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

Abstract

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 19F spectra which consist of two sets of peaks whose chemical shifts and intensities are concentration dependent. The 19F 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 m2 s-1) is found to be 1 order of magnitude lower than that in LiFOS system (5.7 × 10-11 m2 s-1), reflecting the dynamic slowdown due to the formation of threadlike micelles.