What Is a Plasma Membrane?
A polymer membrane containing an ionic group and having selective permeability for ions in a solution. In general, its ion-selective permeability is mainly used in applications, so it is also called an ion-selective permeability film. In 1950, W. Zhu Da first synthesized an ion exchange membrane. In 1956, it was successfully used in the electrodialysis desalination process for the first time.
- Chinese name
- Ion exchange membrane
- Foreign name
- ion exchange membrane
- Nature
- Excellent electrochemical performance and poor mechanical properties
- Application
- Used for desalination of brackish water and concentration of salt solution
- Preparation
- Manufactured by polymer processing molding method
- A polymer membrane containing an ionic group and having selective permeability for ions in a solution. In general, its ion-selective permeability is mainly used in applications, so it is also called an ion-selective permeability film. In 1950, W. Zhu Da first synthesized an ion exchange membrane. In 1956, it was successfully used in the electrodialysis desalination process for the first time.
Ion exchange membrane type
- Ion exchange membranes are thin films made of polymer materials with ion exchange properties (there are also inorganic ion exchange strands, but their use is not yet common). It is similar to an ion exchange resin in that it attaches an active group to the polymer backbone, but the mechanism, method and effect are different. There are many types of ion exchange membranes on the market, and there is no uniform classification method. Generally divided into three categories according to the macrostructure of the membrane:
1. Heterogeneous ion exchange membrane is made by mixing powdery ion exchange resin with binder, pulling sheet and hot pressing. The resin is dispersed in the binder, so its chemical structure is not uniform.
- 2. Homogeneous ion exchange membrane Homogeneous ion exchange membrane is made by introducing active groups into an inert support. It has no heterogeneous structure and is homogeneous in itself. Its chemical structure is uniform, the pores are small, the membrane resistance is small, it is not easy to leak, the electrochemical performance is excellent, and it is widely used in production. But the production is complicated and the mechanical strength is low.
3. Semi-homogeneous ion exchange membranes are also made by introducing active groups into a polymer support. However, the two do not form a chemical bond, and their properties are between the homogeneous ion exchange membrane and the heterogeneous ion exchange membrane. [1]
- In addition, according to different functions and structures, ion exchange membranes can be divided into five types: cation exchange membranes, anion exchange membranes, amphoteric exchange membranes, mosaic ion exchange membranes, and polyelectrolyte composite membranes. The structure of the ion exchange membrane is the same as that of the ion exchange resin, but it is in the form of a membrane.
Preparation method of ion exchange membrane
- Ion exchange membranes are divided into two types: homogeneous membranes and heterogeneous membranes. They can be manufactured by polymer processing and molding methods.
- The homogeneous membrane is first made of polymer materials such as styrene-butadiene rubber, cellulose derivatives, polytetrafluoroethylene, polytrifluorochloroethylene, polyvinylidene fluoride, polyacrylonitrile, etc., and then introducing monomers such as benzene Ethylene, methyl methacrylate, etc. are polymerized into polymers in the membrane, and then through chemical reactions, the required functional groups are introduced. Homogeneous membranes can also be obtained by direct polymerization of monomers such as formaldehyde, phenol, phenolsulfonic acid, and the like.
- The heterogeneous membrane is mixed with ion-exchange resin with a particle size of 200 to 400 mesh and ordinary film-forming polymer materials, such as polyethylene, polyvinyl chloride, polyvinyl alcohol, fluorine rubber, etc., and then processed into a film.
- Whether it is a homogeneous membrane or a heterogeneous membrane, it will lose water and dry in the air and become brittle or cracked, so it must be stored in water.
Ion exchange membrane properties
- Homogeneous membranes have excellent electrochemical properties, but poor mechanical properties, and often require other fibers for reinforcement. Heterogeneous membranes have worse electrochemical performance than homogeneous membranes, and have better mechanical properties. Due to the weak adhesion between hydrophobic polymer film-forming materials and hydrophilic ion exchange resins, gaps often exist to affect ions. Choose permeability.
- The membrane resistance and selective permeability of an ion exchange membrane are important indicators of the electrochemical performance of the membrane. The order of cation permeability in the cation is: Li +> Na +> NH4 +> K +> Rb +> Cs +> Ag +> Tl +> UO (What is this?)> Mg2 +> Zn2 +> Co2 +> Cd2 +> Ni2 +> Ca2 +> Sr2 +> Pb2 +> Ba2 +
- The order of permeability of anions in the anion is: F-> CH3COO-> HCOO-> Cl-> SCN-> Br-> CrO > NO > I-> (COO) (oxalate)> SO
- Ion exchange membrane
- The permeability of water in the membrane is the amount of water carried by the ions as they pass through the membrane. In practice, water permeability is a property of the membrane. The larger the value, the greater the water loss during electrodialysis. Generally, the water permeability of a hydrophobic polymer film is much lower than that of a hydrophilic polymer film.
