Janus membrane with sandwich structure and preparation method

Abstract

The invention discloses a Janus membrane with a sandwich structure and a preparation method thereof, the Janus membrane comprises a base membrane, a super-hydrophobic layer and a hydrophilic layer, and the base membrane is located between the super-hydrophobic layer and the hydrophilic layer and closely attached to the super-hydrophobic layer and the hydrophilic layer. One side of the Janus membrane has super-hydrophobicity and the other side of the Janus membrane has hydrophilicity, so that a multi-element interface with an asymmetric infiltration characteristic is formed, namely, the membrane hydrophobicity requirement for a membrane distillation process, particularly an air gap membrane distillation process, is met, the contact angle of water in air of a super-hydrophobic layer is larger than 150 degrees, and the contact angle of water in air of a hydrophilic layer is smaller than 90 degrees; meanwhile, the hydrophilic layer has the characteristics of high heat efficiency and acceleration of gas phase condensation; the Janus membrane disclosed by the invention is good in wettability resistance, high in interception efficiency, good in thermal efficiency, namely membrane distillation efficiency, and wide in application prospect; and the film forming process of the Janus film is simple and convenient to operate, and industrial production is easy to realize.

Description

Technical Field

[0001] This invention relates to the preparation technology of membrane distillation membrane materials, specifically to an anti-wetting, high thermal efficiency sandwich structure composite Si-PVDF-GO / PAN Janus membrane and its preparation method. Background Technology

[0002] Membrane distillation (MD) is a novel membrane separation technology that organically integrates evaporation and membrane processes. The driving force of membrane distillation is the vapor pressure difference across the membrane. The membrane used in membrane distillation is a porous, non-wetting hydrophobic membrane. One side of the membrane is the hot aqueous solution to be treated, which is in direct contact with the membrane. Due to the hydrophobicity of the membrane, the aqueous solution does not pass through the membrane pores. However, due to the vapor pressure difference of volatile components across the membrane, the volatile components vaporize on the membrane surface on the feed (high temperature) side. The vapor is transferred through the membrane pores to the other side (low vapor pressure side) and condenses into liquid. Membrane distillation is carried out almost at atmospheric pressure, and its equipment is simple and easy to operate. Because only water vapor can pass through the membrane pores, the pollutant rejection rate of membrane distillation can reach almost 100%, producing very pure water. In addition, membrane distillation can treat high-concentration saline solutions and can even concentrate the solution to a supersaturated state, causing crystallization, a process known as membrane distillation crystallization technology.

[0003] Membrane materials used in membrane distillation processes should meet two basic requirements: hydrophobicity and porosity, to ensure that water does not penetrate into the micropores and to maintain a high permeate flux. Simultaneously, they should also meet the requirements of anti-wetting and high thermal efficiency to improve the stability and efficiency of membrane distillation (MD). With the continuous expansion of membrane technology applications, the demands on membrane performance are becoming increasingly stringent. Single membrane materials can no longer meet the requirements of industrial water treatment, making the preparation of composite membrane materials of great significance.

[0004] Currently, the fabrication of composite membranes typically employs a "base membrane + functional layer" approach. The base membrane is often prepared via phase inversion or electrospinning to achieve the desired pore structure and mechanical properties. Functional layers are frequently introduced through interfacial polymerization, coating, electrospraying, sol-gel, or in-situ reactions. Examples include constructing hydrophobic layers to improve anti-wetting properties, or loading hydrophilic / thermally conductive layers to reduce temperature polarization and enhance thermal efficiency. Nanofiller doping (such as SiO2, TiO2, GO, MOFs, etc.) is also widely used to improve the membrane's hydrophobicity, antifouling properties, and heat transfer performance.

[0005] While these methods can effectively achieve multifunctional integration and performance optimization, some shortcomings remain: phase transformation and electrospinning processes are difficult to control precisely in terms of pore size and structure, and their large-scale production costs are high; interfacial polymerization and coating processes are highly sensitive to operating conditions, easily leading to insufficient membrane uniformity or decreased stability; sol-gel and in-situ deposition, although imparting stronger heat resistance and catalytic properties, are complex processes with high energy consumption and costs; uneven dispersion or agglomeration of nanofillers may cause membrane performance degradation. Therefore, the preparation of composite membranes in the field of membrane distillation still faces the challenge of balancing membrane performance improvement with process controllability, scalability, and cost-effectiveness.

[0006] Janus membrane refers to an asymmetric functional membrane material with significantly different physical or chemical properties on both sides. The membrane body includes at least a first functional layer and a second functional layer along the thickness direction. The first functional layer and the second functional layer have significant differences in at least one of wettability, thermal conductivity, pore structure, surface energy or chemical composition, so that the two sides of the membrane perform different functional roles respectively.

[0007] In the prior art, Janus membranes used for membrane distillation are usually achieved by constructing functional layers with different surface properties on the same side of the same membrane substrate. The preparation methods mainly include: selective coating on the same side of the pre-formed base membrane by spraying, dip coating, or blade coating; achieving unilateral wettability control through surface chemical modification methods such as plasma treatment, chemical grafting, or in-situ polymerization; controlling phase transformation conditions during membrane formation to induce the formation of an intrinsic asymmetric structure in the membrane; and constructing a multifunctional composite layer structure through layer-by-layer assembly or composite molding.

