The term permselectivity has been used a lot recently. It means that in some low molecular mixtures, only low molecules with specific components can penetrate the membrane.Specifically, the selection of permeability can be expressed in two ways: permeability coefficient or permeability velocity;Separation coefficient.
Chinese name
Selective permeability
Foreign name
permselectivity
Meaning
The property that low molecules can penetrate the membrane
filmA double electric layer is formed on the interface with the aqueous solution. As a result, the coating film has the property of selecting one of the anion and cation to make it penetrate.The charge on the interface is caused by the difference of dielectric constant between the coating film and the solution and the existence of polar groups on the coating film.In the film former for coating, exceptUrea formaldehyde resinIn addition to special varieties, they are almost cation permeable.However, there are both positive and negative charges in pigments (see Figure 1).
When the current flows through the film, the easily permeable ions mainly conduct electricity. As mentioned earlier, the result is electroosmosis.If the concentration ofelectrolyte solutionSeparated by coating film, the solution with high concentration will be transferred to the one with low concentrationspreadHowever, if only the easily permeable ions diffuse first, it is impossible to maintain electrical neutrality, so the electrolyte diffusion actually occurs at the diffusion rate of the hard permeable ions.Therefore, a potential difference is generated between the solutions on both sides of the coating film to control the diffusion direction of easily permeable ions.This potential difference is called membrane potential.The membrane potential is expressed by the following formula:
EM=(RT/F)(1-2t-)ln(aone/atwo)
Where, EMIs the film potential, T is the absolute temperature, R is the gas constant, F is the Faraday constant, t-Is the migration number of anions in the film;aoneIs the activity of electrolyte in concentrated solution, atwoIs the activity of electrolyte in dilute solution.
Figure 2
Figure 2 is an example of measuring the membrane potential of various coatings.Red lead, iron oxide powder and aluminum powder all have positive electricity. However, because the surface of the film is covered by film formers, it seldom contacts with the solution directly, so the film shows cationic permeability.In this experimental range, only the film of vinyl chloride resin and aluminum powder can penetrate anions.This may be because the pigment is not fully wetted by the film former.This experiment is carried out with solutions of N/32 and N/64 concentrations, and the selectivity is very close, but if the concentration increases, the selectivity will decrease[1]。
Figure 3
Figure 3 shows the use ofion selective electrodeExamples of the results of the determination of chloride ion penetration rate.Cellulose acetateIt has a fixed penetration rate, but the penetration rate of epoxy resin/polyamide curing agent (1:1) increases with time,polyvinyl chloride/Polyvinyl acetate Copolymers slow down the penetration rate with the increase of time, and they are not fixed.When the curing agent of epoxy resin and polyamide is 2:1, the penetration rate will slow down with time, and the penetration amount will gradually decrease, so that it is difficult to measure.Cellulose acetate rarely interacts with water and can maintain a fixed penetration rate even after a long time.When the curing agent of epoxy resin/polyamide is 1:1, because of the excess curing agent and the existence of free amino group (positive charge), the hydrophilicity is very strong.With the increase of contact time with water, the coating film begins to plastic due to water absorption, and the position where the polar base can contact with water increases, so the penetration force of ions increases.When the epoxy resin/polyamide curing agent is 2:1, because the amount of resin and curing agent is equivalent, there is no free amine group, so the hydrophilicity is low.The film is negatively charged.It is estimated that the permeability of PVC/polyvinyl acetate copolymer decreases due to the formation of water groups in the film.
As mentioned above, the film has interface charges in the aqueous solution, which will be the reason for the electroosmosis caused by the corrosion current, and accelerate the aging of the film along with the corrosion under the film.There have been many reports about the influence of the selective permeability of the film to ions on the corrosion reaction under the film.There are various existing arguments, for example, some people believe that it is advantageous to form a coating film that is difficult to penetrate cations in order to prevent the dissipation of iron ions, the product of anodic reaction, and inhibit the anodic dissolution reaction of iron. Others believe that the anode part is dehydrated due to the ability to conduct electroosmosis, so it is advantageous to consider the coating film that can penetrate cations,In addition, some people also argue that mixing cation and anion in the coating film is beneficial to preventing ion penetration, and so on.Although these studies are very interesting, they fail to grasp the key points.They are all related to the analysis of water, reaction products and the supplement and dissipation of substances needed for the reaction. Therefore, it is necessary to further explore the role of interfacial charges on the film in the permeation mechanism of water and ions to the film.However, if the coated steel is applied with electrochemical protection or there is a large amount of current flowing out (or flowing in) from the coated object through the coating film for some reason, the interface charge of the coating film may have a great impact[2]。
Selective permeability of liquid mixtures
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takeM-xyleneAndParaxyleneFor mixtures with similar boiling points such as mixtures, azeotropic mixtures such as water dioxane mixtures, and general organic solvent mixtures such as benzene and cyclohexane mixtures, the so-called "total evaporation" separation method is used to determine the selective permeability of liquid mixture membrane system.This test method is to send the mixed solution to one side of the membrane, depressurize the other side of the membrane, and then cool and liquefy the vapor penetrated to the low-pressure side before quantitative analysis.Sometimes, it is also used to permeate steam with non condensable carrier gas and conduct analysis after cooling and liquefaction.Regardless of which method, the selection of permeability generally refers to the permeation rate and separation coefficient of all the liquid permeated from the membrane.
Measuring device: the vacuum pipeline that uses the high vacuum method to measure the permeability of gas mixture can be used.In order to send the liquid mixture to the infiltration tank, it is necessary to use an infiltration tank that can be fully stirred frequently.Figure 4 shows the simplest measuring device.In this device, the mixed liquid entering the infiltration tank is about 250cmthreeThe diameter of the membrane is 3-5 cm, and the condensation collector is filled with dry ice acetone.After measuring the weight and volume of the permeation steam collected in condenser A or B, the total permeation speed can be calculated, and then the separation coefficient can be calculated after the refractive index, ultraviolet absorption, gas chromatography and other analyzers quantify each component.
Figure 5
The device shown in Figure 4 has two characteristics.One feature is that the mixed liquid sent to the membrane can be circulated;Another feature is that nitrogen can be used as the carrier gas to transport the permeation steam to the condensation collector.Figure 5 is a sectional view of the infiltration tank, which is connected with the circulating pump through the connection port 10, from which liquid flows in and out[3]。
