Orientation effects on bipolar and other asymmetric membranes as observed by concentration potentials

Suffcient experimental material has been collected to show that the concent ration potential E-c of every asymmetric membrane, with a gradient of morph ology and/or composition, must be different in dependence on the orientatio n of the membrane sides towards the more or less concentrated contacting so lution. Experiments proving the orientation effects were carried out with s tepwise annealed cellulose acetate membranes for ultrafiltration and revers e osmosis, with polyelectrolyte complex membranes, with maleimide-based cop olymer membranes, with membranes of blends from polysulfone with sulfonated poly(phenylene oxide) and, last but not least; with bipolar membranes. We denote as (+) orientation of a monopolar membrane the situation when its mo re dense side (skin) is in contact with the more concentrated solution. A b ipolar membrane is in (+) orientation when its positively charged (anion-ac tive) layer is in contact with the more concentrated solution. Generally, t he values of E-c are smaller with monopolar membranes and less positive wit h bipolar membranes in (+) orientation than in (-) orientation. This can be qualitatively explained by the well-known decrease in the E-c values, if t he activity of the solutions in contact with an ion-exchange membrane incre ases. The contribution of the dense part of an asymmetric membrane is decis ive for the overall value of the membrane E-c. Therefore, the decrease in E -c of this layer is also decisive for the decrease in the E-c value of the membrane. With bipolar membranes, a similar principle is valid. In the (+) orientation, the overall charge land the E-c value) of a bipolar membrane m ust be less positive than in the other because the contact of the positivel y charged layer with the more concentrated solution leads to a screening of the positive charge by the electrolyte and vice versa. Thus, with the chan ge of the orientation, a change in the absolute value and often also in the sign of the oriented concentration potential occurs. Quantitative relation ships for oriented concentration potentials based on a Donnan analysis were derived. Estimates of ion activities in the junction between two monopolar layers of various bipolar membranes were calculated. (C) 2001 Elsevier Sci ence B.V. All rights reserved.