J. Phys. Chem. B 2008, 112, 8549–8557
8549
Electron Spin Resonance Investigation of Microscopic Viscosity, Ordering, and Polarity in Nafion Membranes Containing Methanol-Water Mixtures Jamie S. Lawton, Eugene S. Smotkin, and David E. Budil* Northeastern UniVersity Center for Renewable Energy Technology (NUCRET), Department of Chemistry and Chemical Biology, Northeastern UniVersity, Boston, Massachusetts 02115 ReceiVed: January 9, 2008; ReVised Manuscript ReceiVed: April 2, 2008
Electron spin resonance (ESR) was used to monitor the local environment of 2,2,6,6-tetramethyl-4-piperidone N-oxide (Tempone) spin probe in water and methanol mixtures in solution and in Li+ ion exchanged Nafion 117 membranes. Solution spectra were analyzed using the standard fast-motion line width parameters, while membrane spectra were fitted using the microscopic order macroscopic disorder (MOMD) slow-motional line shape program of Freed and co-workers. The 14N hyperfine splitting, aN, which reflects the local polarity of the nitroxide probe, decreases with increasing methanol concentration, consistent with the decrease in solvent polarity. The polarity depended only weakly on composition in the Nafion membrane, but was noticeably more temperature-dependent. The microviscosity of the membrane aqueous phase as reflected by the rotational correlation time (τc) of the probe, was nearly 2 orders of magnitude longer in the membrane than in solution and varied by an order of magnitude over the composition range studied. The probe exhibits significant local ordering in the aqueous phase of Nafion membranes that is diminished with increasing methanol concentration. Introduction Direct methanol fuel cells (DMFCs) offer many attributes that suit them nearly ideally for portable power applications.1–5 A limitation to realizing the full potential of DMFCs is methanol diffusion across the proton exchange membrane (PEM) of the cell. Crossover methanol is catalytically oxidized by oxygen at the cathode, causing a mixed potential and reducing available catalytic sites for oxygen reduction. This lowers the cell voltage and reduces fuel and oxygen utilization.6 The best PEMs currently available for application in DMFCs are perfluorinated sulfonic acid polymers, such as Nafion, a product of Dupont. The structure of hydrated Nafion has been studied extensively and many descriptions of its microscopic phase structure have been formulated. It has been established that the ionic groups at the end of the side chains of the perfluorinated backbone form clusters within the perfluorinated polymer matrix.7,8 These clusters form a network of aqueous phase channels that permit significant swelling by polar solvents and efficient ionic transport through the nanometer-scale domains. Nevertheless, details of the microstructure of Nafion at the molecular level that may influence methanol crossover remain undetermined. Because of its potential application in DMFCs, special interest has been given to studies of the structure of Nafion solvated with methanol and in the presence of methanol-water mixtures. Atomic force microscopy (AFM) studies have shown that a nearly smooth surface is observed on Nafion soaked in methanol while a rough surface dotted with ionic clusters is observed on Nafion soaked in water.9 Small-angle X-ray scattering (SAXS) data suggest that one contrast between Nafion membranes swollen with water and Nafion membranes swollen with methanol is that methanol diffuses into the hydrophobic region of the perfluorinated ether side chains and causes swelling which limits the size of the ionic clusters.10 Radial distribution * To whom correspondence should be addressed. Fax: 617-373-3697. E-mail:
[email protected].
