Nafion^®, the most commonly employed ion exchange polymer, serves as both a separator and a sensor platform. After more than 30 years of use, properties of Nafion have been measured and various models of Nafion have been proposed.

Many properties of Nafion have been measured as a function of water content and hydration [1]. This includes selectivity of alkali metal cations over protons, glass transition temperatures, decomposition temperatures, mobilities, transport numbers, conductivity, solvent swelling, gas solubilities, mechanical properties, and density.

Here, a simple thermodynamic sketch of Nafion is presented that characterizes its varied properties.

Density specifies water content that varies with the intercalated cations in both Nafion membranes and recast Nafion films [2,3]. Nafion is a known biphasic material composed of interconnected solvent filled domains and fluorocarbon domains. Because the dielectric constant reflects the ability of a matrix to support ionization, ions are found in the solvent filled domains. From the density and equivalent weight of Nafion, ion concentrations in the solvated domains are found.

Electrochemical potential μ_j^α for the ions is calculated based on the ionic strength in the solvated domains.

μ_j^α = μ_j^α,0 + RT ln a_j^α + z_jFφ = μ_j^α,0 + RT ln Υ_j^αc_j^α + z_jFφ

Because common models such as Debye Hückel, extended Debye Hückel, and Davies are restricted to ionic strengths well below those found in the solvated domains of Nafion, later models such as those of Stokes and Robinson are used to derive an expression for the activity coefficient.

The activity coefficient equation is used to fit literature data for selectivity, solubilities, swelling, and thermal, transport, and mechanical properties. Self consistent values for waters of hydration (and solvent molecules of solvation) are found for monocations across the literature on numerous measured properties reported as a function of solvent content.

This thermodynamic sketch of Nafion allows calculation of Nafion properties based on activity coefficients and electrochemical potential. The model also provides a practical means for design of Nafion materials with selected properties.

References

[1] J. Leddy, Modification of Nafion Membranes: Tailoring Properties for Function, ACS Symposium Series, Nanomaterials for Energy, edited by J. Liu, S. Bashir, and L.-D. Chen (2015).

[2] K.J. Oberbroekling, D.C. Dunwoody, S.D. Minteer, J. Leddy, Anal. Chem. 2002, 74, 4794-4799.

[3] L.A. Zook, J. Leddy, Anal. Chem. 1996, 68, 3793-3796.