[0008] Existing Janus membranes typically involve first spraying a superhydrophobic coating onto the surface of the base membrane on one side, and then spraying a hydrophilic coating onto the surface of the superhydrophobic coating, forming a single-sided asymmetric structure with "two layers superimposed". Although this type of structure (i.e., hydrophilic layer-superhydrophobic layer-base membrane layer) can combine wetting prevention and wetting regulation functions to a certain extent, it is difficult to achieve both high-efficiency heat insulation and condensation enhancement simultaneously because the two functional layers are concentrated on one side of the membrane, while the other side remains an exposed base membrane.

[0009] In contrast, the "sandwich" structure Janus membrane designed in this application uses a porous hydrophobic base membrane as the middle layer, with a superhydrophobic layer and a hydrophilic layer sprayed on both sides of the base membrane: a low thermal conductivity superhydrophobic evaporation layer is constructed on the feed side to inhibit liquid penetration and reduce ineffective heat transfer; a highly hydrophilic condensation layer is constructed on the permeate side to promote condensate film formation and rapid discharge, shorten the vapor diffusion path, and reduce air gap thermal resistance. Through this highly asymmetrical design of heat insulation on one side and enhanced condensation on the other, the "sandwich" structure Janus membrane (i.e., hydrophilic layer-base membrane layer-superhydrophobic layer) is particularly suitable for air gap membrane distillation processes, and can significantly improve thermal efficiency and flux while ensuring anti-wetting performance. Summary of the Invention

[0010] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a sandwich-structured composite Si-PVDF-GO / PAN Janus membrane with a simple, scalable, and high-performance fabrication process. This Janus-type distillation membrane has an asymmetric structure in the thickness direction, with one side being superhydrophobic and the other hydrophilic. The base membrane is located between the superhydrophobic and hydrophilic layers, exhibiting both good anti-wetting properties and high thermal efficiency. Based on commercial membranes, rapid membrane preparation can be achieved through a simple spraying method, solving the problem of commercial membranes being easily wetted by low surface tension substances. This also improves the thermal efficiency of membrane distillation, making it well-suited for membrane distillation processes and suitable for treating high-salt wastewater containing surfactants.

[0011] The technical solution of the present invention to solve the above-mentioned technical problems is as follows:

[0012] One aspect of the present invention provides a Janus membrane with a sandwich structure, comprising a base membrane, a superhydrophobic layer and a hydrophilic layer, wherein the base membrane is located between the superhydrophobic layer and the hydrophilic layer and is tightly bonded together.

[0013] The base membrane has a superhydrophobic layer sprayed on one side and a hydrophilic layer sprayed on the other side. The superhydrophobic layer and the hydrophilic layer are located on the two sides of the base membrane, respectively.

[0014] In particular, the base membrane is a hydrophobic membrane, preferably a PVDF membrane, a PTFE membrane, or a PP membrane, and more preferably a PVDF membrane.

[0015] In particular, the superhydrophobic layer is a fluorinated Si-NPs coating; the hydrophilic layer is a GO-containing PAN coating.

[0016] In particular, the superhydrophobic layer is prepared according to the following method:

[0017] Inorganic nanoparticles were dispersed in anhydrous ethanol to prepare an inorganic nanoparticle dispersion. Then, ultrapure water, ammonia, and heptadecafluorodecyltrimethoxysilane were added to the dispersion and stirred to obtain a superhydrophobic coating solution. The superhydrophobic coating solution was then sprayed onto one side of the surface of the base film and dried to form a superhydrophobic layer.

[0018] In particular, the inorganic nanoparticles are nano-silica particles, nano-alumina (Al2O3), nano-titanium oxide (TiO2), nano-zinc oxide (ZnO), or nano-magnesium oxide (MgO), preferably nano-silica particles.

[0019] In particular, the mass ratio of inorganic nanoparticles to anhydrous ethanol is 1:(130-205), preferably 1:(135-202.5).

[0020] Specifically, the mass ratio of nano-silica (Si-NPs) to ultrapure water is 1:(20-25), preferably 1:22.5; the mass ratio of Si-NPs to NH3·H2O is 1:(11-21), preferably 1:20; the mass ratio of Si-NPs to FAS-17 is 1:(3-5), preferably 1:4; the content of Si-NPs in the superhydrophobic coating liquid is 0.2wt%-0.6wt%, preferably 0.4wt%; the content of FAS-17 in the superhydrophobic coating liquid is 1.4wt%-1.8wt%, preferably 1.6wt%.

[0021] In particular, the NH3·H2O is concentrated ammonia solution with a concentration of 25-28%, preferably 25%.

[0022] In particular, the hydrophilic layer is prepared according to the following steps: mixing an organic polymer, a dispersion of high thermal conductivity nanomaterials, and an organic solvent, stirring, dissolving, and preparing a hydrophilic layer spraying solution; then spraying the hydrophilic layer spraying solution onto the surface of the other side of the base film, and drying to form a hydrophilic layer.

[0023] In particular, the organic polymer is a copolymer of polyacrylonitrile (PAN) or acrylonitrile (AN), preferably polyacrylonitrile.

[0024] In particular, the high thermal conductivity nanomaterial is a single-layer graphene oxide (GO) or carbon nanotubes (CNTs), preferably GO.

