Effect of the polar–nonpolar liquid mixtures on pervaporative behavior of perfluorinated sulfonic membranes in lithium form

• Nafion and IonClad membranes were studied in contact with polar–nonpolar liquid mixtures.
• Ion-pairs dissociation phenomenon of perfluorinated sulfonic ion-exchange membranes in Li+-form was investigated.
• Higher selectivity of Nafion membranes towards methanol and IonClad membrane towards water was reported.

Two ion-exchange membranes possessing perfluorinated backbone and sulfonic groups (i.e. Nafion® 120 and IonClad™ R4010) with lithium(I) counter-ions were investigated. The interactions between solvents of different polarity and the ion-exchange membranes with various morphologies were taken into account in order to better understand solvation and dissociation phenomena of the ion-pairs.Pervaporation of polar (i.e. water, methanol)–nonpolar (i.e. methyl acetate, dimethyl carbonate) liquid mixtures was carried out. It was revealed that the increase of the polar component concentration above 2 wt% in the feed mixture leads to dissociation of ion-pairs in Nafion membrane, which is reflected by the rapid increase of the polar component partial flux. In the case of IonClad membrane the dissociation of the ion-pairs during pervaporation was observed only when water was a polar feed component.The dissociation of ion-pairs was also evidenced in infrared study by observing the shift of symmetric stretching vibrations (νs) bands of sulfonic groups to the lower wavenumbers, compared to the membrane in the dry state. The symmetric stretching vibrations (νs) bands of the dry Nafion membrane and membrane solvated with water and methanol were equal to 1071 cm−1, 1058 cm−1, and 1054 cm−1, respectively. In the case of IonClad membrane the symmetric stretching vibration (νs) bands changed in contact with water from 1047 cm−1 (dry membrane) to 1037 cm−1. The dissociation of the ion-pairs did not occur in IonClad membrane equilibrated with methanol, which is also consistent with the result obtained during pervaporation.