functions calculated from molecular mechanics simulation studies of Nafion solvated with water and methanol show the methanol oxygen atom in closer proximity than the water oxygen atom to the ether oxygens on the side chains while the distances of water and methanol oxygen atoms to the atoms of the sulfonic end group are similar.11 While SAXS and AFM are surface techniques, electron spin resonance (ESR) is a powerful tool that can be used to study phase behavior of the membrane in the bulk through the use of spin probes or spin labels. Schlick and co-workers have studied the effect of hydration and methanol on the morphology of Nafion membranes using copper(II) as a spin probe.12,13 They found ESR line shape changes that indicated Cu2+ aggregation in membranes with methanol-water mixtures containing below 20% methanol by volume, but not above. Since cations are expected to interact directly with the anion in the ionomer, they concluded that the observed aggregation reflects changes in the polymer morphology. The same group has also carried out several studies of hydrated Nafion solutions and membranes using various nitroxide spin probes to characterize local polarity, phase behavior, and probe mobility.14–17 Specifically, the magnetic parameters of the nitroxide, most notably the isotropic 14N hyperfine splitting, a , provide a probe of the effective local N polarity.18–23 Analysis of the ESR line shape provides information on the rotational correlation time of the probe,24 which may be used to estimate the local or microscopic viscosity of the probe’s environment.25 Application of line shape analysis in terms of the model of microscopic order, macroscopic disorder (MOMD)26 can also characterize the degree of local ordering in the polymer or membrane.27,28 Studies by Ren et al.,29 Dohle et al.,30 and Hallinan and Elabd31 have demonstrated that at low methanol concentrations the bulk diffusion coefficient of methanol increases with methanol concentration. However, this result contrasts sharply with tracer diffusion studies of methanol in simple methanolwater mixtures, which show a decrease in the self-diffusion coefficient of methanol with increasing methanol concentration
10.1021/jp800222c CCC: $40.75 2008 American Chemical Society Published on Web 06/26/2008
8550 J. Phys. Chem. B, Vol. 112, No. 29, 2008 up to xMeOH ≈ 0.3.32 To investigate this apparent discrepancy, we have carried out independent measurements of the local viscosity of the liquid phase of hydrated Nafion membranes by measuring the rotational correlation time τc of a nitroxide spin probe from its ESR line width. Our results confirm that the effective local viscosity increases with methanol content at xMeOH < 0.3 but decreases over the same composition range in Nafion membranes. We also present more detailed studies of the Tempone spin probe in Nafion membranes that demonstrate significant variation in local polarity and solvent ordering over the full range of water-methanol compositions. Experimental Section ESR samples were prepared with different methanol concentrations over a range of 0 e xMeOH e 1 with 0.25 mM 2,2,6,6tetramethyl-4-piperidone N-oxide (Tempone) spin probe. A second set of solutions was prepared with 0.5 M LiHSO4, and a third set was equilibrated Nafion 117 membranes (Ion Power, Inc., New Castle, DE). The Nafion 117 membranes were first purified by heating to 75 °C for 1 h in 3% hydrogen peroxide followed by 1 h in deionized water, 1 h in 0.5 M sulfuric acid, and 1 h in deionized water. The membranes were then Li+ ion exchanged by soaking in 0.06 M LiClO4 (Sigma-Aldrich, St. Louis, MO) for 2 weeks. Li+ ion exchange improved the stability of the Tempone, which is less stable in highly acidic environments. This allowed a considerably lower concentration of probe to be used than what is detectable in the acid form of the membrane. Studies of hydrated ion exchanged Nafion have shown that the density of the Li+ Nafion is similar to the H+ Nafion but the Li+ membrane is somewhat less conductive and absorbs on the order of three water molecules more per sulfonate group.33 Despite these differences, SAXS data show that monovalent cations do not significantly change the membrane morphology or microscopic swelling behavior.34 Structural studies of the polymer can therefore be achieved with fully ion exchanged membranes. The membranes were dried for 2 days and soaked in 0.25 mM Tempone solutions with different methanol concentrations over a range of 0 e xMeOH e 1. It has been shown previously that the mole fraction of methanol in Nafion 117 membranes varies linearly with the mole fraction of methanol in the equilibrating methanol-water solution with a slope close to unity.35 We therefore took the composition of the liquid phase of the membrane to be equal to that of the equilibrating solution. The LiHSO4 was