[0025] In particular, the organic solvent is N-dimethylformamide (DMF), N-methylpyrrolidone (NMP) or dimethyl sulfoxide (DMSO), preferably DMF.

[0026] In particular, the content of organic polymer in the hydrophilic layer spray liquid is 2wt%-4wt%; the content of high thermal conductivity nanomaterial in the hydrophilic layer spray liquid is 0.1wt%-0.2wt%; and the mass ratio of organic polymer to high thermal conductivity nanomaterial in the hydrophilic spray liquid is 100:(2.5-10), preferably 100:(5-5.5).

[0027] In particular, the PAN content in the hydrophilic layer spray liquid is 2wt%-4wt%, preferably 3wt%; the GO content in the hydrophilic layer spray liquid is 0.1wt%-0.2wt%, preferably 0.16wt%; the mass ratio of PAN to GO in the hydrophilic spray liquid is 100:(2.5-10), preferably 100:(5-5.5), and more preferably 100:5.2.

[0028] Another aspect of the present invention provides a method for preparing a Janus membrane with a sandwich structure, comprising the following steps:

[0029] 1) Preparation of superhydrophobic coating spray liquid

[0030] Ultrapure water, ammonia and heptadecafluorodecyltrimethoxysilane (FAS-17) were added to the dispersion of inorganic nanoparticles and stirred until homogeneous to obtain a superhydrophobic coating solution for later use.

[0031] 2) Preparation of hydrophilic coating liquid

[0032] An organic polymer, a dispersion of high thermal conductivity nanomaterials, and an organic solvent are mixed, stirred, dissolved, and a hydrophilic coating solution is prepared for later use.

[0033] 3) Spraying superhydrophobic and hydrophilic layers

[0034] A superhydrophobic layer coating liquid and a hydrophilic layer coating liquid are sprayed on both sides of the base film, respectively. After drying, a sandwich structure Janus film is obtained.

[0035] In particular, the inorganic nanoparticles mentioned in step 1) are nano-silica particles (Si-NPs), nano-alumina (Al2O3), nano-titanium oxide (TiO2), nano-zinc oxide (ZnO) or nano-magnesium oxide (MgO), preferably nano-silica particles.

[0036] In particular, the inorganic nano-Si-NPs have a particle size of 10-15 nm.

[0037] In particular, the dispersion of the inorganic nanoparticles is prepared by adding inorganic nanoparticles to anhydrous ethanol and ultrasonically treating them to obtain the dispersion of inorganic nanoparticles.

[0038] In particular, the mass ratio of inorganic nanoparticles to anhydrous ethanol in the inorganic nanoparticle dispersion is 1:(130-205), preferably 1:(135-202.5).

[0039] In particular, the mass ratio of inorganic nano-silica particles to anhydrous ethanol in the inorganic nanoparticle dispersion is 1:(130-205), preferably 1:(135-202.5).

[0040] In particular, the mass ratio of nano-silica (Si-NPs) to ultrapure water is 1:(14-25), preferably 1:(15-22.5); the mass ratio of Si-NPs to NH3·H2O is 1:(11-21), preferably 1:(12.67-20); and the mass ratio of Si-NPs to FAS-17 is 1:(2-5), preferably 1:(3-4).

[0041] In particular, the content of Si-NPs in the superhydrophobic coating liquid is 0.2wt%-0.6wt%, preferably 0.4wt%; the content of FAS-17 in the superhydrophobic coating liquid is 1.4wt%-1.8wt%, preferably 1.6wt%.

[0042] In particular, the NH3·H2O is concentrated ammonia solution with a concentration of 25-28%, preferably 25%.

[0043] In particular, the mass ratio of FAS-17 to ammonia is (15-25):100, preferably 20:100.

[0044] In particular, the organic polymer described in step 2) is a copolymer of polyacrylonitrile (PAN) or acrylonitrile (AN), preferably polyacrylonitrile.

[0045] In particular, the AN content in the acrylonitrile copolymer is greater than 85 wt%.

[0046] In particular, the high thermal conductivity nanomaterial is a single-layer graphene oxide (GO) or carbon nanotubes (CNTs), preferably GO.

[0047] In particular, the organic solvent is N-dimethylformamide (DMF), N-methylpyrrolidone (NMP) or dimethyl sulfoxide (DMSO), preferably DMF.

[0048] In particular, the dispersing solvent in the high thermal conductivity nanomaterial dispersion is the same type as the organic solvent. If the organic solvent is DMF, then the dispersing solvent in the high thermal conductivity nanomaterial dispersion is DMF; if the organic solvent is DMSO, then the dispersing solvent in the high thermal conductivity nanomaterial dispersion is DMSO; and so on.

[0049] In particular, the high thermal conductivity nanomaterial dispersion is a dispersion of GO in DMF, wherein the mass ratio of GO to DMF is (1-3):1000, preferably 2:1000.

[0050] In particular, the PAN content in the hydrophilic layer spray liquid is 2wt%-4wt%, preferably 3wt%; the GO content in the hydrophilic layer spray liquid is 0.1wt%-0.2wt%, preferably 0.16wt%; the mass ratio of PAN to GO in the hydrophilic spray liquid is 100:(2.5-10), preferably 100:(5-5.5), and more preferably 100:5.2.

