Water Absorption and Donnan Equilibria of Perfluoro Ionomer Membranes for the Chlor‐Alkali Process

Abstract

The water absorption and Donnan equilibria of perfluoro ionomer membranes for chlor‐alkali production were thermodynamically analyzed, utilizing an elastic matrix model. To relate these equilibria, two single parameter model equations were developed that express effective fixed ion concentration and swelling pressure. The empirical parameters α and β were chosen to give the best fits to the observed water absorption data by Nelder and Mead's modified simplex method of optimization. The value of α, which is related to ion pair formation and strong association in the ion cluster in the membrane, was in the range of 0.09–0.12 and was fairly uniform among the four membranes investigated. From the calculated value, it was concluded that the degree of ion pair formation increases sharply when the hydration number of sodium ion decreases below 4 and that it is higher for carboxylic acid membrane than for sulfonic acid membrane, increasing with the equivalent weight (EW) of the membrane and the external concentration. The parameter β, expressing the resistance of the polymer matrix against swelling, ranged from 0.14 to 0.55 and was larger for the carboxylic acid membrane than the sulfonic acid membrane. It was larger for the membrane with shorter and less bulky pendant structure to which the ion‐exchange group is connected. Conversely, it was found to be smaller for the pretreated membrane which exhibits a greater rate of swelling. Calculated swelling pressure was in the expected range, and sorbed electrolyte concentration agreed quite well with the observed value. The factors to be considered in membrane design and use in the chlor‐alkali process are discussed on the basis of these results.