titrated from molar equivalents of LiOH (Sigma-Aldrich, St. Louis, MO) and H2SO4. The samples were deoxygenated in a glovebag by bubbling with ultrahigh-purity argon that had been passed through an oxygen/moisture trap (model OT-4-SS, Agilent). The deoxygenated samples were placed in sealed quartz sample tubes and ESR spectra obtained on an X band Bruker EMX spectrometer with variable temperature control (Bruker ER 4111 VT). Spectra were collected for each sample in the range of 30-80 °C. For each spectrum, three scans of 2048 points were averaged using magnetic field modulation of 0.2 G at 100 kHz; a time constant of 20.48 ms, and a conversion time of 81.92 ms. Spectra were analyzed by two methods. Solution spectra in the fast-motion limit were analyzed using MATLAB36 to carry out least-squares fitting of three lines with the Voigt line shape37 to the mI ) 1, 0, and -1 14N hyperfine lines of the spectrum. The Voigt lines were calculated by direct numerical convolution of Lorentzian lines with a Gaussian inhomogeneous distribution, and the fitting procedure included small corrections for possible distortions from the spectrometer time constant and microwave
Lawton et al. phase. The three Lorentzian linewidths T2-1(mI) were used to calculate the standard A, B, and C parameters for fast-motional nitroxides38 using the definition T2-1(mI) ) A + BmI + CmI2. Analysis of the B/C ratio using the allowed-value equation of Kowert39 indicated nearly isotropic rotational diffusion of the spin probe. The isotropic 14N coupling constant aN was calculated as half the separation of the outer peaks of the spectrum. To calculate rotational correlation time from the fast-motion line width parameters, it was necessary to estimate the values of the magnetic tensors at each sample composition. As discussed below, the value of the isotropic 14N hyperfine splitting, aN, was observed to depend significantly upon the sample composition in both solution and membrane samples. This is consistent with the well-known sensitivity of both the nitrogen hyperfine interaction22 and the electronic g-values18,21,23 of nitroxides in solution to the polarity of a nitroxide’s local environment. Several groups have observed that the relation between the isotropic g-value, giso, and aN (or that between gxx and Azz) is approximately linear.18,19,21,23 This can be used to estimate all of the g and A tensor components for a given composition, since the only tensor components that are appreciably affected by the local polarity are gxx, gyy, and Azz.40 We make the approximation that ∆gx ) (gx - gz) is linearly related to ∆gy ) (gy - gz). Such a linear relationship is evident from the extensive data tabulation of Lebedev for different nitroxide spin probes,41 for which a linear fit gives ∆gy = 0.20∆gx. Using these linear relationships, values of gxx, gyy, and Azz were estimated from aN for a given sample composition. Reported slopes of g0 vs aN in protic solvents18,19,21,23 range from -3.2 × 10-4 to -4.0 × 10-4 G-1. [References 23–25 actually report the slope of gxx vs Azz; this may be normalized to the slope of giso vs aN using the factor of 1.20 to take into account the contribution of gyy to giso assuming the linear relationship between gxx and gyy described above.] Once principal values have been estimated for the A and g tensors, the isotropic rotational correlation time τc may be calculated from the B line shape parameter using the following relation38
B)
{
}
4 µBB0 3 g A τ 1 + (1 + ω02τ02)-1 + 15 p2 0 0 0 4
[
{
2g2A2τ2 1 +
}] (1)
3 (1 + ω02τ22)-1 4
where A0 and A2 refer to the spherical hyperfine tensor components:
A0 )
23 (A
zz -
1 (A + Ayy) ; 2 xx
)
1 A2 ) (Axx - Ayy) (2) 2
When ω02τ02 and ω02τ22 , 1 and in the case of isotropic motion where τ0 ) τ2 ) τc, eq 1 can be solved for τc.
τc )
15p2 B 7µBB0[g0A0 + 2g2A2]
(3)
Slow-motional spectra from the Nafion membrane were analyzed using a MATLAB-based version of EPRLL, the slowmotional line shape program of Schneider and Freed,42 which includes the MOMD model. The parameters that were varied during the fitting procedure included the isotropic Gaussian inhomogeneous line width parameter, the gxx, gyy, and Azz tensor components, the isotropic rotational diffusion constant R, and the orienting potential parameter c20. The remaining magnetic
Nafion Membranes with Methanol-Water Mixtures
J. Phys. Chem. B, Vol. 112, No. 29, 2008 8551 composition at which the polarity appears independent of T shifts to about xMeOH = 0.6. Rotational Correlation Time. According to the StokesEinstein equation for rotational diffusion,43 the rotational correlation time is related to the effective local viscosity η via the equation
τc )
Figure 1. Spectra of Tempone in Li+-exchanged Nafion membranes equilibrated with water-methanol mixtures at the indicated mole fractions of methanol, at room temperature.
