TABLE OF CONTENTS Ionomer membranes allow several mass transport modes, not involving a net electrical current flowing through the membrane. Depending on whether the fluid on one or both sides of the membrane is in liquid or gas form, one can distinguish between dialysis, pervaporation and gas diffusion. These three modes of mass transport are possible in ionomer membranes as well as in membranes made of uncharged polymers. Donnan dialysis, on the other hand, is a special case of dialysis unique to ionomer membranes. All these processes are driven by differences in concentration (or partial pressure in the case of gases or vapors), rather than by differences in electrical potential or total pressure.
In Donnan dialysis, ions of the same charge move in opposite direction through an ionomer membrane driven by a difference in concentration of one or more species. The rates of transport have to be in balance to maintain electrical neutrality (zero current). However, even though there are no electrodes involved, electrical potential differences are still a factor, as illustrated in the following example [56]: Bandini describes a process for conversion of sodium phenoxide into undissociated phenols, in which a solution of the sodium phenoxide is placed on one side of a Nafion membrane and dilute hydrochloric acid on the other. Both sodium and hydrogen ions can diffuse through the membrane driven by the concentration gradients. Because the hydrogen ion is about 7 times as mobile as the sodium ion, its movement generates an electrical potential difference between the...
