Method and device for testing surface grafting density of charged composite membrane
By measuring the flow potential of the surface of the charged composite film and calculating the zeta potential and charge density, the problem of difficulty in accurately measuring the graft density of the film surface in the prior art is solved, and the precise regulation of the charge density of the film surface is achieved, and the film performance is improved.
Patent Information
- Application Number
- CN202510385490.X
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Priority Date
- 2024-12-02
- Filing Date
- 2025-03-29
- Publication Date
- 2025-05-30
AI Technical Summary
It is difficult for the prior art to accurately measure the graft density on the surface of the charged composite film, which affects the regulation of the positive charge density on the surface of the film.
By measuring the flow potential of the composite film surface before and after grafting, the zeta potential and charge density are calculated, and the graft density on the surface of the film is then calculated.
Accurate determination of the graft density of the charged composite film surface is achieved, helping to regulate the charge density of the film surface and improving the film performance.
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Abstract
Description
Technical Field
[0001] The present invention belongs to the technical field of membrane separation, and particularly relates to a method for testing the surface grafting density of a charged composite membrane and a testing device therefor. Background Art
[0002] Charged nanofiltration membranes, especially positively charged nanofiltration membranes, have wide application values in fields such as hard water softening and heavy metal wastewater treatment. At present, positively charged nanofiltration membranes are generally prepared by interfacial polymerization of polyamines and polyacyl chloride monomers. A commonly used polyamine monomer is polyethyleneimine. However, after the interfacial polymerization reaction, a large amount of acyl chloride groups remaining on the membrane surface are easily hydrolyzed to form carboxyl groups, causing the membrane surface to be locally negatively charged, thereby shielding part of the positive charges and reducing the positive charge density. Generally, surface grafting modification is required to increase the positively charged density on the membrane surface. The degree of membrane surface grafting is an important index for systematically studying the properties of grafting monomers, grafting rate, and the relationships among the charged structure on the membrane surface, charged performance, and the separation performance of the grafted chain and the membrane. It is crucial for optimizing the grafting process and preparing high-performance charged nanofiltration membranes.
[0003] The grafting rate of the membrane can be calculated by an elemental analyzer through measuring the elemental content on the membrane surface at different grafting times. However, the grafting rate measured by this method is the overall grafting rate of the membrane cortex and cannot reflect the grafting degree on the membrane surface. The grafting of the membrane usually occurs on the surface of the cortex. Accurately measuring the grafting degree on the membrane surface is beneficial for better regulating the charge density on the membrane surface, but currently, there is a lack of a simple method for measuring this grafting degree. Summary of the Invention
[0004] In view of the technical problem in the prior art that the grafting degree on the cortex surface of a charged composite membrane cannot be accurately measured, the present invention proposes a method for calculating the surface grafting density of a charged composite membrane. The technical solution of the present invention is as follows: On the one hand, the present invention discloses a method for testing the surface grafting density of a charged composite membrane, which tests the grafting density of a charged modification layer chemically grafted and combined on the surface of a nascent separation cortex generated by interfacial polymerization, and includes the following steps: First step, respectively place the composite membranes before and after grafting into a membrane cell, and measure the streaming potential on the membrane surface flowing through the membrane cell channel under different inlet and outlet pressure differences; Second step, calculate the zeta potential on the membrane surface before and after grafting from the streaming potential and the inlet and outlet pressure differences; Third step, calculate the charge density on the membrane surface before and after grafting from the zeta potential; Fourth step, calculate the surface grafting density of the membrane from the charge densities on the membrane surface before and after grafting.
[0005] Preferably, the method for testing the surface grafting density of the charged composite membrane described in the present invention is a method for testing the surface grafting density of a positively charged polyamide composite membrane, which measures the grafting density of the positively charged modification layer chemically grafted onto the surface of the nascent separation cortex formed by interfacial polymerization. The density of the positively charged groups on the membrane surface after grafting is the sum of the density of the positively charged groups on the membrane surface before grafting and the density of the grafted positively charged groups; the density of the negatively charged groups on the membrane surface after grafting is the difference between the density of the negatively charged groups on the membrane surface before grafting and the density of the consumed negatively charged groups during grafting. The positive charge density on the membrane surface after grafting is the difference between the density of the positively charged groups and the density of the negatively charged groups on the membrane surface after grafting.
[0006] Charge density on the membrane surface before grafting:
[0007] where n I+ and n I- are the number of positively charged groups and the number of negatively charged groups on the effective surface area (A) of the membrane, respectively; N 0 is Avogadro's constant; F is the Faraday constant.
[0008] Charge density on the membrane surface after grafting:
[0009] where n II+ and n II- are the number of positively charged groups and the number of negatively charged groups on the effective surface area (A) of the TFC-II membrane, respectively.
[0010] For the polyamide composite membrane, one grafting molecule can react with a acyl chloride groups on the membrane surface, thus eliminating a negative charges; and one grafting molecule can provide an additional b amine groups that can absorb protons. The number of grafting molecules grafted on the effective surface area (A) of the membrane is n g . Then the number of positively charged groups and negatively charged groups on the membrane surface after grafting are:
[0011]
[0012] Therefore, the charge density on the membrane surface after grafting is calculated as:
[0013] From this, the surface grafting density of the membrane can be obtained:
[0014] On the other hand, the present invention discloses a device for measuring the surface grafting density of a charged composite membrane, which includes a feed tank, a feed pump, a cross-flow membrane cell, electrodes upstream and downstream of the membrane cell, as well as a pressure gauge and a potentiometer. The electrolyte solution in the feed tank is pumped into the membrane cell at a certain pressure by the feed pump. The potential between the electrodes upstream and downstream of the membrane cell is measured by the potentiometer, and the pressure of the inlet and outlet of the membrane cell is measured by the pressure gauge, so as to calculate the zeta potential and charge density on the membrane surface, and then the grafting density is calculated from the zeta potential and charge density on the membrane surface before and after grafting.
