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

Abstract

Abstract Sufficient experimental material has been collected to show that the concentration potential E c of every asymmetric membrane, with a gradient of morphology and/or composition, must be different in dependence on the orientation of the membrane sides towards the more or less concentrated contacting solution. Experiments proving the orientation effects were carried out with stepwise annealed cellulose acetate membranes for ultrafiltration and reverse osmosis, with polyelectrolyte complex membranes, with maleimide-based copolymer 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 more dense side (skin) is in contact with the more concentrated solution. A bipolar membrane is in (+) orientation when its positively charged (anion-active) layer is in contact with the more concentrated solution. Generally, the values of E c are smaller with monopolar membranes and less positive with bipolar membranes in (+) orientation than in (−) orientation. This can be qualitatively explained by the well-known decrease in the E c values, if the activity of the solutions in contact with an ion-exchange membrane increases. The contribution of the dense part of an asymmetric membrane is decisive 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 (and the E c value) of a bipolar membrane must be less positive than in the other because the contact of the positively charged layer with the more concentrated solution leads to a screening of the positive charge by the electrolyte and vice versa. Thus, with the change of the orientation, a change in the absolute value and often also in the sign of the oriented concentration potential occurs. Quantitative relationships 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.