The influence of absorbed methanol on the conductivity and on the microstructure of ion-exchange membranes

In this paper, we have measured the conductivity of two cation-exchange membranes (Nafion®117 and CM2), as a function of methanol content (XMeOH) and lithium chloride concentration in the external solution. The experimental data are treated and analyzed using the microheterogeneous model, leading to the determination of volume fractions of different structural regions: the gel phase containing the polymer chains with fixed sites and only counter-ions, the inter-gel phase formed by the electroneutral electrolyte solution filling central regions of the pores, and the hydrophobic domains of polymer chains free from functional sites. The variations of the membrane conductivity and of the inter-gel volume fraction (f2) versus XMeOH are explained by different physicochemical interactions between water, methanol, counter-ions, functional sites, and membrane material. The decrease in the membrane conductivity is attributed to the diminution of the dielectric constant in presence of methanol, and so to the formation of ion-pairs between counter-ions and functional sites in the gel phase and between counter-ions and co-ions in the inter-gel phase. We have confirmed the higher acidity of the functional groups in Nafion compared to the CM2 membrane. For low XMeOH values, the decrease of f2 versus XMeOH is attributed to the transfer of methanol molecules from the inter-gel phase to the gel phase in order to maintain the solubility of the different ions (functional sites and counter-ions). For high XMeOH values, due to the formation of ion-pairs, the gel phase becomes almost uncharged, and so methanol molecules come back to the inter-gel phase, leading to an increase of the f2 values.