H and 19F NMR Study of the Counterion Effect on the Micellar Structures Formed by Tetraethylammonium and Lithium Perfluorooctylsulfonates. 1. Neat Systems

The micellar structures formed by the perfluorooctylsulfonate ion (FOS - ) in combination with the tetraethylammonium (TEA + ) and lithium (Li + ) counterions are studied by measuring the NMR chemical shifts and self-diffusion coefficients of both TEA + and FOS - at 30 °C. Spherical micelles generated by LiFOS are characterized by a lifetime of 10 -6 s, an aggregation number of about 36, and a cmc value of 7.16 mM. The fluorine spectrum of the FOS - ion in the micellar solutions of LiFOS consists of one set of peaks whose chemical shifts are averaged between the monomer and micellar states. When combined with the hydrophobic TEA + counterion, the FOS - ions show the 19 F spectra which consist of two sets of peaks whose chemical shifts and intensities are concentration dependent. The 19 F spectroscopic features are discussed by assuming the appearance of small aggregates at high surfactant concentrations. The binding coefficient (0.80) of the TEA + counterions to FOS - micelles is determined on the basis of the self-diffusion coefficients, measured for both TEA + and FOS - . The micellar self-diffusion coefficient in the TEAFOS system (8.4 × 10 -12 m 2 s -1 ) is found to be 1 order of magnitude lower than that in LiFOS system (5.7 × 10 -11 m 2 s -1 ), reflecting the dynamic slowdown due to the formation of threadlike micelles.