parameters were fixed at the values gzz ) 2.0023, Axx/γe ) 5.0 G, and Ayy/γe ) 5.5 G. The isotropic rotational correlation time was calculated from R as τc ) (6R)-1. Results ESR spectra of Tempone in the Nafion membrane at 303 K are presented in Figure 1. The broad line shape observed in water and at lower methanol concentrations gradually narrows as methanol is added, reflecting an increase in the rate of rotation with increasing methanol concentration. Particularly at the slower motions, the outer peaks of the spectrum are significantly broader than the central line and manifest an asymmetry that is typical of microscopic probe ordering (i.e., the MOMD model). Effective Polarity. Figure 2 shows the dependence of the isotropic 14N hyperfine splitting aN as a function of composition for both solution and membrane samples. The aN measured in solutions containing LiHSO4 are not shown in Figure 3 but were slightly higher, on average by about 0.014 G, than those measured in solution without LiHSO4. A significant decrease in aN is observed with increasing methanol concentration, consistent with the expectation that aN should decrease upon going from the highly polar water solvent to the less polar methanol. In comparison, the local polarity in Nafion membranes reflected by the aN parameter exhibits a somewhat weaker dependence on composition. At low concentrations of methanol in the membrane, the effective local polarity is slightly lower than that for similar concentrations in solution, whereas, at high concentrations, the effective polarity is higher. The membrane also confers a slight but resolvable temperature dependence on the effective local polarity. The solution-phase data exhibit a slight temperature dependence that is barely resolvable beyond the experimental uncertainty. At low methanol concentrations the effective polarity of the solution decreases slightly with increasing temperature, whereas at xMeOH = 0.5 the effective polarity is essentially independent of T and at higher concentrations the effective polarity increases with increasing temperature. These weak trends are somewhat amplified in the membrane, and the
4π re3η 3kBT
(4)
where η is the effective local viscosity, re is the hydrodynamic radius of the rotating probe, and kB is Boltzmann’s constant. This relation is tested in Figure 3, which shows a plot of the τc measured for Tempone vs η/T for all temperatures and compositions of water-methanol mixture studied. Values for η were interpolated from the tabulation of bulk shear viscosity of water-methanol mixtures vs composition, temperature, and pressure by Kubota et al.44 Data for pure water are indicated respectively by squares in Figure 3. Although, eq 4 predicts a linear relationship between τc and η/T, the data exhibit marked nonlinearity. As discussed below, this curvature indicates that the effective local viscosity, or “microviscosity”, experienced by the probe differs from the bulk shear viscosity that is plotted in Figure 3. The solid line in Figure 3 shows the best-fit second-order polynomial function to the data, and the dotted line represents the limiting slope of this function as η/T f 0, which is approximately 4.23 × 10-10 s K cP-1. This corresponds to an effective hydrodynamic radius, re, ) 2.4 ( 0.3 Å. The best-fit line to the data for pure water (squares in Figure 3) has a slope of 4.06 × 10-9 s K cP-1, which is quite comparable to the limiting slope observed above, and yields essentially the same hydrodynamic radius. Once the hydrodynamic radius of the probe is determined, eq 4 can be used to predict the microviscosity of its environment from the rotational correlation time, τc. The τc values measured for the spin probe in Nafion membranes are nearly 2 orders of magnitude longer than those measured in solution, reflecting a correspondingly higher microviscosity in the aqueous phase of the membrane. More significantly, in the membranes τc decreases monotonically by an order of magnitude with increasing methanol concentration at all temperature studies, as shown in Figure 4C. This contrasts strongly with the observed behavior in solution, where the τc values follow the qualitative behavior of the bulk viscosity, initially increasing with methanol concentration and going through a maximum at approximately xMeOH = 0.3. The composition dependence is much more dramatic in the membrane as well: for example, at 30 °C τc varies by approximately (30% over the full range of compositions in solution, whereas in the membrane, τc drops by more than a factor of 10 upon going from pure water to pure methanol. As a control to test whether changes in viscosity are attributable to the ionic groups in the membrane, measurements were also carried out for solutions containing Li+ and HSO4ions at a concentration (0.5 M) comparable to that of the ionic groups in the membrane. As Figure 4B shows, τc exhibits behavior that is very similar to that in simple water-methanol solution, although the correlation times are slightly longer at a given temperature. Ordering. Figure 5 shows the ordering parameter c20 of the Tempone spin probe in the Nafion membrane as a function of solution composition. In pure water and at low methanol concentrations, significant ordering of the probe is observed (c20 > 0), while at high methanol the ordering essentially disappears. Comparison to Figure 4C shows that there is an inverse relationship between mobility and ordering of the probe: at
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Figure 2. Isotropic 14N splitting aN of the Tempone spin probe as a function of xMeOH in Li+-exchanged Nafion membranes (dotted lines) and water-methanol mixtures (dashed lines) at different temperatures, as indicated in the legend.
