Nanosheet layer-by-layer self-assembled ultrathin polyelectrolyte nano layered composite membrane and preparation method thereof

Abstract

The invention discloses a nanosheet layer-by-layer self-assembled ultrathin polyelectrolyte nano layered composite membrane and a preparation method thereof, and belongs to the technical field of functional polymer materials. The invention provides a novel two-dimensional layer-by-layer self-assembly strategy which is used for manufacturing the ultrathin polyelectrolyte layered nano composite membrane. A TpEB COF nanosheet and an MXene nanosheet with opposite charges are subjected to layer-by-layer self-assembly on an hPAN substrate through electrostatic interaction, and the polyelectrolyte-like COF / MXene layered nano composite film with the thickness lower than 50 nm can be prepared through soaking and flushing. The composite membrane has ultra-thin thickness, abundant interlayer and in-plane water channels, small tortuosity, precise size screening nano-channels, and strong electronegativity. Therefore, the permeability of the composite membrane is remarkably improved, the nanofiltration performance is excellent, good long-term operation stability and pressure resistance are shown, the preparation process is simple and easy to operate, and the composite membrane is expected to be applied to the field of nanofiltration separation.

Description

Technical Field

[0001] This invention belongs to the field of functional polymer materials technology, and relates to a nanosheet layer-by-layer self-assembled ultrathin polyelectrolyte nanolayered composite film and its preparation method. Background Technology

[0002] With rapid industrial development, water scarcity and energy crises have become major problems facing human society. Membrane separation technology, with its high separation efficiency and low energy consumption, is widely used in water treatment. Membrane separation technology provides an effective solution for water purification. Nanofiltration membranes, as separation materials with nanoscale pores, have demonstrated significant application value in seawater desalination, wastewater treatment, biopharmaceutical separation, and fine chemical industries. As a core component of precision separation technology, the continuous improvement of nanofiltration membrane performance is crucial for reducing energy consumption and costs in water treatment and material separation. Membrane thickness is one of the key factors determining water flux; theoretically, a thinner separation layer means lower transmission resistance and higher permeation efficiency. Therefore, the preparation of ultrathin (e.g., less than 50 nanometers thick) and defect-free nanofiltration membranes has become a cutting-edge direction in this field, pursuing the goal of "ultra-fast permeation."

[0003] Currently, the main method for constructing nanofiltration separation layers based on two-dimensional nanomaterials (such as MXene, COF, and graphene) is vacuum filtration. Vacuum filtration is a mature technology that can form a separation layer with high rejection rate through the close stacking of nanosheets, and the resulting membrane often exhibits high pure water flux. However, this method has limited precision in controlling membrane thickness, and the nanosheets are prone to disordered over-stacking, resulting in a separation layer thickness that typically far exceeds 100 nm. This makes it difficult to achieve extreme thinning of the structure, fundamentally limiting further improvement in its permeation performance.

[0004] To achieve precise and controllable reduction in membrane thickness, layer-by-layer self-assembly technology has attracted attention due to its ability to construct layers sequentially through intermolecular forces, theoretically allowing for molecular-level control. However, those skilled in the art face a long-standing but unresolved contradiction in practice: traditional layer-by-layer self-assembly methods construct separation layers by alternating electrostatic adsorption of polyelectrolytes with opposite charges. However, the selective layers formed by the close packing of polyelectrolyte molecular chains often suffer from small free volumes, leading to high membrane permeation resistance and low water permeability. Although the separation layer thickness is significantly reduced to ultrathin ranges (e.g., <100 nm, or even <50 nm), the pure water flux of the membrane often shows a sharp decline instead of an increase. This phenomenon contradicts the traditional theoretical expectation that "thinning increases flux," indicating that at the ultrathin scale, microstructural factors such as the tortuosity and regularity of water transport channels within the composite membrane, and the intrinsic hydrophilicity of the membrane surface, replace thickness as the more critical factors dominating permeation performance. In existing technologies, when using polyelectrolytes to prepare nanofiltration membranes through layer-by-layer self-assembly, it is generally impossible to maintain or optimize these microstructures while achieving extreme thinning. As a result, the prepared ultrathin films are mostly "inefficient" or "failed" membranes, and cannot achieve both "ultrathinness" and "high flux". This constitutes a major obstacle to the practical application of this technology.