[0051] In particular, the base membrane mentioned in step 3) is a hydrophobic membrane, preferably a PVDF membrane, a PTFE membrane or a PP membrane, and more preferably a PVDF membrane.

[0052] In particular, the base film has a size of 10*10 cm.

[0053] In particular, the superhydrophobic coating liquid is sprayed as follows: the superhydrophobic coating liquid prepared in step 1) is sprayed on one side of the base film. After the solvent evaporates, the hydrophobic inorganic nanoparticles adhere to the surface of the base film to form a superhydrophobic layer.

[0054] In particular, a spray gun was used to apply the superhydrophobic coating liquid, with the spray gun needle 10±1 cm away from the base film surface; the amount of superhydrophobic coating liquid applied was per 1 m³ 2 The superhydrophobic coating liquid is sprayed onto the surface of the base film in amounts of 500-700 ml, preferably 600 ml.

[0055] In particular, the nozzle diameter is 0.2±0.1 mm; the gas pressure is 0.4±0.05 MPa.

[0056] In particular, the base film coated with the superhydrophobic layer spraying liquid is dried. The Si-NPs and FAS-17 in the superhydrophobic layer spraying liquid adhere to the surface of one side of the base film and partially enter the pores on the surface of the base film to form a mechanically interlocking composite interface. After the superhydrophobic layer-base film is obtained, the hydrophilic layer spraying liquid is then sprayed.

[0057] In particular, the drying temperature is 45±2 ℃ and the drying time is 25-35 min.

[0058] In particular, the hydrophilic layer spraying liquid is sprayed as follows: the hydrophilic layer spraying liquid prepared in step 2) is sprayed on the other side of the base film. After the organic solvent evaporates, the organic polymer and high thermal conductivity nanomaterials in the hydrophilic layer spraying liquid adhere to the other side of the base film to form a hydrophilic layer.

[0059] In particular, a spray gun was used to apply the hydrophilic coating liquid, with the spray gun needle 15±1cm from the base film surface; the spray volume of the hydrophilic coating liquid was 1m. 2 The hydrophilic layer spraying liquid is sprayed onto the surface of the base film in the amount of 500-700 ml, preferably 600 ml.

[0060] In particular, the hydrophilic layer coating liquid is sprayed in 2-4 coats (preferably 3 coats), and each 1 m 2 The total amount of the hydrophilic coating liquid sprayed onto the surface of the base film is 500-700 ml, preferably 600 ml.

[0061] In particular, the interval between two consecutive sprayings is 20±5 min, allowing the organic solvent to evaporate, thus obtaining the hydrophilic layer.

[0062] In particular, the nozzle diameter of the spray gun is 0.2±0.1 mm, and the gas pressure of the spray gun is 0.2±0.01 MPa.

[0063] In particular, it also includes rinsing the base film with anhydrous ethanol (EtOH) at least twice, allowing the EtOH to evaporate and the film to dry, and then spraying a superhydrophobic layer coating liquid and a hydrophilic layer coating liquid to obtain a clean base film.

[0064] The anti-wetting, high thermal efficiency Janus membrane of the present invention has the following characteristics:

[0065] The sandwich-structured composite Si-PVDF-GO / PAN Janus membrane of this invention has a thickness of 130-145 μm, a porosity of 75%-80%, a pore size of 0.45-0.75 µm, a water contact angle greater than 150° for the superhydrophobic layer, and a contact angle less than 90° for the hydrophilic layer. In the air-gap membrane distillation desalination process of brine with a sodium chloride content of 3.5%, its permeate flux can reach 15-17 kg / (m³). 2 The desalination efficiency is greater than 99.9% (·h). An anti-wetting test was conducted using air-gap membrane distillation with 0.02 mmol of low-surface-tension sodium dodecyl sulfate (SDS) surfactant added to a 3.5% sodium chloride solution; the permeate flux remained at 10–16 kg / (m²). 2 ·h). Its thermal efficiency index, temperature polarization coefficient, is greater than that of the PVDF base membrane (<0.96), reaching 0.98. The larger the temperature polarization coefficient, the closer the membrane surface temperature is to the bulk fluid temperature, and the weaker the temperature polarization effect, which is beneficial for maintaining a high transmembrane vapor pressure difference and improving the mass transfer efficiency of the membrane distillation process.

[0066] The thermal conductivity (measured by a thermal conductivity meter) of the hydrophilic layer of the Janus membrane of this invention is greater than that of the base membrane (0.0408 W / mK), reaching 0.0427 W / mK. The higher the thermal conductivity of the hydrophilic layer, the more conducive it is to the rapid heat dissipation and condensation of steam on the membrane permeation side, thereby reducing the condensation thermal resistance and promoting the enhancement of heat transfer on the condensation side, thus improving the thermal efficiency of the membrane distillation process. Moreover, the thermal conductivity of its superhydrophobic layer is much lower than that of the base membrane, as low as 0.0393 W / mK. The superhydrophobic layer has a low thermal conductivity, which is beneficial to suppress ineffective heat conduction on the feed side, reduce heat conduction loss, thereby maintaining a high transmembrane temperature difference and improving the thermal efficiency of the membrane distillation process.

