Solubility and transport behavior of water and alcohols in Nafion™

Sorption isotherms, concentration dependent diffusion coefficients and immersion solubilities were determined for water and three alcohols (methanol, ethanol, propanol) at 32 degrees C in acid-form Nafion(TM) films. Apparent diffusion coefficients for water from vapor sorption kinetics exhibit a maximum at low concentrations, are higher for desorption than sorption and are below the reported immersion value. This anomalous behavior is attributed to nonisothermal conditions arising from the heat of condensation of water vapor and the time-dependent volume response to changes in vapor concentration, as well as, mass transfer limitations. To overcome these complications, diffusion coefficients were determined from steady state permeabilities combined with solubilities. Nominal diffusion coefficients, D-nom, from boundary layer corrected membrane resistances, exhibited characteristic differences in the magnitude and concentration dependence of the diffusion coefficient for water and the three alcohols, but D-nom for the liquids were far higher than for unit activity vapor. More detailed corrections for changes in the sweep and carrier vapor concentrations along the sample length and across the boundary layer were carried out to obtain effective diffusion coefficients, D-eff, With improved agreement between vapor and liquid. For water, D-eff increases smoothly and continuously with concentration. For the alcohols, D-eff exhibits three distinct regions of differing slope. These comparisons and the marked solubility increase with small amounts of water in the alcohols, suggest that water interacts more strongly with the sulfonic acid residues, while the alcohols preferentially solvate the fluoroether side chain and cause structural change which is responsible for the rapid D-eff increase in a narrow concentration range. (C) 2000 Elsevier Science Ltd. All rights reserved.