Abstract
We report on QuasiElastic Neutron Scattering (QENS) investigations of the dynamics of protons and water molecules confined in nanostructured perfluorinated sulfonic acid (PFSA) materials, namely a commercial Aquivion membrane and the perfluorooctane sulfonic acid (PFOS) surfactant. The former is used as electrolyte in low-temperature fuel cells, while the latter forms mesomorphous self-assembled phases in water. The dynamics was investigated as a function of the hydration level, in a wide time range by combining time-of-flight and backscattering incoherent QENS experiments. Analysis of the quasielastic broadening revealed for both systems the existence of localized translational diffusive motions, fast rotational motions and slow hopping of protons in the vicinity of the sulfonic charges. The characteristic times and diffusion coefficients have been found to exhibit a very similar behaviour in both membrane and surfactant structures. Our study provides a comprehensive picture of the proton motion mechanisms and the dynamics of confined water in model and real PFSA nanostructures.
Highlights
Phase interface, and iii) structural diffusion occurring by continuous breaking and forming of H-bonds in the dense water molecules network [10]
We performed Inelastic Fixed Window Scans (IFWS) [31] on IN16B to evaluate the two-component hypothesis on the perfluorooctane sulfonic acid (PFOS) and Aquivion samples
We have analysed quantitatively the dynamics of water molecules and protons confined in the Aquivion membrane and PFOS surfactant system, on an extended hydration range and time scale
Summary
Phase interface, and iii) structural diffusion occurring by continuous breaking and forming of H-bonds in the dense water molecules network [10]. The QENS technique provides a unique insight into molecular dynamics, as a natural counterpart of the numerous numerical works In their pioneering QENS study, Volino et al [24] analysed the motions of water molecules inside the Nafion polymer matrix with a model for the diffusion inside a sphere [25]. The QENS spectra over a wide energy range are obtained by combining time-of-flight (ToF) and backscattering (BS) experiments and analysed by a phenomenological multi-Lorentzian analysis approach based on microscopic models This provides a detailed picture of the molecular mechanisms and delivers quantitative parameters such as characteristic times, confinement sizes, jump distances and diffusion coefficients
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