Combined effect of salt concentration and pressure gradients across charged membranes

The combined effect of both concentration and pressure differences on electrical potential (Deltaphi) for two ion-exchanger membranes, one positively charged (AE) and another negatively charged (CE), measured with the membranes in contact with NaCl solutions was studied. Results show a linear dependence between Deltaphi and pressure, independently if DeltaC and DeltaP have the same or opposite directions.

The ratio of the streaming potential for cation/anion exchange membranes is r = (2.1+/-0.4). A "bipolar" membrane (BM) was obtained by joining together both ion-exchanger membranes. In order to correlate the behaviour of the BP membrane with that corresponding to each sublayer, the same kind of measurements was carried out for both opposite external conditions, this means, applying the pressure on the cation exchanger (CABM) or on the anion exchanger membrane (ACBM), respectively.

From values obtained at DeltaP = 0, the counter-ion transport number in each ion-exchange membrane was obtained and the contribution of membrane potential on Deltaphi values can be evaluated. Results show clear differences on both the membrane potential and the effect of pressure in the bipolar membrane depending on these two external situations, and in this case the ratios are: r((CABM/ACBM)) = (2.2 +/- 0.4), r((CABM/CE)) = (5.0+/-1.2) and r((ACBM/AE)) = (4.9+/-0.3); higher influence of the anion exchange sublayer on the electrical potential difference values was obtained.

A reduction around 75-85% in the transport number of the ions in the bipolar membrane with respect to that corresponding at each charged sublayer was estimated.