Semi permeable membrane is a kind of membrane that only allows certain molecules and ions to diffuse in and out. Generally, semi permeable membrane only allows ions and small molecular substances to pass throughBiomacromoleculeMaterials cannot pass through the semipermeable membrane freely, because the pore size of the semipermeable membrane is larger than that of ions and small molecules, but smaller than that of biological macromolecules such as protein, starch, etc., such as parchment, cellophane, etc. belong to the semipermeable membrane.
Chinese name
Semipermeable membrane
Foreign name
semipermeable membrane
Application
Pharmaceutical industry, beverage industry, electronics and power industry
Introduction
Membrane structure allowing only ions and small molecules to pass through
One day in 1748, French physicist Noelle designed such an experiment to improve the production level of wine: fill a glass cylinder with alcohol, seal it with a pig bladder, and then immerse the cylinder in water.When he was about to do the next step, he suddenly found that the pig bladder began to expand outward, and then found that water penetrated into the cylinder through the bladder, and finally the bladder was broken.This is the record of the earliest discovery of membrane and its penetration in modern scientific experiments.
In 1830, French physiologist Dutroxia conducted an osmotic pressure test inside and outside the membrane. He used a bell jar shaped glass container, sealed the bottom with parchment paper, inserted a long glass tube from the top, put solutions of different concentrations and substances into the container, and then immersed it in the water tank.Then the liquid level in the glass tube was observed to rise, and it was found that the rising value was proportional to the concentration of the solution.He explained that the pressure was caused by the water escaping from the outside to the solution through the parchment, and named this phenomenon "penetration".Until 1854, British scientist Graham found in the experiment that the crystal placed on one side of the semi permeable membrane would diffuse to the other side faster than the colloid, and applied it to the design of ultrapure water machine, and proposed the concept of dialysis.At this time, people became interested in semi permeable membrane, and German biochemist Traub Moritz produced the first artificial membrane in human history - copper ferrocyanide membrane in 1864.
The year 1960 is of epochal significance for the history of membrane technology development.In the same year, human beings finally realized their dream of producing fresh water from brackish water. Scientists working at the University of California, Los Angeles, developed the world's first asymmetric cellulose acetate reverse osmosis membrane. This membrane has the same high desalination rate as the previous homogeneous cellulose acetate reverse osmosis membrane. The difference is that it is asymmetric in morphology and structure,And the amount of water penetration has increased nearly ten times.The successful development of this reverse osmosis membrane has made the reverse osmosis process from laboratory to industrial application.At the same time, this new process of making asymmetric separation membrane by phase inversion has attracted extensive attention in academia, technology and industry.[1]
characteristic
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Semipermeable membrane A thin film that selectively penetrates substances.Generally, only solvents or solvents and small molecular solutes are allowed to penetrate, but too large molecular solutes are not allowed.For example, cellophane only allows water to pass through sucrose solution, while sucrose molecules cannot;The animal bladder allows water to pass through, but not alcohol molecules;Hot palladium or platinum allows hydrogen to penetrate, while argon molecules cannot.Semi permeable membrane can be made of a variety of high molecular materials to separate substances with different molecular weights, constant osmotic pressure and gas partial pressure.Semi permeable membrane mainly refers to reverse osmosis and ultrafiltration in membrane separation technology.When reverse osmosis process is applied, it is called reverse osmosis membrane. It is a membrane with water-based groups. The membrane not only has the role of screening and filtration, but also has the priority of adsorption of water molecules.Often inreverse osmosisThe films of PAM include cellulose acetate film, aromatic polyamine film, polybenzimidazole film, etc.Semi permeable membrane can be made into plate, tube and hollow fiber, and also used for diffusion dialysis.The micropore diameter of the epidermis of the membrane is 0.6~0.9 nm, and the critical pore diameter is 1.3 nm.The semipermeable membrane with larger pore diameter is used for ultrafiltration, called overfilter membrane, and it is 0.07~0.7MPa (0.7~7kgf/cmtwo）Working under pressure, it is used to separate molecules and particles with a diameter of less than 10nm. Its permeability belongs to the screening principle.Membrane processes used in sewage treatment include electrodialysis, reverse osmosis and ultrafiltration, all of which are semi permeable membranes.Semi permeable membrane is applied to industrial wastewater treatment, some of which have already been produced in scale, and some are still in the stage of laboratory research.[2]
Common semipermeable membrane
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Cellulose acetate membrane
The membrane is made by dissolving cellulose acetate with solvent, adding pore forming agent, making the membrane, steaming off the solvent, and heat treating to a certain extent.The solvent used is acetone, and dioxane is also used. The pore forming agent is Mg (ClOfour)two、ZnCltwoAnd HSPOfourEtc.The membrane can be divided into plate type, tube type, spiral roll type and hollow fiber type.The composite cellulose acetate membrane is introduced below.
