Nafion hydrated ionic clusters

The transport properties of hydrophobic-hydrophilic ion-exchange membranes such as Nafion are likely to alter with time when operated at high current densities. The thermodynamic tendency of the system to form large hydrated ionic clusters is realized by increasing the molecular energy (kT) of the polymer chains to such an extent that the activation... 

Figure 3 is a plot of the 23Na chemical shift change for Nafion (1100 equiv wt) vs. % water and the water-to-sodium molar ratio. Since changes in the first hydration sphere of the cation would be expected to cause the largest chemical shift changes, it is possible to estimate the number of water molecules in the first hydration sphere of Na+ (more properly, a Na+-S03" ion pair) in Nafion. One obtains an estimate of 3-4 water molecules per Na+ ion from the plot in Figure 3. Essentially the same value can be obtained from a similar plot for the linewidth data (Figure 4). Similar behavior has been observed recently for the 23Na linewidth in the sodium diisooctyl sulfosuccinate/water/heptane reversed micellar system (21) and in the caprylic acid/sodium caprylate/ water system (22). Hence, the results obtained here provide support for the presence of hydrated ionic clusters in Nafion.

Preliminary results of the MIR investigation of the molecular nature of the ionic cluster regions of Nafion perfluorosul-fonate membranes have been presented. An advantage of the MIR probe is the ability to monitor short-range dynamic aspects of the ionic-hydrate molecular architecture for equilibrium states. It has also been demonstrated how the resultant concepts can be translated into a molecular-based model that may be significant in providing a base for mathematical representations of various macroscopic physical properties. 

DPD simulations have been analyzed in view of the microsegregated structure of hydrated Nafion at various X (Hayashi et al., 2003 Vishnyakov and Neimark, 2005 Yamamoto and Hyodo, 2003). A typical structure is depicted in Figure 2.11. Size and separation distance of ionic clusters were found to increase approximately linearly with X. Upon increasing X, the membrane undergoes a percolation transition from isolated hydrophilic clusters to a 3D network of randomly interconnected water channels. Wu et al. (2008) applied extensive DPD simulations to compare the morphologies of Nafion, short sidechain (SSC) PFSA PEMs of Solvay-Solexis, and 3M... 

Nafion membranes are composed of hydrophobic-hydrophilic comonomers. When the ionic form of Nafion is exposed to water, some swelling takes place. However, in order to minimize the free energy of the system, water will be taken up in such a way as to avoid contact with the hydrophobic TFE units of the copolymer. At equilibrium, for a fixed structure, there will be a balance between the free energy of dilution (osmotic swelling) and the free energy of hydration of the TFE units. As a result of these two opposing effects, water of hydration is contained in ion clusters which reduces the necessity of water-TFE interaction and yields a system of minimum energy. Several models... 

The physical structure of Nafions salts has been explored by neutrons (SANS) and X rays (SAXS). The former method is sensitive to fluctuations in the coherent neutron scattering cross section while the latter detects fluctuations in the electron density. Such fluctuations arise in the Nafions from partial crystallization of the samples and from clustering of the ionic groups or water molecules in hydrated s lmples. 

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