Structure and Dynamics of Micellar Aggregates in Aqueous Nafion Solutions Reported by Electron Spin Resonance and Fluorescence Probes

Abstract

Aqueous solutions of Nafion, an ionomer consisting of a perfluorinated backbone and pendant sulfonic groups, were studied by the electron spin resonance (ESR) spin probe method and by fluorescence spectroscopy. The spin probes used were 4-(N,N-dimethyl-N-alkyl)ammonium-2,2,6,6-tetramethyl-piperidine-1-oxyl cations (CATn) with n, the number of carbon atoms in the alkyl group of the probe, equal to 1 (CAT1), 8 (CAT8), and 16 (CAT16). The ESR spectra of the probes were analyzed in terms of line shapes as a reflection of the local dynamics and 14N hyperfine splittings as indicators of the local polarity. The line shapes point to slower dynamics for the larger probes, CAT8 and CAT16, suggesting that these probes are located deeper inside the aggregates compared to CAT1. The high local polarity reflected in the magnetic parameters was explained by assuming water penetration into the aggregates. The local polarity was estimated from the intensity ratio, R, of the third to the first vibronic peaks (R = I III/I I) in the fluorescence spectrum of pyrene (P) as a probe; R is the polarity index. In the Nafion micelles R = 0.64, indicating a polar local environment, in accord with the ESR results. By comparison, R is 0.53 in water, 0.75 in methanol, and 1.91 in a perfluorinated oil. The fluorescence data also suggest that the upper limit of the critical micelle concentration, cmc, for Nafion is 0.01% w/w. The local viscosity was estimated to be 74 cP at ambient temperature from the fluorescence spectrum of 1,3-bis(1-pyrene)propane (P3P), based on a previously determined calibration curve of the intensity ratio of the excimer-to-monomer emissions, I E/I M, vs viscosity for 14 nonaqueous solvents of known viscosities.

ESR study of aqueous micellar solutions of perfluoropolyether surfactants with the use of fluorinated spin probes

“…4, likely reflect two distinct locations of the nitroxide group (in environments of different molecular packing) and/or different conformations of the probe chain intercalated between surfactant chains. Similar spectral features have been reported previously for doxyl spin probes in poly(ethylene-methacrylic acid) ionomer membranes [23] and for cationic spin probes, with the alkyl substituent of different chain-lengths, in aqueous micelles of Nafion [24].…”

Colloidal Dispersion of a Perfluorosulfonated Ionomer in Water–Methanol Mixtures

“…Therefore, its membrane properties may be strongly dependent on the dispersion state of PFSI before casting. So far, many studies were carried out on the dispersion state of PFSI in aqueous dispersion as well as in polar dispersants [ 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 ]. However, the very strong opposite interactions and the electrostatic repulsion and hydrophobic attraction among PFSI particles in the dispersion make the characterization of the PFSI dispersion difficult.…”

Monitoring pH-dependent conformational changes in aqueous solutions of poly(methacrylic acid)-b-polydimethylsiloxane copolymer based on fluorescence spectra of pyrene and 1,3-bis(1-pyrenyl)propane

“…This process is expected to be more efficient in a more fluid environment; for this reason the intensity ratio of the excimer to monomer emission maxima, I E =I M ; in the P3P fluorescence spectra can be used to estimate the local viscosity in the host [3,4]. This approach was used in recent work from our laboratories, where both P3P and P were used to examine self-assembling of ionomers in aqueous solutions [5,6].…”