The structure of the composite cellulose acetate membrane consists of a surface layer and a porous layer (bottom layer).The surface layer (0.1~0.3 μ m thick) has a fairly fine microporous structure (pore diameter<50 angstrom), which is called a semipermeable membrane. The bottom layer is a spongy porous structure, which is 200-500 times thicker than the surface layer. The hole is large (pore diameter is about 400 angstrom), and it is elastic, playing the role of supporting the surface layer.During reverse osmosis operation, the solution must be in contact with the surface layer and cannot be inverted.Acetate fiber membrane is suitable for solutions with pH range of 3-7 (long-term use of Fangei, pH is about 4.5).
Polyamine membrane
Aliphatic polyamide films, such as nylon-66 and nylon-6, were mainly made before 1970. These films have poor water permeability.Later, the aromatic polyamide film was made, which has good water permeability, demineralization rate, mechanical strength and chemical stability.It can be used in the pH range of 4~10 (long-term use range of pH5~9).Aromatic polyamide film is mainly made of Zhongyi fiber.The casting solution of this kind of membrane is usually composed of aromatic polyamide, solvent and salt additive (as cosolvent).The hollow fiber membrane is made by solution spinning Rubozhi: a certain concentration of aromatic polyamide spinning solution is sprayed out through a circular mesopore spinneret at a certain temperature (such as 80~140 ℃), and then baked, evaporated, and soaked.
Preparation method
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Collodion semipermeable membrane
1. Add 10mL concentrated nitric acid (14.1g/mL) into a 100mL beaker to prepare a mixed solution and cool it to about 20 ℃.Immerse a small piece of absorbent cotton in the mixed solution for 8~10min, take it out, wash it thoroughly, and dry it to obtain cellulose nitrate.
2. Take 10mL of ether and 10mL of ethanol respectively in the reagent bottle, add 1g of the above dry cellulose nitrate, shake constantly, and place for 1-2 days to generate transparent collodion solution.
3. Take 3mL of the above 5% self-made or commercially available collodion solution and add it into a dry 150mL conical flask. Turn the conical flask crosswise and continuously to make the inner wall and mouth of the bottle evenly stained with collodion solution.Pour out the excess collodion solution, and then invert it for about 1 min to allow ether and ethanol to evaporate continuously until it is dry.Gradually peel off the film at the mouth of the bottle, and inject distilled water along the film seam of the bottle wall to gradually separate the film from the bottle wall. Take it out gently and immerse it in distilled water for standby.
Parchment semipermeable membrane
Put the quantitative filter paper into a beaker containing 2:1 water sulfuric acid, press it gently with a glass rod to make it immersed evenly (neither the glass rod nor the filter paper can contain water).After soaking for 60~90s, take it out, immediately wash it with distilled water, then put it into 1:1 ammonia monohydrate aqueous solution (i.e. ammonia water), neutralize the residual acid, take it out and dry it in air.
be careful:
1. The conical bottle for making the rubber cotton semi permeable film must be dry, otherwise it is difficult to form the film.
2. The collodion semi permeable membrane is easy to break, so please keep it in distilled water before experiment.
3. When making parchment, the concentration of sulfuric acid and soaking time must be controlled, and the filter paper must not overlap during soaking.