[0067] Beneficial effects

[0068] Building upon existing research, this invention innovates the composite membrane preparation structure by employing a spray coating method to prepare a sandwich-structured composite Si-PVDF-GO / PAN Janus membrane with asymmetric wetting properties at multiple interfaces. This preparation method is simple, convenient, and easily scalable for industrial production. Furthermore, due to the asymmetric wetting properties of the multiple interfaces, the superhydrophobic layer exhibits strong anti-wetting properties, while the hydrophilic layer demonstrates high thermal conductivity, significantly improving the stability, durability, and efficiency of membrane distillation, thus possessing broad application prospects. Attached Figure Description

[0069] Figure 1 The images shown are scanning electron microscope (SEM) images of the Janus superhydrophobic membrane prepared using the method described in Example 1. The left image is a low-magnification SEM image, and the right image is a high-magnification SEM image.

[0070] Figure 2 The images shown are scanning electron microscope (SEM) images of the hydrophilic layer of the Janus membrane prepared using the method in Example 1. The left image is a low-magnification SEM image, and the right image is a high-magnification SEM image.

[0071] Figure 3 The images shown are scanning electron microscope (SEM) images of the Janus membrane cross-section prepared using the method in Example 1. The left image is a low-magnification SEM image of the overall membrane cross-section, and the right image is a high-magnification SEM image of a local section of the cross-section.

[0072] Figure 4 This is a photograph of the air contact angle of the Janus membrane superhydrophobic layer prepared using the method in Example 1.

[0073] Figure 5 Photographs showing the air water contact angle of the Janus membrane hydrophilic layer prepared using the method in Example 1.

[0074] Figure 6 The thermal conductivity data are for the Janus membrane base, superhydrophobic layer, and hydrophilic layer prepared using the method in Example 1.

[0075] Figure 7 This diagram illustrates the wettability flux and salt rejection performance of the Janus membrane distillation process using experimental methods.

[0076] Figure 8 TPC data graphs for testing the Janus membrane and base membrane distillation process using experimental methods. Detailed Implementation

[0077] The principles and features of the present invention are described in conjunction with the following embodiments. The examples are only used to explain the present invention and are not intended to limit the scope of the present invention.

[0078] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0079] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar to or equivalent to those described herein may be used in the implementation or testing of this invention.

[0080] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0081] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0082] Example 1: Preparation of sandwich-structured composite Si-PVDF-GO / PAN Janus membrane

[0083] (1) Preparation of superhydrophobic coating liquid

[0084] 1-1) Add nano-silica particles (Si-NPs, 0.4 g) to anhydrous ethanol (EtOH, 81 g), sonicate for 30±5 min (usually 20-40 min), mix thoroughly to ensure Si-NPs are uniformly dispersed in EtOH, wherein:

[0085] The mass ratio of Si-NPs to EtOH is 0.4:81 (i.e., 1:202.5, usually (1:(130-205)), that is, every 1 g of Si-NPs is mixed with 202.5 g of EtOH, that is, every 1 g of Si-NPs is uniformly dispersed in 202.5 g of EtOH, and usually every 1 g of Si-NPs is dispersed in 130-205 g of EtOH;

[0086] 1-2) After ultrasonic treatment, add ultrapure water (9 g), NH3·H2O (ammonia water, 8 g), and FAS-17 (1.6 g), and stir evenly with a magnetic stirrer to obtain a superhydrophobic coating solution for later use.

[0087] The stirring rate is 400±10 r / min, and the stirring time is 2 hours (usually 1-3 h).

[0088] The mass ratio of nano-silica (Si-NPs) to ultrapure water is 0.4:9 (i.e., 1:22.5, usually (1:(14-25)).

[0089] The mass ratio of Si-NPs to NH3·H2O is 0.4:8 (i.e., 1:20, usually 1:(11-21)).

[0090] The concentration of NH3·H2O is 25% (usually 25-28%); concentrated ammonia solution;

[0091] The mass ratio of Si-NPs to FAS-17 is 0.4:1.6 (i.e., 1:4, usually 1:(3-5)).

[0092] The content of Si-NPs in the superhydrophobic coating liquid is 0.4wt% (usually 0.2wt%-0.6wt%).

[0093] The content of FAS-17 in the superhydrophobic coating spray is 1.6 wt% (usually 1.4 wt%-1.8 wt%).

[0094] The introduction of ammonia into the FAS-17 fluorinated nano-silica system aims to provide a mild alkaline environment to catalyze the hydrolysis and subsequent condensation reaction of the ethoxy groups in FAS-17. This promotes the effective condensation of the generated silanol groups with the hydroxyl groups on the surface of nano-SiO2, forming stable Si-O-Si covalent bonds. This process not only significantly improves the grafting efficiency and uniformity of fluorosilanes on the nanoparticle surface and inhibits ineffective self-polymerization reactions, but also improves the dispersion state of nano-SiO2, thus facilitating the construction of a structurally stable and durable superhydrophobic functional layer.