compositions for which there is significant ordering, the correlation time is also the largest, whereas short correlation times are observed in the absence of ordering. Discussion Polarity of Nafion Membrane vs Solutions. The most striking effect of the membrane on the local polarity of the nitroxide probe evident from Figure 2 is that it produces a much weaker dependence of the aN parameter on the sample composition than is observed either in simple water-methanol mixtures or in water-methanol-LiHSO4 solutions. The apparent local polarity in membranes equilibrated with pure water appears somewhat reduced relative to liquid solution. This result is unusual since one would expect the water in the membrane to have higher ionic strength due to the exchange sites in the membrane, consistent with our observation that aN does increase slightly in 0.5 M LiHSO4 solutions. One possible explanation of the decreased aN in the membrane relative to water solution is that it results from ordering of the probe relative to the charged groups in the membrane. Schwartz et al.22 have shown that electric charges placed at fixed locations relative to the NsO group do alter aN in a way that depends upon the distance of the charge from the NsO group, its orientation, and sign. Thus, at low methanol content where probe ordering is observed, partial alignment of Tempone relative to ionic groups in the membrane could account for the reduction in average aN in the membrane relative to water solution. Changes in the effective local dielectric constant of the solvent could also arise from alignment of the solvent dipoles in the solvation shells around the charged groups. Alternatively, the decreased aN in the hydrated membrane at low methanol concentrations could reflect the presence of two phases in the membrane with significantly different polarities. We discuss this possibility in greater detail below, after considering the membrane aN data at high methanol concentrations.
In contrast to the behavior at low methanol concentrations, the apparent polarity of the probe environment as reflected by aN appears to be significantly higher in the membrane than that in bulk liquid at high methanol concentrations. The apparent increase in polarity in the membrane was much larger than the small increase observed in 0.5 M LiHSO4. This indicates that the effect not only from the presence of charged groups but also from the structural features of the membrane. As noted above, the membrane seems to buffer the influence of composition on the apparent local polarity reflected by aN, maintaining it in a range intermediate to the values observed in pure liquid water or methanol. This could reflect an averaging of the aN value resulting from chemical exchange of the probe between membrane phases of significantly different polarities. However, two features of the data are not fully consistent with such an interpretation. First, the spectra do not reflect any relatively low mobility phase, such as would be expected for membranes equilibrated either with pure water or pure methanol. In the case of pure water, a less polar phase would include the hydrophobic chains of the Nafion polymer, which should be present in a glassy or highly viscous liquid (plastic) state over the temperature range studied. In the case of membranes equilibrated with pure methanol, some water molecules are retained by the dried membrane,45 remaining tightly bound and essentially immobilized in the solvent shells around the ionic groups. In both cases, one would anticipate a component in the spectrum that reflects motion that is much slower than what is observed experimentally. Second, the observation of an averaged spectrum rather than a superposition of spectra at high methanol concentrations imposes significant constraints on a two-phase model. Specifically, the exchange would have to be quite rapid, and the mobilities of the probe in each of the phases would have to be very similar in order to produce the observed spectrum. While
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J. Phys. Chem. B, Vol. 112, No. 29, 2008 8553
Figure 3. Correlation time τc of the Tempone spin probe as a function of η/T of a water-methanol mixture for all compositions and temperatures studied. Data from pure water (squares) are indicated by squares. The dashed curve is a second-order polynomial fit to the data, and the dotted line represents the limiting slope of the polynomial as η/T f 0, or 4.23 × 10-9 K s cP-1.