[0005] Therefore, the urgent technical problem to be solved in the field of nanofiltration membranes is: how to develop a method for preparing an ultrathin separation layer that can effectively construct such a layer. This method must be able to overcome the bottleneck of "thickness reduction accompanied by deterioration of permeability" that is common in existing layer-by-layer self-assembly technologies, so as to truly achieve the original goal of improving membrane permeability efficiency through structural thinning. Summary of the Invention

[0006] To address the aforementioned problems, the present invention aims to provide a nanosheet-based self-assembled ultrathin polyelectrolyte nanolayered composite membrane and its preparation method, thereby solving the problem that existing nanosheets are difficult to effectively improve both the pure water flux and Congo red rejection rate when preparing ultrathin nanofiltration membranes using the layer-by-layer self-assembly method.

[0007] To achieve the above objectives, the present invention first provides a method for preparing an ultrathin, polyelectrolyte-like nanolayered composite film by self-assembly of nanosheets, comprising the following steps: (1) The porous support membrane is first immersed in NaOH solution to obtain a hydrolyzed, negatively charged porous support membrane hPAN; (2) The hPAN obtained in step (1) is floated face down on the surface of the positively charged nanosheet dispersion to assemble nanosheets. After being removed, it is rinsed with deionized water to obtain (P / N). 0.5 -hPAN membrane; (3) Take the (P / N) obtained in step (2) 0.5-hPAN membranes were floated face down on the surface of a negatively charged nanosheet dispersion for nanosheet assembly. After removal, the membranes were rinsed with deionized water to obtain (P / N) nanosheets. 1.0 -hPAN membrane; (4) Alternately repeat steps (2) and (3) to perform layer-by-layer self-assembly, obtaining (P / N). n -hPAN membrane, the subscript "n" represents the number of double layers.

[0008] In one embodiment of the present invention, the porous support membrane is one of polyvinylidene fluoride, polytetrafluoroethylene, polyethersulfone, mixed cellulose microporous membrane, polyacrylonitrile ultrafiltration membrane, and cellulose acetate ultrafiltration membrane; since the pore structure of the polyacrylonitrile ultrafiltration membrane is suitable and the charge can be easily controlled by hydrolysis, the polyacrylonitrile ultrafiltration membrane is further preferred.

[0009] In one embodiment of the present invention, in step (1), the mass concentration of the NaOH solution is 0.1-40 mg / mL, preferably 0.1-5 mg / mL, and the soaking time is 0-60 min, not 0, preferably 2-10 min, and more preferably 5 min.

[0010] In one embodiment of the present invention, the positively charged nanosheet is a TpEB COF nanosheet. The TpEB COF nanosheet is prepared by the following method: 1, 3, 5 Triformylphloroglucinol (Tp) and ethidium bromide (EB) are dissolved in dimethyl sulfoxide. After dissolution, the Tp solution is added dropwise to the EB solution and stirred for 24-72 hours to obtain a TpEBCOF nanosheet dispersion.

[0011] In one embodiment of the present invention, the mass ratio of Tp to EB is 1 to 3.

[0012] In one embodiment of the present invention, the negatively charged nanosheets are selected from MXene nanosheets. The preparation method of the MXene nanosheet dispersion includes: firstly, dissolving LiF in HCl solution, then adding Ti3AlC2, stirring for 24-48 hours, centrifuging and washing several times at 3500 r / min until the pH value of the supernatant reaches neutral; then, ultrasonically treating and exfoliating to obtain a monolayer MXene nanosheet dispersion.

[0013] In one embodiment of the present invention, the mass concentration of the dissolved LiF is 67-167 mg / mL, and the concentration of the HCl solution used is 5-9 mol / L.

[0014] In one embodiment of the present invention, in steps (2) to (4), the positively charged nanosheet dispersion is obtained by dissolving positively charged nanosheets in ultrapure water, and the negatively charged nanosheet dispersion is obtained by dissolving negatively charged nanosheets in ultrapure water.

[0015] In one embodiment of the present invention, in steps (2) to (4), the concentration of the positively charged nanosheet dispersion is 0.01 to 1 mg / mL, the concentration of the negatively charged nanosheet dispersion is 0.01 to 1 mg / mL, preferably 0.01 to 0.05 mg / mL, and the soaking time is 0 to 60 min, and not 0, preferably 1 to 5 min. Further, the concentration of the positively charged nanosheet dispersion is preferably 0.03 mg / mL, the concentration of the negatively charged nanosheet dispersion is preferably 0.03 mg / mL, and the soaking time is preferably 2 min.