Application of ion exchange membrane
- The ion exchange membrane can be assembled into an electrodialyzer for desalination of brackish water and concentration of a salt solution. The degree of desalination of the electrodialysis device (see picture) can reach the purity of primary distilled water. It can also be used for desalination of glycerin and polyethylene glycol, separation of various ions and radioactive elements, isotopes, and fractionation of amino acids. In addition, ion exchange membranes are also used in the purification of organic and inorganic compounds, the treatment of radioactive waste liquids in the atomic energy industry, and the preparation of nuclear fuel, as well as in fuel cell membranes and ion-selective electrodes. Ion exchange membranes occupy an important position in the field of membrane technology, and it will also play an important role in the research of biomimetic membranes.
Performance index of ion exchange membrane
- The performance of ion exchange membranes is multi-faceted. Comprehensive evaluation and analysis of membranes must be performed according to the electrochemical, chemical and physical and mechanical properties of membranes. General commercial films often provide the following performance indicators. 1. Exchange capacity Exchange capacity is the key parameter of ion exchange membrane, and its unit is mmol / g. Generally, the membrane with high exchange capacity has good permeability and strong conductivity. However, since the active group is generally hydrophilic, when the content of the active group is high, the moisture and swelling degree in the film will increase accordingly, thereby affecting the strength of the film. Sometimes the membrane structure is too loose, which reduces the selectivity of the membrane. Generally, the exchange capacity of the membrane is about 2-3mmol / g. 2. The water content refers to the internal water combined with the active group in the membrane, which is expressed by the grams of water per gram of dry membrane (%). The water content is related to its exchange capacity and degree of crosslinking. As mentioned above, as the exchange capacity increases, the water content increases. Due to the structure of the membrane with a high degree of cross-linking, the water content will also decrease accordingly. Increasing the water content of the membrane can increase the conductivity of the membrane, but due to the swelling of the membrane, the selectivity of the membrane will decrease. Generally, the moisture content of the membrane is about 20% -40%. 3. Electrical conductivity (film resistance) is generally expressed by electrical conductivity (.cm) or specific resistance (.cm), and it is also commonly used to express the surface resistance, that is, the resistance per unit film area (.cm). The representation of resistance varies by application. Generally speaking, the smaller the resistance is, the better it is without affecting other performance to reduce power consumption. Membrane resistance is related to membrane structure and film thickness, but also to external solutions and temperature. Generally, 25C, the membrane conductance measured in a 0.1mol / L KCL solution or a 0.1mol / L NaCL solution is used as a comparison standard. 4. Selective permeability reflects the selective permeability of the membrane to different ions. The number of ion migration (t) is used. And membrane transmittance (p). The number of ion migration in the membrane is the ratio of the migration amount of a certain kind of ions in the membrane to the migration amount of all ions in the membrane. Alternatively, it can also be expressed as the ratio of the amount of charge carried by ion migration. For an ideal ion exchange membrane, the migration number of counter ions is 1, and the migration number of the same name ions is 0. Actually, due to various factors, the actual migration of counter ions in the membrane may reach 1. There are two methods to obtain the ion migration number of the membrane. One is the membrane potential method, which measures the membrane potential of the membrane in two different concentrations of similar electrolytes, and then calculates the migration number from the membrane potential. Another method is to directly measure the migration number of the membrane in an electrodialysis cell under an external DC electric field. Generally, the practical ion exchange membrane has a permeability of greater than 85% and a counterion migration number of greater than 0.9. It is hoped that good selective permeability is still present in high-concentration electrolytes. 5. Mechanical strength The mechanical strength of the film includes the burst strength and tensile strength of the film, as well as the bending strength and flexibility. Bursting strength refers to the highest pressure that the membrane can withstand when subjected to vertical pressure. It is measured by hydraulic blasting method and expressed in pressure per unit area (MPa). It is an important indicator of the mechanical strength of the membrane. Tensile strength refers to the highest tensile force of the membrane when the membrane is subjected to tensile force in equal directions, expressed in terms of the relay force per unit area (MPa). The mechanical strength of the film mainly determines the chemical structure and reinforcing materials. The enhanced degree of cross-linking can increase the mechanical strength of the membrane, while increasing the exchange capacity and water content will reduce the strength. Generally the tip of the film used is greater than 0.3MPa. 6, expansion performance (dimensional stability) film expansion and contraction should be as small and uniform as possible. Otherwise, it will not only cause assembly, but also cause bad phenomena such as increased head loss, water leakage, leakage, and current rate reduction. 7, chemical properties refers to the film's resistance to acids and alkalis, solvents, oxidation, radiation, temperature, organic pollution and other properties