these conditions might be realized in a membrane containing two liquid phasessfor example, a water-rich and a methanolrich onesthey are unlikely when one of the phases exhibits low mobility. However, the results do not rigorously rule out the possibility of rapid exchange between two mobile phases in the membrane. A more definitive test of this possibility would be to utilize perdeuterated probes with substantially narrower inhomogeneous line widths that allow better resolution of small polaritydependent changes in the magnetic parameters. Better resolution could also be achieved by observing the spectrum at high ESR frequency, which would also be more sensitive to rapid exchange processes and also enable better resolution of polarity effects on the magnetic parameters of the spin probe. The observation of a weak temperature dependence of aN in solution is consistent with previous measurements of Schlick and co-workers,16 who attributed it to the known temperature dependence of the dielectric constant of water. The somewhat stronger temperature dependence that we observe in Nafion membranes is also consistent with their previous measurement in 6.6% (w/w) aqueous solutions of Nafion. This effect could result from the influences of ordering and reduced mobility that may reduce the value of aN in the membrane environment relative to water. The higher activation energy for solvent rotation in the membrane produces greater temperature dependence in the dielectric constant. Also in the membrane temperature would affect the motion of the polymer matrix, causing larger temperature dependence on the local electric field. Microviscosity in Nafion Membranes vs Solutions. As noted in the Results, the experimental data for methanol-water mixtures deviate significantly from the Debye-Stokes-Einstein equation (Figure 3). The observed curvature can be explained in terms of differences between the bulk shear viscosity of the
solution that is measured experimentally and the microviscosity that is experienced by the spin probe. Although the τc and bulk viscosity both exhibit similar trends, going through a maximum near xMeOH ≈ 0.3, the variation experienced by the spin probe is less extreme than that exhibited by the bulk solution. This increase in the bulk viscosity at xMeOH ≈ 0.3 has been attributed to incomplete mixing of methanol and water at the molecular level,46 which results in clustering of the water and methanol molecules, leading to increased frictional drag between the clusters. Clustering in water-methanol mixtures has been inferred from thermodynamic considerations,47 tracer measurements of mutual water-methanol diffusion,32 theoretical calculations,48,49 and experimental observations by a number of different methods.47,50,51 Perhaps the most detailed picture comes from neutron diffraction studies of solutions at xMeOH ) 0.7; intermolecular interactions between the water and the methanol lead to clustering of the methanol methyl groups, which were bridged together by the hydroxyl group hydrogen bonding with the water molecules.50 The bulk shear viscosity thus reflects changes in the size of the molecular clusters that occur with composition. In contrast, the viscosity sensed by the probe is a “microviscosity”. At the molecular level, the solvent is not a homogeneous environment and may not be as strongly influenced by solvent structures that are of a size comparable to or larger than the probe itself. Probe rotation is nearly 2 orders of magnitude slower in the membrane compared with solution, indicating correspondingly higher microscopic viscosity in the aqueous membrane phase. The relatively small effect of the ions on probe mobility in 0.5 M LiHSO4 solutions indicates that τc in the membrane is dominated by structural influences rather than the presence of charged groups in the membrane. The much larger microviscosity in the membrane also indicates that the small enthalpic
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Figure 4. Rotational correlation time, τc, of the Tempone spin probe as a function of temperature and liquid composition in (A) water-methanol, (B) water-methanol with 0.5 M LiHSO4, and (C) Li+-exchanged Nafion 117 membrane with water-methanol. Temperatures are indicated in the legend.
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Figure 5. Ordering potential coefficient c20 of the Tempone spin probe as a function of temperature and liquid composition in Li+-exchanged Nafion membranes, in energy units of kT. Local ordering of the probe is observed at lower concentrations of methanol but disappears as the concentration of methanol increases.