[0015] Preferably, the cross-flow membrane cell can be used to hold a flat membrane or a hollow fiber membrane.
[0016] The technical solution of the present invention has achieved remarkable technical effects and progress, and has substantial features.
[0017] The technical feature of the present invention is that by using the relationship between the grafted molecules and the original charged functional groups on the membrane surface, the grafting density on the membrane surface can be directly obtained from the difference in the number of charge functional groups on the membrane surface eliminated by the grafted molecules and the amount of hetero-charged functional groups increased by the grafted molecules and the surface charge density before and after grafting. This method has a simple process.
[0018] Through the above technical innovation, the present invention has made remarkable technical progress and has excellent application prospects in the field of separation of charged membranes. Detailed implementation mode
[0019] The present invention will be further described below through specific examples.
[0020] The base membrane is a polysulfone flat ultrafiltration membrane with a cut-off molecular weight of 50,000 Daltons; The polyamine compound used is polyethyleneimine (PE I ) The aromatic triacyl chloride used is 1,3,5-benzenetricarbonyl chloride (TMC); The surface modifier used is diethylenetriamine (DETA); Ethanol is used as the solvent for the modifier; n-Hexane is the solvent of trimesoyl chloride. Example
[0022] The PE The positively charged product is prepared by interfacial polymerization of an aqueous solution and a n-hexane solution of TMC. polyamide membrane; Take Positively charged The polyamide membrane is soaked in a diethylenetriamine / ethanol solution for grafting modification; it is assumed that each diethylenetriamine molecule can react with a residual acyl chloride group, thereby eliminating a potential negative charge, and furthermore, the grafted diethylenetriamine has two amino groups that can be positively charged in water, equivalent to grafting two positive charges.
[0023] Step 1: Place the composite membranes before and after grafting into the membrane cell respectively, and measure the streaming potential on the membrane surface flowing through the liquid channel in the membrane cell under different inlet and outlet pressure differences; Step 2: Calculate the zeta potential on the membrane surface before and after grafting from the streaming potential and the inlet and outlet pressure differences, which are 20.0 and 18.1 mV respectively; Step 3: At pH = 7, calculate the charge density on the membrane surface before and after grafting from the zeta potential, which are 1.18 and 0.91 mC m -2 ; Step 4: Calculate the grafting density of DETA:
[0024] It should be noted that the above embodiments are only specific preferred embodiments of the present invention and do not constitute a limitation to the present invention. Any embodiment falling within the protection scope of the present invention formed by the features of the claims of the present invention or equivalent features constitutes an infringement of the patent right of the present invention.
Claims
1. A method for testing the grafting density on the surface of a charged composite membrane, which is to calculate the grafting density of a charged modification layer grafted by chemical bonds on the surface of a primary separation cortex generated by interfacial polymerization, characterized in that: The steps include: In the first step, the composite membranes before and after grafting were placed in the membrane pool, and the streaming potential of the membrane surface flowing through the liquid channel of the membrane pool was measured under different inlet and outlet pressure differences. In the second step, the zeta potential of the membrane surface before and after grafting was calculated from the streaming potential and the inlet and outlet pressure difference; The third step is to calculate the charge density of the membrane surface before and after grafting by zeta potential; The fourth step is to calculate the grafting density on the membrane surface based on the charge density on the membrane surface before and after grafting.
2. The method for testing the grafting density on the surface of a charged composite membrane according to claim 1, characterized in that: The invention relates to a method for testing the surface grafting density of a positively charged polyamide composite membrane, and the grafting density of a positively charged modification layer grafted by chemical bonds on the surface of a primary separation cortex generated by interfacial polymerization is tested, wherein the surface grafting density is the number of grafted molecules per unit membrane surface area, the density of the positively charged groups on the membrane surface after grafting is the sum of the density of the positively charged groups on the membrane surface before grafting and the density of the grafted positively charged groups; the density of the negatively charged groups on the membrane surface after grafting is the difference between the density of the negatively charged groups on the membrane surface before grafting and the density of the negatively charged groups consumed by grafting; and the positive charge density on the membrane surface after grafting is the difference between the density of the positively charged groups on the membrane surface after grafting and the density of the negatively charged groups consumed by grafting.
3. A device for measuring the grafting density on the surface of a charged composite membrane, characterized in that. It includes a feed tank, a feed pump, a cross-flow membrane pool for clamping flat membranes or hollow fiber membranes, electrodes upstream and downstream of the membrane pool, a pressure gauge, and a potentiometer. The feed pump transports the electrolyte solution in the feed tank to the membrane pool at a certain pressure, the potentiometer measures the potential between the upstream and downstream electrodes of the membrane pool, and the pressure gauge measures the pressure in and out of the membrane pool, thereby calculating the zeta potential and charge density of the membrane surface, and then calculating the grafting density based on the zeta potential and charge density of the membrane surface before and after grafting.