Carboxylate membranes

Asahi s investigations showed that a Na+ concentration of 1.1 N was necessary in compartment II to maintain a current efficiency of 96% in the carboxylic membrane during operation with 3.5 N brine in compartment I and 32% caustic soda in compartment III. The Na+ concentration in compartment II was generally far lower than that in compartment I, and clearly indicates the tendency for depression of the Na+ concentration at the interface of the sulphonic and carboxylic layers in the normal... 

Carboxylic Membranes. PTFE films were irradiated in 50% aqueous solutions of acrylic acid. To prevent homopolymerization, 0.25% ferrous... 

Perfluorinated carboxylate membranes were introduced about seven years ago. These membranes can be synthesized by a variety of methods or by various chemical conversions from the Nafion polymer.Composite membranes which contain both sulfonate and carboxylate functional groups have also been produced (see Section IV.l for more details). These carboxylate membranes have been widely employed in the advanced membrane chlor-alkali cells. This major chemical technology is in the process of being revolutionized by the use of these materials, a remarkable accomplishment for such a small group of polymers. ... 

Potential applications of perfluorinated carboxylate membranes have been focused to date on the chlor-alkali process. It has been pointed out previously that these polymers in acid form are not desirable for electrochemical applications because of rather high resistance. ... 

Ion clusters are commonly observed in the ionized forms of the perfluorinated membranes. The size of the clusters appears to be larger for sulfonate than for carboxylate membranes." " The size increases in the order Na, and Cs" and decreases with increasing number of functional groups per chain and with increasing temperature.As in the case of ethylene ionomers, the perfluorinated carboxylic acid membranes do not form ion clusters, at least in the dry state." The electrostatic interaction may be too weak to form ionic clusters. These observations are expected according to the Eisenberg theory (see Section II.2). 

Figure 5 shows the electrolyte uptake of the sulfonic acid membrane as a function of electrolyte concentration for three different acid electrolytes. The electrolyte content of the membrane decreases with increasing acid concentration. As shown in Table 2, both sulfonate and carboxylate membranes sorb large amounts of other solvents as well, particularly alcohols and other protic solvents. "" " ... 

The self-diffusion coefficients of sodium and cesium ions have been measured for perfluorinated sulfonate (Nafion ) and carboxylate membranes of similar structure. The exchange-site con-... 

Figure 17. Sodium ion transport number vs. caustic catholyte solution for a perfluorinated carboxylate membrane ( ) anolyte is 5 M NaCl and (O) anolyte and catholyte are identical concentrations of NaOH. (Ref. 149 reprinted by permission of the publisher, The Electrochemical Society, Inc.)...

One can control an equilibrium amount of water uptake by the membranes and also the size of the ionic clusters by changing relative humidity of the membranes or by immersing the membranes in aqueous solutions of sodium chloride with different concentrations (11.). Figure 13 shows change of SAXS profiles upon immersing the sodium-sulfonated (a) and -carboxylated membranes having 1100 E.W. (b) into the aqueous sodium-chloride solutions with various concentrations. 

Upon swelling the carboxylic-acid and carboxylated membranes, the scattering angle 0max at which the Hy scattered intensity... 

Figure 15. Hv light-scattering patterns for the carboxylic acid and sodium-carboxylated membranes having 1100 EW under room-temperature dry state, and those immersed in HzO and C2H5OH.

It should be noted that the change of the spherulite size is reversible. That is, as shown in Figure 18 for the carboxylated membranes, the Hy pattern is expanded upon immersing the membranes in methanol and is contructed to original size upon deswelling the membranes. Upon deswelling, the stress is released to result in reconstitution of the spherulitic orientation correlation in the disordered peripheral regions. 

Figure 18. Reversible change of the Hv light-scattering pattern upon swelling and deswelling the sodium-carboxylated membranes having 1100 EW with CH3OH.

I, 1980, 76, 2558-2574 L.Y. Levy, A. Jenard and H.D. Hurwitz, Hydration and ion-exchange process in carboxylic membranes. Part 1. Infrared spectroscopic investigation of the acid membranes, J. Chem. Soc. Trans. 1, 1982, 78, 29-36 M. Falk, Infrared spectra of perfluorosulfonated polymer and water in perfluorosulfonate polymer, Perfluorinated Ionomer Membranes, ed. A. Eisenberg, H.L. Yeager, ACS Symposium Series, American Chemical Society, Washington DC, 1982, p. 139 C. Heitner-Wirguin and D. Hall, An infrared study of an anion exchange membrane,... 

I. Alexandrova and G. Iordanov, Transport of nickel and copper against a concentration gradient through a carboxylic membrane, based on poly(vinyl chloride)/ poly(methylmethacrylate-co-divinylbenzene), J. Appl. Polym. Sci., 1997, 63, 9-12. 

Bilayer carboxylate membranes can be produced by surface modification of Nafion-type membranes. In a process used by Asahi Chemical the sulfonate on a Nafion-type surface is reduced to sulfinic and sulfenic acids, then oxidized to a carboxylate layer of 2-10 pm thickness ... 

Aciplex (bilayered perfluorosulfonate and carboxylate membrane developed by Asahi Chemical (now Asahi Kasei)). 

As the ion-exchange capacity increases, the water content increases [51], as shown in Fig.4.8.7, the sulfonate membranes having a higher equilibrium water content compared to the carboxylate membrane. Shorter chain lengths in carboxylate membranes also result in lower water content. 

FIGURE 4.8.13. Dependence of condiictivity of carboxylate membranes on caustic concentration [51]. (With permission from Elsevier.)... 

At pH < 2, can convert active group on carboxylate membrane into nonconductive -COOH... 

Operation with Acidified Brine. Acidification of the feed brine improves the performance of the cells. As explained in Section 7.5.6.1, the improvement results partly from the elimination of carbonate ion, a precursor of CO2, and partly fix)m neutralization of some of the hydroxyl ion that passes through the membrane fiom the catholyte. At the same time, overacidification is a danger, because carboxylic membranes can protonate below pH 2. The feed brine can be at a lower pH. It is the pH of the anolyte that must be limited. Since carbonate decomposes at about pH 3, the benefits of acidification come in two stages. 

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