Microstructure and filtration principle of ion exchange membrane

The working principle of ion exchange membranes (IEMs) is based on their unique molecular structure, which allows the selective transmission of certain ions while excluding others. It is the microstructure and chemical properties of these membranes that play a crucial role in selectivity and permeability. important role.

The molecular structure of ion exchange membrane can be broken down into the following aspects:

1. The polymer skeleton of the membrane. At the macro level, the ion exchange membrane IEM is a solid, but at the micro level, it is composed of a network ("skeleton") formed by polymer chains. Its main function is to provide mechanical strength to the membrane.

2. In addition to the supporting skeleton, connected to the polymer main chain are some ionizable functional groups. In cation exchange membranes, these functional groups are negatively charged. Typical representatives are sulfonic acid groups, sulfonic acid groups in ion exchange membranes. Usually attached to the polymer matrix, forming a network of ion exchange sites throughout the membrane, the sulfonic acid group is a functional group consisting of one sulfur atom bonded to three oxygen atoms and one hydrogen atom (SO3H). Due to the presence of acidic hydrogen atoms, when in aqueous solution, the dissociation of hydrogen atoms results in the formation of negatively charged sulfonate ions (SO3-), that is why it is negatively charged, the negatively charged sulfonate group attracts the positively charged ions or Cation, due to the electrostatic attraction between the negative charge of the sulfonate group and the positive charge of the cation, it has a strong affinity for cations, especially metal cations. This characteristic makes the sulfonate group commonly used in cation exchange materials, including cation exchange resins. and cation exchange membranes that selectively interact with and transport cations while blocking anions.

Like cations, the functional groups of the anion exchange membrane are positively charged, such as quaternary ammonium groups. The quaternary ammonium groups are functional groups composed of nitrogen atoms bonded with four organic substituents or groups. They exist with the positively charged atoms or groups. Four covalent bonds and therefore have a positive charge, these positively charged quaternary ammonium groups attract negatively charged ions or anions due to electrostatic attraction and can interact with a variety of anions, including halides (e.g., chloride, bromide, iodine compounds), sulfates, nitrates and other negatively charged substances.

Therefore, from the above analysis, it can be seen that the cation exchange membrane has negatively charged functional groups, and from an application perspective, it can attract positively charged ions; conversely, the anion exchange membrane has positively charged functional groups, so it can attract negatively charged ions.

3. So how does the ion exchange membrane realize ion conduction? When an electric field is applied across the membrane, or there is a difference in ion concentration on both sides of the membrane, the bound ions will be prompted to move. This movement of ions generates an ionic current, making the membrane conductive, and the ions will jump from one functional group through the membrane. to another functional group, thereby forming the conduction movement of ions.

4. The functional principle of ion exchange membrane selectivity is that the size, charge and hydration energy of ions determine their ability to pass through the membrane. For example, when a cation exchange membrane attracts larger cations, it is more likely to allow small cations. By; Likewise, ions with lower hydration energies (such as less tightly bound water molecules) also move more easily than ions with higher hydration energies because they interact more easily with functional groups in the membrane.

5. The membrane excludes common ions and non-ions. This is what we call the part that is not allowed to pass. The charge of the membrane prevents ions with the same charge as the functional group (common ion) from passing through. The cation exchange membrane has a negatively charged Functional groups, so it repels anions. Similarly, some nonionic substances, compounds or molecules that do not have a net charge, are neither positive nor negative, are electrically neutral, and are usually excluded because they cannot be combined with Charged functional groups interact.

Through the above comprehensive analysis, the selective ion transport in the ion exchange membrane is a complex process, which may also be affected by many factors, including the properties of the polymer and functional groups, the density of the functional groups, the hydration level of the membrane, the The type and concentration of ions, the transition solution and operating conditions (such as temperature or electric field strength), etc., but the structure and chemical properties of the material determine its unique properties, which also reminds our manufacturers and users about ion exchange The importance of membrane material selection and design to ion exchange membrane performance.