[0016] In one embodiment of the present invention, in step (4), the number of double layers "n" is 0 to 10, preferably 2.5.

[0017] In one embodiment of the present invention, the layer-by-layer self-assembled ultrathin film is a layered nanocomposite membrane formed by the electrostatic attraction assembly of positively charged nanosheets and negatively charged nanosheets, which can be constructed within 10 seconds at room temperature. The selected TpEB COF nanosheets have abundant in-plane pores, and the resulting layered nanocomposite membrane has an ultrathin thickness and abundant intralayer and in-plane water channels with low tortuosity, which facilitates the rapid transport of water molecules within the separation membrane during nanofiltration. Simultaneously, thanks to the strong electrostatic interaction between the positive and negative nanosheets and good mechanical strength, the membrane also achieves excellent stability.

[0018] Based on the above method, this invention provides a nanosheet-layered self-assembled ultrathin polyelectrolyte nanolayered composite membrane.

[0019] The present invention also provides the application of the above-mentioned nanosheet layer-by-layer self-assembled ultrathin polyelectrolyte nanolayered composite membrane in nanofiltration separation.

[0020] The present invention also provides the application of the above-mentioned nanosheet layer-by-layer self-assembled ultrathin polyelectrolyte nanolayered composite membrane in nanofiltration technology.

[0021] Beneficial effects: (1) Traditional polyelectrolyte layer-by-layer self-assembly methods for preparing ultrathin nanofiltration membranes result in a sharp decline in water flux due to the dense selective layer structure and high mass transfer resistance, making it impossible to achieve both ultrathinness and high water flux. This invention successfully overcomes this bottleneck by employing electrostatic layer-by-layer self-assembly of TpEB COF nanosheets with vertical mass transfer channels and MXene nanosheets. The constructed ultrathin (10-50 nm) separation layer is not a traditional dense polymer layer, but rather a nanosheet layered structure with regular, short-path vertical channels and a suitable hydrophilic surface. This allows water molecules to be rapidly transported through the pores of the COF nanosheets while significantly reducing the membrane thickness, greatly reducing mass transfer resistance. The composite membrane of this invention simultaneously achieves high water flux and high Congo red rejection rate, with a pure water flux as high as 105 L / m³. 2 h 1 bar 1 .

[0022] (2) This invention introduces a negative charge onto the surface of the supporting membrane through hydrolysis. This step is the basis for the subsequent successful layer-by-layer self-assembly, ensuring that positively charged COF nanosheets and negatively charged MXene nanosheets can be deposited alternately, uniformly, and firmly on the supporting membrane through strong electrostatic interaction, forming a structurally stable and precisely controllable ultrathin composite layer. The inherent ordered channels of TpEB COF nanosheets, perpendicular to the plane of the sheet, provide a fast transport channel with low tortuosity for water molecules, which is the structural source of obtaining ultra-high water flux. At the same time, the rigid structure and suitable size of COF nanosheets, together with MXene nanosheets, construct a compact stacked structure with sub-nanometer precise interlayer spacing, ensuring excellent size sieving ability, thereby achieving a retention rate of up to 99.8% for Congo red.

[0023] (3) This invention achieves precise control over the thickness, density, and channel length of the separation layer by adjusting key process parameters such as the number of assembled layers, the concentration of the nanosheet dispersion, and the assembly time. By optimizing process parameters such as the number of layers, the optimal balance point of water flux can be found while ensuring a high retention rate, avoiding the problems of excessively long channels and increased resistance due to too many layers, or excessive defects and decreased retention due to too few layers.

