Combined effect of salt concentration and pressure gradients across charged membranes

The combined effect of both concentration and pressure differences on electrical potential (Df ) 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 D f a nd pressure, independ ently if D C a nd D P h ave 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 measurem ents 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 ), respec tively. From values obtained at D P = 0 , the counter-ion transport number in each ion-exchange membrane was obtained and the contribution of membrane potential on D f v alues 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.

Concentration and pressure effect on two ionexchange membranes were separately measured by using NaCl solutions. The combined effect of both concentration and pressure gradients on two oppositely charged membranes and a bipolar one obtained by joining together both single charged membranes was studied. Streaming potential coefficient and ion transport numbers for the two opposite orientations of the bipolar membrane have been correlated with those obtained for each single charged layer. Results indicate a greater influence of the negatively charged sublayer on the behaviour of the bipolar membrane.