Ion exchange membranes are considered the "heart" of electrodialysis and are a membranous ion exchange resin. There are many types of ion exchange membranes, and their structures can be simply divided into two parts: base membranes (polymer compounds) and active groups (including fixed groups and counter ions).

According to the types of active groups contained in the membrane, it can be divided into three categories: cation exchange membranes, anion exchange membranes, and special membranes.

1. Cation exchange membrane (abbreviated as cation membrane) contains acidic active groups in the membrane body, which can selectively penetrate cations without allowing anions to penetrate. These active groups mainly include sulfonic acid group (- SO: H), phosphate group (- OPOH), phosphite group (- POzHz), oxalic acid group (COOH), phenolic group (- CH4OH), etc.

2. Anion exchange membrane (referred to as anion membrane) contains alkaline active groups in the membrane body, which can selectively penetrate anions without allowing cations to penetrate. These active groups mainly include: quaternary ammonium group [- N (CH3) 3OH], primary amine group (- NHz), secondary amine group (- NHR), tertiary amine group (- NR2), etc.

3. Special membranes include a zwitterionic ion exchange membrane consisting of uniformly distributed cationic and anionic active groups within a membrane, a composite ion exchange membrane (also known as a double 92 polar membrane) with positively charged and negatively charged membranes attached together, an embedded ion exchange membrane with some positive and some negative charges coexisting in the thick seat direction of the membrane, and a surface layer membrane coated with an anion or cation exchange resin on the surface of the positive or negative membrane.

According to the distribution of active groups in the base membrane, ion exchange membranes can be divided into three categories: heterogeneous membranes, homogeneous membranes, and semi homogeneous membranes. It is a thin film made by processing a mixture of fine powder of ion exchange agent and adhesive,

1. Heterogeneous membrane

The adhesive part containing ion exchange active groups and film-forming structure forms a discontinuous membrane chemical structure, hence it is called heterogeneous membrane or heterogeneous membrane. This type of film is easy to manufacture and inexpensive, but generally has poor selectivity and high film resistance.

2. Homogeneous membranes are membranes made directly from polymer materials with ion exchange groups, or membranes made by directly attaching active groups to the polymer membrane base. The ion exchange active groups in this type of membrane chemically combine with the film-forming polymer material, resulting in a completely uniform composition, hence it is called a homogeneous membrane. This type of membrane has excellent electrochemical and physical properties, and is the main development direction of ion exchange membranes in recent years.

3. Semi homogeneous membrane: The film-forming polymer material of this membrane is uniformly combined with ion exchange active groups, but there is no chemical binding between them. For example, a semi homogeneous membrane is formed by dissolving ion exchange resin and film-forming polymer binder in the same solvent, and then using a casting method to form the membrane. Its appearance, structure, and performance are all between heterogeneous films and homogeneous films.

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