[0024] (4) This invention is simple to operate. Through simple soaking and layer-by-layer self-assembly, an ultrathin polyelectrolyte-like nanolayered composite membrane is successfully prepared. The thickness of the composite membrane can be precisely controlled between 10-50 nm. The all-nanosheet membrane utilizing electrostatic interactions not only provides sub-nanometer-level interlayer spacing for separation but also achieves a compact interlayer structure for excellent stability. This invention exhibits excellent performance, with the COF / MXene composite membrane achieving a pure water flux of up to 105 L / m³. 2 h 1 bar 1 It achieved a rejection rate of 99.8% for Congo red, which is superior to most reported two-dimensional nanofiltration membranes. Attached Figure Description

[0025] Figure 1 This is a scanning electron microscope image of the TpEB COF nanosheets prepared in Example 1 of this invention; Figure 2 This is a scanning electron microscope image of the MXene nanosheets prepared in Example 1 of the present invention; Figure 3 (COF / MXene) prepared in Example 1 of this invention 2.5 Scanning electron microscope (SEM) images of the hPAN composite membrane (a) surface and (b) cross-section. Detailed Implementation

[0026] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0027] Example 1 (1) Preparation of TpEB COF nanosheet dispersion: 6.3 mg Tp and 17.88 mg EB were dissolved separately in 5 mL of dimethyl sulfoxide. After dissolution, the Tp solution was added dropwise to the EB solution, and the mixture was stirred for 24 hours to obtain a TpEB COF nanosheet dispersion. (2) Preparation of MXene nanosheet dispersion: First, 2g of LiF was dissolved in 30mL of 6mol / L HCl solution, and then 2g of Ti3AlC2 was added. After stirring at 35℃ for 24h, the reaction mixture was transferred to a centrifuge tube and washed multiple times with water at 3500r / min until the pH of the supernatant reached neutral. Then, it was ultrasonically exfoliated in an ice bath under flowing nitrogen for 1h. Subsequently, unexfoliated Ti3C2T was removed by centrifugation at 3500r / min for 40min. x A single-layer MXene nanosheet dispersion was obtained.

[0028] (3) (COF / MXene) 2.5Preparation of the hPAN composite membrane: First, the PAN membrane was immersed in a 0.2 mg / mL NaOH solution for 5.0 minutes, and then rinsed with deionized water to obtain the hPAN membrane. The hPAN membrane was then immersed in a 0.03 mg / mL TpEB COF dispersion for 2 minutes, followed by rinsing with deionized water to remove excess TpEB COF nanosheets. After air drying, the membrane was suspended face down in a 0.03 mg / mL MXene solution for 2 minutes, and then rinsed with deionized water to remove excess MXene nanosheets. The above steps were repeated for 2.5 cycles to obtain a (COF / MXene) membrane with a thickness of 42 nm. 2.5 -hPAN composite membrane.

[0029] The membrane's separation performance was tested, and its pure water flux reached 105 L / m³. 2 h 1 bar 1 The rejection rate for Congo red at a concentration of 50 ppm was 99.8%. Characterization of nanosheet-layer-by-layer self-assembled ultrathin polyelectrolyte-like nanolayered composite films The (COF / MXene) prepared in Example 1 2.5 The -hPAN composite membrane was characterized, and the results are shown in the figure. Figure 1 ~ 3.

[0030] Figure 1 The image shows a scanning electron microscope (SEM) image of TpEB COF nanosheets, revealing their nanosheet morphology.

[0031] Figure 2 The image shows a scanning electron microscope (SEM) image of MXene nanosheets, revealing their nanosheet morphology.

[0032] Figure 3 (COF / MXene) 2.5 Scanning electron microscope images of the hPAN composite film (a) surface and (b) cross section show its dense, defect-free structure and ultra-thin thickness.

[0033] Example 2 The difference between Example 2 and Example 1 is that the layer-by-layer self-assembly was repeated for 2.0 cycles to obtain a (COF / MXene) layer with a thickness of 35 nm. 2.0 -hPAN composite membrane. The membrane's separation performance was tested; its pure water flux was 144 L / m³. 2 h 1 bar 1 The rejection rate for Congo red was 95.8%.

[0034] Example 3 The difference between Example 3 and Example 1 is that the layer-by-layer self-assembly was repeated for 3.0 cycles to obtain a (COF / MXene) layer with a thickness of 49 nm. 3.0 -hPAN composite membrane. The membrane's separation performance was tested; its pure water flux was 54 L / m³. 2 h 1 bar 1 The rejection rate for Congo red was 99.8%.

[0035] As can be seen from Examples 1-3, with the increase of the number of assembled layers, the retention performance of the composite membrane for Congo red gradually improves and then stabilizes, but the pure water flux gradually decreases. When the number of layers is 2.5 bilayers, the composite membrane has better separation performance.