[0095] (2) Preparation of hydrophilic coating liquid

[0096] Polyacrylonitrile (PAN, 3 g), graphene oxide dispersion (GO dispersion, 7.5 g), and N,N-dimethylformamide (DMF, 89.5 g) were shaken at room temperature and pressure for 24 hours (usually 20-30 hours) until completely dissolved to obtain a hydrophilic coating solution for later use. The solution contained in the following is also prepared:

[0097] The GO dispersion is a solution of GO and DMF, wherein the mass ratio of GO to DMF is 2:1000;

[0098] The mass ratio of PAN to DMF is 3:97 (i.e., 1:32.3, usually 1:(30-35)).

[0099] The mass ratio of PAN to GO in the GO dispersion is 3:0.015 (i.e., 1:0.005, usually 1:(0.003-0.006)).

[0100] The PAN content in the hydrophilic layer spray liquid is 3 wt% (usually 2 wt%-4 wt%); the GO content in the hydrophilic layer spray liquid is 0.16 wt% (usually 0.1 wt%-0.2 wt%); the mass ratio of PAN to GO in the hydrophilic spray liquid is 100:5.2 (usually 100:(2.5-10), preferably 100:(5-5.5)).

[0101] (3) Base film cleaning treatment

[0102] The PVDF base film surface is rinsed three times (usually at least twice) with anhydrous ethanol (EtOH). After the EtOH evaporates and the film dries, a clean base film is obtained.

[0103] (4) Apply hydrophobic coating liquid

[0104] Using a 0.2±0.01 mm nozzle spray gun, at a distance of 10±1 cm from the substrate surface, and at an absolute spray pressure of 0.4±0.05 MPa, the superhydrophobic coating liquid was sprayed onto one side of the clean substrate film. The spray volume of the superhydrophobic coating liquid was 600 mL / m. 2 (Typically (500-700) mL / m) 2 Then, the base film coated with the superhydrophobic coating liquid is placed in an oven for drying. The drying temperature is 45±2 ℃ and the drying time is 30 min (usually 25-35 min). The Si-NPs and FAS-17 in the superhydrophobic coating liquid adhere to one side of the base film surface and partially enter the pores of the base film surface to form a mechanically interlocked composite interface, thus obtaining the superhydrophobic layer-base film. The spray gun coating is uniform and delicate and does not cause clogging.

[0105] (5) Apply hydrophilic coating liquid

[0106] Using a 0.2±0.01 mm nozzle spray gun, at a distance of 15±1 cm from the base film surface, and at an absolute spray pressure of 0.2±0.01 MPa, the hydrophilic layer coating liquid was sprayed onto the other side of the superhydrophobic layer-base film. The spraying volume of the hydrophilic layer coating liquid was 600 mL / m. 2(Typically (500-700) mL / m) 2 The hydrophilic layer coating is applied in three coats (usually two to four coats), with each coat containing 200 mL / m³. 2 The process involves spraying 600 mL of hydrophilic layer spray solution per square meter of base film surface onto the other side of the base film surface in three applications. After each application of the hydrophilic layer spray solution, the base film is placed in an oven for drying at a temperature of 60±2 ℃ for 20 min (usually 15-25 min). The GO and PAN in the hydrophilic layer spray solution adhere to the other side of the superhydrophobic layer-base film, forming a "sandwich" structure, namely hydrophilic layer-base film layer-superhydrophobic layer, thus preparing the Janus membrane of the present invention.

[0107] High-resolution electron micrographs of the superhydrophobic and hydrophilic surfaces of the Janus membrane of this invention are shown below. Figure 1 and Figure 2 As shown, the electron microscope image of the hydrophilic layer of the Janus membrane cross-section is as follows. Figure 3 As shown.

[0108] When characterizing the cross-sectional morphology of Janus films using scanning electron microscopy, it is difficult to simultaneously and clearly present the complete structure of the superhydrophobic layer, the base film, and the hydrophilic layer within the same field of view. Therefore, it is necessary to photograph each functional layer separately. The main reasons are as follows: Firstly, Janus films typically exhibit a significant asymmetric layered structure, with each layer differing significantly in thickness, pore structure characteristics, and surface roughness. The superhydrophobic and hydrophilic layers are mostly nanometer to micrometer-scale thin layers, while the base film is relatively thick, making it difficult to capture both the overall field of view and local details at a single magnification. Secondly, the differences in material composition and electrical properties among different functional layers lead to inconsistent imaging contrast and focusing conditions under electron beam illumination. If the entire layer is photographed, problems such as local overexposure, insufficient resolution, or blurred interfaces often occur. Therefore, electron microscopy characterization typically employs independent cross-sectional imaging of the superhydrophobic layer, the base film, and the hydrophilic layer to ensure the clarity of the structural features of each layer and the reliability of the characterization results.

[0109] The hydrophilicity and hydrophobicity of both sides of the Janus membrane were tested separately, and it was found that the water contact angle of the superhydrophobic layer exceeded 150°. Figure 4 The water contact angle of the hydrophilic layer is less than 90°, see Figure 5 Its hydrophilicity and hydrophobicity meet Janus membrane standards.

[0110] The thermal conductivity of the membrane at 25°C was determined using a Hot Disk thermal constant analyzer (model TPS2500S) from Hot Disk GmbH, Sweden. Thermal conductivity data for the base membrane, superhydrophobic layer, and hydrophilic layer are as follows: Figure 6 As shown.

