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3. Overview of Ion Exchange Membranes Ion exchange membranes (IEMs) represent the key components of all electromembrane processes. IEMs are typically thin polymeric films containing fixed charged groups which are ionisable in water [10, 57]. On the basis of the presence of positive or negative charged groups, these membranes can be firstly classified into anion and cation exchange membranes, respectively. A cation exchange membrane (CEM) is characterized by the presence of fixed negative charges. Thus it is able to let positive ions (counter-ions) move across it and to block anions (co-ions). Conversely, the anion exchange membrane (AEM) blocks cations allowing the transport of anions. This exclusion principle, firstly theorized by Donnan [58], represents the fundamental IEM feature that makes the operation of all electromembrane processes possible. The following sections are intended to give a general overview of IEMs fundamental theory, manufacturing and characterisation. For a more detailed insight on these topics, readers can refer to some more specifically focused reviews [59–61]. 3.1 Donnan equilibrium and membrane potential When an IEM is in contact with an electrolyte solution, the fixed charges on the surface of the IEM attract counter-ions by Coulomb forces and generate an electrical field. The Donnan exclusion [49, 62] causes a sharp change of concentration at the IEM-solution interface, thus generating a very thin charged region called electrical double layer (nanoscale, Debye length), where the counter-ions neutralize the fixed charges, and their concentration is much higher than the co-ions concentration (Figure 4). Therefore, counter-ions tend to diffuse from the IEM to the electrolyte solution while co- ions diffuse in the opposite direction. However, the large gradients of chemical potential are counter- balanced by a large gradient of electrical potential, the so-called Donnan potential, so that the electrochemical potential is conserved (Donnan equilibrium) and the net flux of ions is null. 11PDF Image | Electrodialysis for water desalination
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