Nafion alcohol uptake

In Fig. 6.15 are summarized the sorption isotherms of methanol, ethanol and propanol by Nafion [272,281 ], along with the data for water. It can be observed that for alcohol activity higher than 0.5 the alcohol sorption follows the order methanol > ethanol > propanol, as expected according to the size of the sorbate. At low alcohol activities the alcohol uptakes were nearly identical. The behavior of water uptake is different and could be due to the fact that in one case [272] the membrane was used without treatment, while Zhao et al. [281] performed the measurements on the expanded form. In the last case the sorption of water is higher than methanol, while for the untreated membrane more methanol is sorbed in the membrane as compared to water. The data by Jalani et al. [276] for a cast Nafion membrane, not shown in Fig. 6.15, indicates a similar behavior, with the uptake order at high activities being water tv methanol > ethanol > propanol. 

Fig. 6.16 Alcohol uptake of Nafion 117 at room temperatiue as a fimction of the alcohol concentration (O) t-hutanol ( ) 2-propanol (A) ethanol (V) methanol. (Adapted fiom Ref. [282])...

The authors explained this behavior by resorting to differences in viscosity between methanol and higher alcohols, but a look to Fig. 6.16 showing the alcohol uptake of Nafion 117 membranes would indicate that the solubility of the alcohols could be also responsible for the observed dependence of permeability on alcohol concentration. 

Mechanical Tensile Studies. The stress vs. strain curves for membranes having various silicon oxide solid uptakes for the four alcohol solvents are displayed in Figures 3-6. Also shown, for reference, on each of these figures is the tensile behavior of untreated Nafion in the dry add form. 

Datta and coworkers measured the sorption of methanol from vapor by Nafion all over the range of methanol activities at 25 °C [274] and 30 °C [275]. The uptake of ethanol and propanol from vapor by Nafion as a function of the alcohol activities were determined at 20 °C [272] and 30 °C [276]. Zhao et al. [281] reported uptake of methanol and ethanol by Nafion, at 40 and 60 °C, up to alcohol activities close to unity. 

Although the former results for the uptake of pure alcohols by Nafion give us an insight of the behavior of this membrane in relation to its use in DAFC, more clear evidence can be obtained by analyzing the uptake from a liquid water-alcohol mixture, and also firom the thermodynamic parameter known as the partitiOTi coefficient, defined by Eq. 6.5. 

Saarinen et al. [282] observed that the total uptake by Nafion 117 from a water-methanol solution reaches a maximum at x = 0.8, that is, shifted toward more concentrated methanol solutions as compared with former results. However the treatment of the membrane before measurements was not reported in this case. These authors performed uptake measurements for ethanol, 2-propanol and tert-butanol aqueous solutions. The maximum total sorption decrease inversely with the alcohol size (x 0.7 for ethanol, x = 0.25 for 2-propanol, and x = 0.10 for t-butanol), as can be observed in Fig. 6.16. The total mass up taken from the ethanol-water mixture is similar to that reported by Song et al. [286] for dilute solutions (x < 0.2). Godino et al. [287] have studied the liquid uptake of methanol, ethanol, 1-propanol, and 2-propanol aqueous mixtures and up to molar fractions 0.36 for methanol, 0.28 for ethanol, and 0.23 for propanol. The results for methanol and ethanol agree with the previous ones, while in the case of 2-propanol they observed a maximum total uptake at x w 0.15, and for 1-propanol the maximum seems to be close to x = 0.23. 

Verma and coworkers [90-92] studied the methanol and ethanol uptake in Nafion/Nd203, Nafion/Nd triflate, and Nafion/Er triflate. In all the cases they observed a slight increase of the mass total uptake with alcohol concentration (between 0.5 M and 2 M) and a very small increase of the uptake with the inorganic... 

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