[0111] Example 2: Preparation of sandwich-structured composite Si-PTFE-GO / PAN Janus membrane

[0112] (1) Preparation of superhydrophobic coating liquid

[0113] 1-1) Add 0.6 g of Si-NPs to 81 g of EtOH, and ultrasonically vibrate for 30 min (usually 20-40 min) to mix thoroughly, ensuring that the Si-NPs are uniformly dispersed in the EtOH.

[0114] The mass ratio of Si-NPs to EtOH is 0.6:81 (i.e., 1:135, usually (1:(130-205)).

[0115] 1-2) After ultrasonic treatment, add 9 g of ultrapure water, 7.6 g of NH3·H2O and 1.8 g of FAS-17, and stir evenly with a magnetic stirrer to obtain a superhydrophobic coating solution for later use.

[0116] The stirring rate is 400±10 r / min, and the stirring time is 2 hours (usually 1-3 h).

[0117] The mass ratio of nano-silica (Si-NPs) to ultrapure water is 0.6:9 (i.e. 1:15, usually (1:(14-25)).

[0118] The concentration of NH3·H2O is 25% (usually 25-28%); concentrated ammonia.

[0119] The mass ratio of Si-NPs to NH3·H2O is 0.6:7.6 (i.e., 1:12.67, usually 1:(11-21)).

[0120] The mass ratio of Si-NPs to FAS-17 is 0.6:1.8 (i.e., 1:3, usually 1:(2-5)).

[0121] The content of Si-NPs in the superhydrophobic coating liquid is 0.6wt% (usually 0.2wt%-0.6wt%).

[0122] The content of FAS-17 in the superhydrophobic coating spray is 1.8wt% (usually 1.4wt%-1.8wt%).

[0123] (2) Preparation of hydrophilic coating liquid

[0124] The same as step (2) in Example 1.

[0125] (3) Base film cleaning

[0126] The same as step (3) in Example 1.

[0127] (4) Apply hydrophobic coating liquid

[0128] The same as step (4) in Example 1.

[0129] (5) Apply hydrophilic coating liquid

[0130] The same as step (5) in Example 1.

[0131] Experimental example:

[0132] The performance of the Janus membrane in Example 1 was determined using the following method:

[0133] 1. Water contact angle: The hydrophilicity and hydrophobicity of the membrane under different conditions were evaluated using the OCA50 water contact meter (WCA) from Dataphysics GmbH, Germany. The wetting behavior of 0.8 μL ultrapure water droplets at the initial contact moment (t=0 s) with the membrane surface was recorded under constant temperature conditions. The static water contact angle was calculated by automatically fitting the droplet profile using the Young-Laplace equation.

[0134] The results of the water contact angle measurement are as follows: Figure 4 , 5 .

[0135] The test results show that the superhydrophobic layer of the prepared Janus membrane has a contact angle greater than 150° (151.5°) and a hydrophilic layer contact angle less than 90° (~62°), which meets the Janus membrane standard, i.e., it has asymmetric hydrophilicity and hydrophobicity.

[0136] 2. The thermal conductivity of the membrane at 25 °C was determined using a Hot Disk thermal constant analyzer (model TPS2500S) from Hot Disk GmbH, Sweden.

[0137] The measurement results are as follows Figure 6 .

[0138] The measurement results show that the thermal conductivity of the superhydrophobic layer of the Janus membrane prepared in this invention (0.0393 W / mK) is lower than that of the base membrane (0.0408 W / mK), which can isolate ineffective heat transfer on the hot side. The thermal conductivity of the hydrophilic layer (0.0427 W / mK) is higher than that of the base membrane, which can facilitate rapid heat dissipation and condensation on the permeation side of the membrane, thereby reducing the condensation thermal resistance and promoting enhanced heat transfer on the condensation side.

[0139] 3. Membrane distillation experiments were conducted using the Janus membrane prepared in Example 1:

[0140] The performance of Example 1 and the base PVDF membrane was evaluated using an air-gap membrane distillation process.

[0141] Before conducting the air-gap membrane distillation experiment, the Janus membrane prepared according to this invention is cut to a size that matches the membrane module, with the superhydrophobic layer of the Janus membrane facing the hot-side feed liquid and the hydrophilic layer facing the cold-side air-gap condenser. Subsequently, the Janus membrane is reliably bonded to the membrane module through a sealing structure and connected to a hot and cold feed liquid circulation device for air-gap membrane distillation operation.

[0142] The hot side used a 3.5wt% sodium chloride aqueous solution at 50 ℃ (simulating seawater salinity), and the cold side used deionized water at 10 ℃. The flow rate on the hot side was controlled at 2 L / min, and the flow rate on the cold side was controlled at 3 L / min. The effective membrane area was... The membrane exhibits good stability, with a salt rejection rate of nearly 100%, and the pure water flux remains relatively stable.

[0143] Membrane antiwetting test: hot side 50 ℃, cold side 10 ℃ deionized water, hot side flow rate controlled at 2 L / min, cold side flow rate controlled at 3 L / min, effective membrane area is Initially, 0.02 mmol SDS was added to a 3.5 wt% NaCl salt solution, and the pure water flux and salt rejection of the original PVDF membrane and the prepared Janus membrane were tested using the same method. The test results for both membranes (the Janus membrane prepared in this embodiment of the invention, and the PVDF base membrane) are as follows: Figure 7 As shown, only the Janus membrane achieved a salt rejection rate of nearly 100% within 8 hours of operation, while the base membrane completely lost its membrane distillation function after 3 hours of operation.

