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 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.

Linear Viscoelastic Behavior of Perfluorooctyl Sulfonate Micelles Stabilized with Tetraethylammonium and Tetramethylammonium Cations

“…The bare radius of the spherical FOS micelles (not including the TEA radius), was r FOS ≅ 1.6 nm, 5,16) and each micelle is composed of ≅ 16 FOS units. 18) This r FOS value was close to that estimated from the packing structure of the perfluorooctyl groups in the micelle, and r FOS hardly changed in the presence of other cations (Li). 6) Thus, the spherical FOS micelles in our FOSTEA/FOSTMA solutions should be similar, in size and structure, to those in the pure FOSTEA solutions.…”

“…These results suggest that the In the pure FOSTEA solution, the surface of the FOS micelles are saturated with a fraction of the existing TEA cations and the remaining, non-bound (free) TEA cations catalyze the thermal scission through their exchange with the bound TEA. [17][18][19] We expect a similar situation in our FOSTEA/FOSTMA binary solutions: Namely, the TMA cations should be preferentially bound on the FOS micelles, and some TEA cations can be also bound if a vacancy is left on the micelle surface; cf. Fig.3b.…”

“…Experiments [17][18][19] indicated that the FOS surface in the value. Thus, we utilize this value to formulate a simple model for τ in the binary solutions, as explained below.…”

“…16) In attempt to clarify the thermal scission mechanism of the FOS threads, detailed NMR 18,19) and rheological 17) studies were conducted for the pure FOSTEA solutions as well as for binary solutions of FOSTEA and lithium perfluorooctyl sulfonate (FOSLi). The NMR studies 18,19) revealed that a fraction of the TEA cations in the FOSTEA solutions is bound on the spherical FOS micelles and that the non-binding Li cations 6) in the FOSTEA/FOSLi solutions enhance preferential binding of the coexisting TEA cations on the FOS micelles.…”

“…18,19) Furthermore, the rheological study 17) indicated that the Maxwell-type terminal relaxation time τ (= thermal scission time) of the FOS threads having the surface saturated with the bound TEA cations is uniquely determined by a concentration of the coexisting, nonbound TEA cations, C TEA *. Specifically, τ was found to be proportional to {C TEA *} .…”