Properties in Concentrated Solution Environments

Industrial applications of perfluorinated ionomer membranes such as the electrolysis of sodium chloride solution to produce chlorine and sodium hydroxide often involve the use of highly concentrated solutions at elevated temperatures. The optimization of these systems depends upon a sound characterization of membrane transport processes under such conditions. Sodium ion is the major current-carrying species through the membrane in a chlor-alkali cell, and 

The self-diffusion coefficient of sodium ion is plotted in Fig. 11 vs. the reciprocal of absolute temperature for the perfluorosulfo-nate and perfluorocarboxylate membranes which were discussed in Section Here though, the membrane environment con- 

In 5 M NaOH, the carboxylate now shows slightly lower values of the diffusion coefficients compared to the sulfonate membrane. This pattern is repeated for values measured with 4 M and 5 M NaCl solutions as well. At higher NaOH solution concentrations, values for the carboxylate become much smaller, averaging 

The results of ion and water sorption measurements for the two polymers under these solution conditions help to explain this difference. Table 5 lists the concentrations of various sorbed species and the mole ratio of water to cation/anion in the polymer phase for NaCl and NaOH solution environments. This ratio decreases both in the polymers and in solution with increasing concentration. In solution, the ratio varies from 10.8 to 4.0 over the concentration range of 5-12.5 M NaOH, so that ions in the polymer phase exist in a significantly less aqueous environment compared to the solution phase. As noted by Mauritz and co-workers for perfluorosulfonate membranes, these water contents are insufficient to provide even primary hydration spheres for sodium ions, sorbed anions, and exchange sites, and the likelihood 

Equilibrium Sorption of Sodium Ion, Anion, and Water in Perfluorinated Carboxylate and Sulfonate Polymers, 80 C  

---