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Title: Slow Solvation Dynamics in the Microheterogeneous Water Channels of Nafion Membranes
Authors: BURAI, TN
Keywords: Ion-Exchange Selectivity
Normalized Emission-Spectroscopy
Fuel-Cell Membranes
Perfluorosulfonate Ionomer
Reverse Micelles
Biological Water
Proton Transport
Issue Date: 2009
Citation: JOURNAL OF PHYSICAL CHEMISTRY B, 113(49), 15901-15906
Abstract: Solvation dynamics in Nafion membrane is studied using the well-known solvation probe, coumarin 102 (C102). In native Nafion membrane, the fluorescence maximum of C102 occurs at 525 nun. The decays recorded at different wavelengths are superimposable. There is no tithe-dependent Stokes shift (TDSS) in the time scale of the experiment. This is rationalized in light of the strongly acidic environment in Nafion membrane, which causes the C102 molecules to become protonated. The protonated molecules are bound tightly to the negatively charged sulfonate groups. In Na(+)- and Me(4)N(+)-exchanged Nafion membranes, the fluorescence gets blue-shifted by 65 nm, indicating the deprotonation of the cation and formation of neutral C102 in these membranes. TDSS is observed in the picosecond-nanosecond time scale, in the cation-exchanged Nafion membranes, although the amount of Stokes' shift is rather small, as compared to that observed in organic solvents, indicating that a significant amount of the solvation is ultrafast and is missed in the present experiment. The observed solvation dynamics is bimodal with fast (similar to 1 ns) and slow (> 10 ns) components. The ultraslow component is ascribed to the quasi-static water molecules in the Nafion membrane. The difference in the extents of apparently missing ultrafast components, between Me(4)N(+)- and Na(+)-substituted membranes is rationalized by a model involving the difference in distributions of the cations in the water channel.
ISSN: 1520-6106
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