[0144] Figure 7 In the diagram: dots represent flux data, and triangles represent rejection rate data.

[0145] Membrane thermal efficiency experiment: hot side 50 ℃, cold side deionized water 10 ℃, hot side flow rate controlled at 2 L / min, cold side flow rate controlled at 3 L / min, effective membrane area 4.17*10 -3 m 2 The temperatures of the original PVDF membrane and the prepared Janus membrane at the inlet and outlet on the hot and cold sides were measured using type K thermocouples. The average temperature on the hot side was calculated as follows: The average temperature on the cold side is The membrane heating side temperature is The cold side temperature of the membrane is The thermal efficiency of the membrane was characterized using the temperature polarization coefficient. The test results for the two membranes are as follows: Figure 8 As shown. The calculation formula is as follows:

[0146]

[0147] The measurement results show that the TPC (temperature polarization coefficient, 0.98) of the MD process using the Janus membrane of the present invention is greater than that of the PVDF base membrane (<0.96). This indicates that under the same operating conditions, the Janus membrane prepared by the present invention can effectively weaken the temperature polarization effect in the membrane distillation process, making the actual temperature difference on the membrane surface closer to the overall temperature difference of the system, thereby maintaining a higher effective heat transfer driving force and demonstrating the significant advantage of the Janus membrane of the present invention in terms of membrane distillation thermal efficiency.

[0148] The above embodiments of the present invention are merely exemplary and do not constitute any limitation on the scope of the present invention. Those skilled in the art should understand that modifications or substitutions can be made to the details and form of the technical solutions of the present invention without departing from the spirit and scope thereof, but all such modifications and substitutions fall within the protection scope of the present invention.

Claims

1. A Janus membrane with a sandwich structure, characterized in that: It includes a base film, a superhydrophobic layer, and a hydrophilic layer, wherein the base film is located between the superhydrophobic layer and the hydrophilic layer and is tightly bonded together.

2. The Janus membrane as described in claim 1, characterized in that, The base membrane is a hydrophobic membrane.

3. The Janus membrane as described in claim 1 or 2, characterized in that, The superhydrophobic layer is prepared by the following method: inorganic nanoparticles are dispersed in anhydrous ethanol to prepare an inorganic nanoparticle dispersion; then ultrapure water, ammonia, and heptadecafluorodecyltrimethoxysilane are added to the dispersion and stirred to obtain a superhydrophobic layer spray solution; then the superhydrophobic layer spray solution is sprayed onto one side of the surface of the base film, and after drying, the superhydrophobic layer is formed.

4. The Janus membrane as described in claim 1 or 2, characterized in that, The process involves mixing an organic polymer, a dispersion of high thermal conductivity nanomaterials, and an organic solvent, stirring, dissolving, and preparing a hydrophilic layer spray solution. The hydrophilic layer spray solution is then sprayed onto the surface of the other side of the base film, and after drying, the hydrophilic layer is formed.

5. A method for preparing a sandwich-structured Janus membrane, characterized in that, Includes the following steps: 1) Preparation of superhydrophobic coating spray liquid Ultrapure water, ammonia and heptadecafluorodecyltrimethoxysilane FAS-17 were added to the dispersion of inorganic nanoparticles and stirred until homogeneous to obtain a superhydrophobic coating solution for later use. 2) Preparation of hydrophilic coating liquid An organic polymer, a dispersion of high thermal conductivity nanomaterials, and an organic solvent are mixed, stirred, dissolved, and a hydrophilic coating solution is prepared for later use. 3) Spraying superhydrophobic and hydrophilic layers A superhydrophobic layer coating liquid and a hydrophilic layer coating liquid are sprayed on both sides of the base film, respectively. After drying, a sandwich structure Janus film is obtained.

6. The preparation method according to claim 5, characterized in that, The inorganic nanoparticles mentioned in step 1) are nano-silica, nano-alumina, nano-titanium oxide, nano-zinc oxide, or nano-magnesium oxide particles.

7. The preparation method according to claim 5 or 6, characterized in that, The content of inorganic nanoparticles in the superhydrophobic coating liquid in step 1) is 0.2wt%-0.6wt%; the content of FAS-17 in the superhydrophobic coating liquid is 1.4wt%-1.8wt%.

8. The preparation method according to claim 5 or 6, characterized in that, The organic polymer mentioned in step 2) is a copolymer of polyacrylonitrile (PAN) or acrylonitrile (AN), preferably polyacrylonitrile.

9. The preparation method according to claim 5 or 6, characterized in that, Step 2) The high thermal conductivity nanomaterial is a single-layer graphene oxide (GO) or carbon nanotubes (CNTs), preferably GO.

10. The preparation method according to claim 5 or 6, characterized in that, In step 2), the content of organic polymer in the hydrophilic layer spray liquid is 2wt%-4wt%; the content of high thermal conductivity nanomaterial in the hydrophilic layer spray liquid is 0.1wt%-0.2wt%; and the mass ratio of organic polymer to high thermal conductivity nanomaterial in the hydrophilic spray liquid is 100:(2.5-10), preferably 100:(5-5.5).

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