[0036] Example 4 The difference between Example 4 and Example 1 is that the concentration of the TpEB COF dispersion is 0.01 mg / mL, resulting in a (COF / MXene) dispersion with a thickness of 35 nm. 2.5 -hPAN composite membrane. The membrane's separation performance was tested; its pure water flux was 220 L / m³. 2 h 1 bar 1 The rejection rate for Congo red was 60.4%.

[0037] Example 5 The difference between Example 5 and Example 1 is that the concentration of the TpEB COF dispersion is 0.05 mg / mL, resulting in a (COF / MXene) dispersion with a thickness of 47 nm. 2.5 -hPAN composite membrane. The membrane's separation performance was tested; its pure water flux was 44 L / m³. 2 h 1 bar 1 The rejection rate for Congo red was 99.8%.

[0038] Examples 1, 4, and 5 show that as the concentration of the TpEB COF dispersion increases, the retention performance of the composite membrane for Congo red gradually improves and then stabilizes, but the pure water flux gradually decreases. At a concentration of 0.03 mg / mL, the composite membrane exhibits good separation performance.

[0039] Example 6 The difference between Example 6 and Example 1 is that the concentration of the MXene dispersion is 0.01 mg / mL, resulting in a (COF / MXene) dispersion with a thickness of 37 nm. 2.5 -hPAN composite membrane. The membrane's separation performance was tested; its pure water flux was 159 L / m³. 2 h 1 bar 1 The rejection rate for Congo red was 82.5%.

[0040] Example 7 The difference between Example 7 and Example 1 is that the concentration of the MXene dispersion is 0.05 mg / mL, resulting in a (COF / MXene) dispersion with a thickness of 45 nm. 2.5 -hPAN composite membrane. The membrane's separation performance was tested; its pure water flux was 60 L / m³. 2 h 1 bar 1 The rejection rate for Congo red was 99.8%.

[0041] As can be seen from Examples 1, 6, and 7, with the increase of MXene dispersion concentration, the retention performance of the composite membrane for Congo red gradually improves and then stabilizes, but the pure water flux gradually decreases. At a concentration of 0.03 mg / mL, the composite membrane exhibits good separation performance.

[0042] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that the immersion and assembly time for each layer was extended to 4 minutes, while the other operating parameters remained the same as in Example 1. The number of nanosheets assembled in each layer was increased, resulting in a composite membrane thickness of 51 nm. When testing the membrane's separation performance, its pure water flux decreased to 72 L / m³. 2 h 1 bar 1 The rejection rate for Congo red was 99.8%.

[0043] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that the TpEB COF nanosheets in step (1) were replaced with PEI polyelectrolyte. This resulted in blockage of the interlayer channels in the prepared composite membrane, reducing the pure water flux. The composite membrane thickness was 30 nm, and its pure water flux decreased to 11.6 L / m². 2 h 1 bar 1 The rejection rate for Congo red was 99.8%.

[0044] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that the hPAN membrane was first immersed in a 0.5 mg / mL TpEB COF dispersion for 2 minutes, and then rinsed with deionized water to remove excess TpEB COF nanosheets. After natural drying, the membrane was suspended face down in a 0.5 mg / mL MXene solution for 2 minutes, and then rinsed with deionized water to remove excess MXene nanosheets, thus preparing a bilayer composite membrane.

[0045] The composite membrane has a thickness of 42 nm, and its pure water flux is reduced to 19.1 L / m². 2 h 1 bar 1 The rejection rate for Congo red was 99.8%.

[0046] Comparative Example 4 A composite membrane was prepared using vacuum filtration: first, 10 mL of a 5 μg / mL COF nanosheet dispersion diluted with pure water was vacuum filtered; then, 10 mL of a 1 μg / mL MXene nanosheet dispersion diluted with pure water was vacuum filtered to prepare a bilayer composite membrane. Due to the negative pressure during vacuum filtration, the composite membrane had numerous defects. The membrane's separation performance was tested, and its pure water flux was 58 L / m³. 2 h 1 bar 1 The rejection rate for Congo red was 85.6%.

[0047] Comparative Example 5 The difference between Comparative Example 5 and Example 1 is that a polyethersulfone microporous membrane was used instead of a porous supporting membrane. Polyethersulfone membranes have larger pore sizes and weaker electronegativity, resulting in more defects in the composite membrane. The membrane's separation performance was tested, and its pure water flux was 286 L / m³. 2 h 1 bar 1 The rejection rate for Congo red was 16.9%.

