemical shift vs. the inverse of its concentration (1/C), the critical micellar concentrations (cmc) of PFHA was obtained, and is the concentration where the δ-1/C relationship starts to deviate from a linear relationship. cmc of PFHA in C₅-OH and in TFTCE calculated in this study were 0.0195 mol/L and 0.0406 mol/L, respectively. The cmc of PFOA in C₅-OH measured was 0.0547 mol/L. Using the chemical shift vs. 1/C relationship of different ¹⁹F nuclei to calculate cmc resulted in different values, and this might be an advantage for determining cmc using this method. By comparing the δ vs. 1/C relationship obtained experimentally with that calculated by stimulation, equilibrium constant (K) and aggregation number (n) for PFOA in C₅-OH were obtained, and were 21(mol/L)^-(n-1) and 3.9, respectively.

Key words: NMR, ¹⁹F, chemical shifts, surfactant, perfluoroheptanoic acid, perfluorooctanoic acid, critical micellar concentration