Equilibrium and Diffusion of Methanol and Water in a Nafion 117 Membrane

The uptake concentrations of methanol and water in a Nafion membrane are measured as a function of methanol mol fraction from a liquid methanol–water mixture. The results are fitted by a thermodynamically consistent equation, derived from an expression of the Gibbs energy. The uptake of pure methanol and pure water from the vapor phase is measured as a function of vapor pressure. A discontinuity, known in the literature as Schroeder's paradox, between membrane uptakes of pure methanol and pure water from the liquid and vapor phases is noted. Regions of unconditional instability are enveloped by spinodal loci for the ternary mixture in the membrane when equilibrated with liquid mixtures. The effective diffusion coefficient is measured at 60°C for diffusion from a vapor into a saturated Nafion membrane. The effective diffusion coefficients are found to be on the order of 10⁻¹¹ m²/s, which is comparable to other measurements made for diffusion of a vapor through the membrane.