Structural properties of cation exchange membranes: Characterization, electrolyte effect and solute transfer

Experimental investigations have shown that the presence of electrolytes has a strong influence on the transfer of neutral organic solutes through ion exchange membranes used in electrodialysis. It was further demonstrated that this influence is due to the impact of the membrane counter-ions on the noncovalent interactions existing between the hydrated and charged polymer chains in the membrane. The aim of this work is to investigate the structural properties of hydrated CMX membranes equilibrated with different counter-ions. Different methods, such as Infrared spectra (IR), contact angle and Differential Scanning Calorimetry (DSC) measurements, were used to characterize the membrane samples soaked in different electrolytes. In addition, IR spectra were calculated using a quantum mechanics approach and compared with the experimental ones. Shifts of characteristic IR peaks as function of membrane ionic form were observed in both experimental and computed spectra. Both spectra present shifts to lower wavenumber in presence of cations with higher hydration number. The contact angle of CMX membranes also increases in presence of more hydrated ions revealing a decrease of the hydrophilicity of the membrane. Concerning DSC, the freezing temperature of the water entrapped in the membrane polymeric network soaked with different electrolytes was measured. A shift at lower temperature was found for more hydrated trapped ions. The computational and experimental membrane structural properties were correlated with the corresponding transfer properties (sugar fluxes) and a good agreement was obtained.