interactions that produce clustering in liquid solution either cannot be resolved or are negligible in the membrane environment. A number of effects can account for the very high microviscosity that is observed in the membrane. Molecular mechanics simulations have reported a local density of solvent oxygen atoms in the vicinity of the ionic group up to 2.2 times the density of the bulk solvent.11 The solvent molecules are also at least partially immobilized by interaction with the sulfonate groups of the polymer, leading to greater friction with the rotating probe molecule. Finally, the hydrodynamic drag experienced by the probe should be significantly affected by both the size and shape of the aqueous domain in which it is dissolved. A more significant observation is the order of magnitude decrease of τc, over the range of compositions from pure water to pure methanol. The difference in microviscosity in the membrane is much larger than the difference between the viscosities of the pure solvents, indicating that the change observed in the membrane must be a result of structural alterations in the membrane environment that change the free volume accessible to the probe. A possibility for the morphological change in the membrane is that methanol may absorb into the membrane in a different manner than water. Yeager and Steck52 proposed an interfacial phase based on their studies of the diffusion of different ions through the membrane. This interfacial region was described as vacant space with side chain material and residual water and sulfonate groups that were unincorporated into the hydrophilic
phase. Thus, it is possible that hydrophobic interactions with the methyl carbon of methanol cause it to adsorb onto such regions and become partitioned from the water. This model is supported by the SAXS experiments of Haubold et al.,10 which were explained by swelling of the regions around the Nafion side chains and shrinkage of the hydrophilic clusters at higher methanol concentrations. It is also consistent with recent Fourier transform infrared-attenuated total reflectance (FTIR-ATR) measurements by Hallinan and Elabd,31 who suggested that the preferential sorbtion of methanol into Nafion is primarily responsible for increased methanol flux through the membrane. If the main interaction of the hydrophobic regions of the membrane is with the nonpolar carbons of methanol, the nitroxide groups may interact more with the polar hydroxide groups of the methanol, consistent with our observation that the spin label occupies a membrane phase that appears much more polar than liquid methanol even at high methanol concentrations. Local Ordering in Nafion Membranes. The line shape analysis of the spectra from Nafion membranes in terms of the MOMD model provide clear experimental evidence for local ordering of the spin probe in the water phase of the membrane. The probe ordering in turn reflects the strong alignment that may be expected for the water dipoles in the hydration shells around the ionic groups of the membrane. The ordering decreases as methanol content increases and is not evident at higher methanol concentrations.
8556 J. Phys. Chem. B, Vol. 112, No. 29, 2008 The ordering observed at low methanol concentrations is consistent with the higher microviscosity that is observed in the hydrated micelle. The tight packing and ordering of the water molecules helps to restrict the rotation of the probe in aqueous environments. Ordering is not observed in compositions of high methanol content, suggesting that the methanol may disrupt interactions between water and the ionic groups in the membrane. This could occur if the methyl group of the methanol tends to associate with more hydrophobic regions of the Nafion chains while retaining its polar interactions with water, allowing the water to be more uniformly dispersed through the membrane. Schlick and co-workers have studied the effect of hydration and methanol on the morphology of Nafion membranes using copper(II) as a spin probe.12,13 Line shape changes indicated Cu2+ aggregation in membranes with methanol-water mixtures containing less than 20% methanol by volume. Since cations are expected to interact directly with the anions in the ionomer, they concluded that the observed aggregation reflects changes in the polymer morphology due to methanol. Our results are consistent with the possibility of methanol-induced changes in the shape of the inverted micelles in the membrane, although the neutral, soluble Tempone probe is more likely to reflect changes in the solvent structure surrounding the charged groups of the polymer than the cationic Cu2+ spin probe. Conclusion We have utilized a nitroxide spin probe to investigate the nature of the solution-filled nanopores in the hydrophilic region of Nafion 117 as a function of methanol content. Comparison of the probe behavior in the membrane and in solution has provided new insights into the molecular-scale influences of methanol on membrane structure. The local polarity probed by the isotropic 14N hyperfine interaction shows that the probe remains in a relatively polar phase of the membrane at all methanol concentrations. Measurements of probe mobility have confirmed initial measurements of methanol crossover in DMFCs that suggest methanol diffusion coefficients exhibit different dependence on methanol concentration in Nafion membranes relative to simple solutions. The extreme change in microviscosity in the membrane determined from the rotational correlation time indicates that the local structure of the membrane alters dramatically with methanol content. This is likely not a direct effect of the charged groups in Nafion since the presence of Li+ and HSO4- ions in simple methanol-water mixtures does not produce large changes in either the τc or the aN of the spin probe. Microscopic ordering in the membrane reflects preferential alignment in the solvated water shell that is abolished by addition of methanol, which may indicate a change in the size or shape of the inverse micelles with added methanol. The combined results support the idea that methanol fluidizes the membrane and substantially changes the local environment of the charged vesicles. The overall effect is an increase in the effective diffusion coefficients of all mobile species in the membrane. These results suggest a number of new ways to probe the behavior of proton exchange membranes on a molecular scale. A useful monitor of microscopic diffusion is Heisenberg spin exchange between diffusing spin probes, which is easily measured from the ESR spectrum. Comparison of the concentration dependence of HE in the membrane and in simple solution over a range of temperatures should yield new insights into the local morphology and connectivity of solvent domains in the membrane. The possibility remains that the spin probe undergoes rapid exchange between sites in the membrane with
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