[0048] Comparative Example 6 The difference between Comparative Example 6 and Example 1 is that the PAN was not pretreated with sodium hydroxide. This reduced the attractive force of the supporting membrane on the nanosheets, resulting in fewer assembled nanosheets. The composite membrane thickness was 32 nm, and its separation performance was tested, showing a pure water flux of 132 L / m³. 2 h 1 bar 1 The rejection rate for Congo red was 87.3%.

[0049] Comparative Example 7 The difference between Comparative Example 7 and Example 1 is that EB was replaced with p-phenylenediamine (Pa). TpPa COF nanosheets have weak charge, which is not conducive to electrostatic assembly, resulting in more defects in the composite membrane. The membrane's separation performance was tested, and its pure water flux was 118 L / m³. 2 h 1 bar 1 The rejection rate for Congo red was 72.8%.

[0050] The embodiments provided above are not intended to limit the scope of the invention, nor are the described steps intended to limit the order of execution. Any obvious modifications made to the invention by those skilled in the art based on existing common knowledge also fall within the scope of protection defined by the claims.

Claims

1. A method for preparing an ultrathin, polyelectrolyte-like nanolayered composite film by self-assembly of nanosheets, characterized in that, Includes the following steps: (1) The porous support membrane is first soaked in NaOH solution to obtain a hydrolyzed, negatively charged porous support membrane hPAN; (2) The hPAN obtained in step (1) is floated face down on the surface of the positively charged nanosheet dispersion to assemble nanosheets. After being removed, it is rinsed with deionized water to obtain (P / N). 0.5 -hPAN membrane; (3) Take the (P / N) obtained in step (2) 0.5 -hPAN membranes were floated face down on the surface of a negatively charged nanosheet dispersion for nanosheet assembly. After removal, the membranes were rinsed with deionized water to obtain (P / N) nanosheets. 1.0 -hPAN membrane; (4) Repeat steps (2) and (3) alternately to perform layer-by-layer self-assembly and obtain (P / N). n -hPAN membrane, where "n" represents the number of double layers.

2. The preparation method according to claim 1, characterized in that, The porous support membrane is any one of polyvinylidene fluoride, polytetrafluoroethylene, polyethersulfone, mixed cellulose microporous membrane, polyacrylonitrile ultrafiltration membrane, and cellulose acetate ultrafiltration membrane.

3. The preparation method according to claim 1, characterized in that, The NaOH solution has a mass concentration of 0.1-40 mg / mL and a soaking time of 0-60 min, not 0.

4. The preparation method according to claim 1, characterized in that, In step (2), the positively charged nanosheets are TpEB COF nanosheets, and the positively charged nanosheet dispersion is obtained by dissolving the positively charged nanosheets in ultrapure water.

5. The preparation method according to claim 4, characterized in that, The TpEB COF nanosheets were prepared by the following method: 1, 3, 5 Triformylphloroglucinol (Tp) and ethidium bromide (EB) are dissolved in dimethyl sulfoxide. After dissolution, the Tp solution is added dropwise to the EB solution and stirred for 24-72 hours to obtain a TpEB COF nanosheet dispersion. The mass ratio of Tp to EB is 1-3.

6. The preparation method according to claim 4 or 5, characterized in that, In step (2), the concentration of the positively charged nanosheet dispersion is 0.01~1 mg / mL, and the nanoassembly time is 1~5 min.

7. The preparation method according to claim 1, characterized in that, In step (3), the negatively charged nanosheets are selected from MXene nanosheets, and the negatively charged nanosheet dispersion is obtained by dissolving the negatively charged nanosheets in ultrapure water.

8. The preparation method according to claim 1, characterized in that, In step (3), the concentration of the negatively charged nanosheet dispersion is 0.01~1 mg / mL, and the nanoassembly time is 1~5 min.

9. The nanosheet-layer-by-layer self-assembled ultrathin polyelectrolyte nanolayer composite membrane prepared by the preparation method according to any one of claims 1 to 8.

10. The application of the nanosheet layer-by-layer self-assembled ultrathin polyelectrolyte nanolayered composite membrane according to claim 9 in nanofiltration separation and nanofiltration technology.

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