R. Haruki et al., Dynamics of Fe cations in Nafion membranes studied by nuclear resonant quasielastic scattering of synchrotron radiation, J PHYS JPN, 69(12), 2000, pp. 4049-4054
Nuclear resonant quasielastic scattering from ferrous cations in ion-exchan
ge Nafion membranes was measured at room temperature, and the intrinsic dyn
amics of the cations inside the spherical regions surrounded by the sulfoni
c groups of Nafion was studied. For the analysis, the formula of nuclear re
sonant absorption for a particle diffusing in a sphere with an impermeable
surface was derived. The formula indicates that the effect of the surface u
pon the diffusive motion is small if the radius of the sphere is larger tha
n a critical value; in the case of Fe-57 ions, the value is about 2 Angstro
m. As the radius of the spherical regions in our Nafion samples is estimate
d to be about 5 Angstrom, the existence of the surface can be ignored and t
he intrinsic dynamics of Fe cations can be extracted. The diffusion coeffic
ient estimated from the least squares fit was (1.12 +/- 0.13)x10(-5) cm(2)/
s for 15 wt% water content. This value is in good agreement with that of Fe
cations in an HCL solution. As a result, Fe cations in Nafion diffuse as f
reely as those in solutions.