Nafion linewidth

Figure 1 depicts the behavior of the chemical shift with %H 0. The shift change occurs over a range of about 130 ppm, rather large for the sodium ion (7). The behavior of the line-width is qualitatively similar to The shift behavior in Figure 1. For saturated Nafion, the linewidth is about 220 Hz at 30°C, and increases to thousands of Hz for very low (1-2%) water contents. Linewidth and chemical shift changes with decreasing water content are reversed with increasing temperature (9). The above chemical shift and linewidth behavior of the sodiun form of Nafion parallels that of the model salt CF SO Na as a function of...

Unlike the chemical shift, the Na linewidth of saturated Nafion is not close to that of the model salt. The 220 Hz line-width of saturated Nafion is an order of magnitude larger than that of CF SO Na. This suggests that some or all of the sodium... 

The amount of broadening experienced by alkali metal ions upon binding to a polymer such as Nafion will depend on several factors including nuclear spin, electric quadrupole moment, stability of the hydrated ion, and electronic polarizability. This last factor relates to the ease with which electric field gradients can be produced by binding to an anionic site. Based on the relative change in linewidth upon going from saturation to low water contents, the ions studied so far are related... 

State 2 would be expected to have an increased linewidth due to their proximity to anions. Of course, rapid exchange between States 1 and 2 would produce the single, broadened line seen for water-saturated Nafion. 

The variations observed in the spectra with changing water content (figure 4) imply a modification of the environment of the ferrous ion below 6 % water. [Fe(H20)6]2+ complexes are progressively dehydrated, but it is difficult to establish a precise, quantitative correlation between the hyperfine parameters and the number of water molecules in the vicinity of the ion. The influence of the structure of the Nafion itself becomes increasingly felt as the water concentration decreases, and certainly leads to a very broad distribution of environments for the ferrous ion, as may be seen from the variation in linewidth as the water content approaches zero. 

Alkali metal NMR spectra were observed at the appropriate resonance frequencies listed in Table I, using 12-mm tubes and a Varian XL-100 spectrometer with Gyrocode Observe capability. External 19F or internal H field-frequency lock was used. Depending on the linewidth of the resonance being observed, spectral widths of 256 Hz to 12 kHz in 8192 frequency domain points were used. For 23Na and 85Rb, 90° pulses of 50/isec and a 0.1-sec repetition rate were used. For 6Li and 133Cs, the approximately 55° pulses were 30 /xsec, and the pulse repetition rates for the Nafion samples were 60 sec and 1 sec, respectively. 

Table II. 23Na Chemical Shifts and Linewidths for Nafion SO Na+ as a Function of Water Content and Temperature0...

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.

Comparison of the data in Figures 3 and 4 shows that while the chemical shift approaches that of the model salt, the linewidth approaches a value one order of magnitude higher. This behavior suggests that Na+ ions in saturated Nafion are substantially restricted relative to the small molecular weight electrolyte, even when the contribution to the linewidth attributable to bound ions is taken into account. This result bears on the theoretical treatment of Nafion dissociation in terms of a four-state model (3). The bulk of the ions appears to be partially restricted (States 2 and 3 of Ref. 3), with a very small percentage bound as contact ion pairs (State 4 of Ref. 3). It is not known if any of the Na+ ions can be considered totally free. 

Figure 4. Plot of the 23Na resonance linewidth of Nafion (1100 equiv wt) vs. % HzO and the H20/Na molar ratio (20)...

In Table V is the temperature dependence of the 23Na linewidths of Nafion-Na+ for a number of equivalent weights. At all equivalent weights the resonance narrows with increasing temperatures. This behavior is indicative of exchange which is fast on the NMR time scale. With two exceptions (27° and 47°C at 1347 and 1500 equiv wt), the linewidth... 

The larger linewidths observed for the alkali metal forms of Nafion relative to the corresponding model electrolytes indicate that ionic mobility is restricted because of ion binding to the polymer in the aqueous regions. Without additional studies it is not possible to obtain an estimate of relative binding strength from the data in Table VI. However, experiments identical to those for 23Na are possible in order to probe the hydration properties of these other ions in Nafion. 

In Table VII are the relative H chemical shifts of water in Nafion at several water contents. The experiments were conducted on specially prepared Nafion spheres in order to eliminate bulk susceptibility effects. These spheres behaved the same as the corresponding Nafion films and powders in limited 23Na NMR experiments. The H linewidths are sufficiently narrow to allow accurate measurement of the chemical shift. With decreasing water content, the resonance shifts upfield, suggesting the breakup of water-hydrogen bonding as for NaCl. The relative shift of pure water and water in saturated Nafion is not known at this time. The increased linewidth indicates decreased water mobility, as seen for the sodium ions. Additional experiments using model electrolytes and a chemical shift standard are warranted. 

Table VII. Shifts and Linewidths of Water in Nafion—Na+ Spheres (1100 Equiv Wt) °...

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