## 2 Structure of Ion-Exchange Membranes
Sometimes, commercial IEMs contain several layers (anisotropic membrane structure), namely, a thin skin layer and a supporting layer [18] The skin layer serves to give the membrane some special properties: antifouling resistance [19] or monovalent ion permselectivity (e.g., Neosepta monovalent ion permselective ACS and CMS membranes produced by Astom, Japan) [20; 21; 22] or other [23]. Bipolar membranes consist of two layers, a CEM and an AEM [6; 7; 24; 25; 26; 27]. As it is mentioned above, the terms \"homogeneous\" and \"heterogeneous\" IEM are commonly used in the field of IEMs to distinguish between the membranes produced using the paste method and the membranes made via the hot pressing of powdered resins and inert binder. However, \"homogeneous\" membranes are not strictly speaking homogeneous. For instance, Nafion(r) is a perfluorinated sulfonated ion-exchange dense polymer generally considered as homogeneous [28]. Figure 1: SEM images of surface and cross-sections of IEMs; (**a**) a homogeneous Neosepta® AMX (Astom, Toukyouto, Japan, 2016) AEM, a styrene-DVB copolymer [12], (**b**) a heterogeneous MA-40 (Shchekinoazot, Russia, 2016) AEM; 1: particles of ion-exchange resin; 2: polyethylene; 3: reinforcing cloth. Reproduced with permission from [4], [PERSON], 2012. Nevertheless, when swelled in water, it contains hydrophilic pores/channels confined within a hydrophobic matrix [5]. These pores/channels provide for the transport of ion and water through a Nafion membrane. The presence of internal solution within a hydrophobic matrix allows one to speak about phase separation [5]. From this point of view, this membrane is multiphase and heterogeneous, however, this heterogeneity appears on a scale of the order of 10 nm. The main structural feature of the functional part of IEMs (which is an ion-exchange resin in the case of heterogeneous membranes) is the hydrophobic backbone of hydrocarbon or perfluorinated chains, which antagonizes the hydrophilic character of the charged functional groups. Due to the flexibility of the polymer chains, this combination provides self-organization during the membrane formation [29]. Polymer chains form the matrix of the membrane, whereas the functional groups are aggregated in clusters. The size of the clusters is of the order of a few nanometres. Consequently, the hydrophobic components contribute to the morphological stability of the membrane. The hydrophilic domains create a system of conducting channels. When placed in an aqueous solution, hydrophilic functional groups are hydrated and the membrane swells. The size of clusters increases, conducting channels appear between them and they form a percolation system when a certain water content is reached [30]. The water content and degree of swelling depend on the external solution concentration: with increasing solution concentration water content of an IEM decreases [30; 31]. When a membrane contains weakly basic functional groups, such as secondary and tertiary amino groups, there is a dependence of water content on the external solution pH [31; 32; 33]. The model of clusters and channels first proposed by [PERSON] [34] describes these main features and explains correctly the transport properties of IEMs [5; 30; 35]. A rather realistic presentation of two-dimensional structure of Nafion(r), which generally complies with the [PERSON] model, is made by [PERSON] [36]. The clusters, channels, some defects of the structure, spaces between ion-exchange resin particles, the binder and the fabrics of the cloth form a system of pores in an IEMs, whose size varies from a few nm to 1-2 \(\upmu\)m [13; 37; 38; 39]. The investigations of the pore size distribution made using the standard contact porosimetry ([PERSON] et al. [40]; [PERSON] et al. [37] and [PERSON] et al. [38; 39]) and differential scanning calorimetry (DSC)-based thermoporosimetry ([PERSON] et al. [41]; [PERSON] et al. [42]; [PERSON] et al.