Morphological changes and facilitated transport characteristics for nafion membranes of various equivalent weights

Perfluorosulfonic acid materials such as Nafion(1) that have been exchanged with Ag(I) ion have been shown to exhibit high selectivities with respect to membrane separations of mixtures of unsaturated organic compounds. This unusual behavior cannot be explained solely by the interaction of the unsaturate with Ag(I) ion. The membrane morphology appears to also contribute to the observed selectivities, To better understand the connection between morphology and membrane selectivity, Nafion materials with equivalent weights (EW) ranging from 1100 to 1500 g/equiv were characterized through their uptake of water and through small-angle X-ray scattering (SAXS) studies for materials in the dry, Ag+ forms and hydrated, Na+-forms. The water contents decreased with increasing EW from 34% to 10%. The SAXS data indicated that the ionic cluster sizes were similar (1.6 +/- 0.1 nm) for the different materials in their dry, Ag+ forms but decreased slightly with increasing EW in the hydrated forms from 4.4 to 3.6 nm. Fluxes of 1,5-hexadiene and 1-hexene through the hydrated Ag+ forms of these materials decreased with increasing EW, but both the ideal and observed separation factors, while different, maintain approximately the same values for each membrane evaluated. The amount of alkenes absorbed into these materials from single-component or mixed feed solutions per mol of Ag+ sites was virtually the same for all materials. This result appears to be the basis for the observed high selectivities. Calculations based on the water contents and SAXS data show that for the series of Nafion materials the distances between ionic clusters vary by only 6% for the dry membranes and by only 3% for the hydrated membranes.