Method for synthesis of ionic liquid nanodroplets by solute-induced phase separation

Abstract

The application provides a method for synthesizing ionic liquid nanodroplets through solute-induced phase separation, and relates to the technical field of nanomaterials. The method generates a hydrophobic ionic liquid in situ through an ion exchange reaction in a homogeneous system, and constructs nanodroplets by using the nucleation and growth law of the ionic liquid from nothing to something. The method effectively solves the technical problems of wide droplet distribution, large size and easy coalescence of droplets in the traditional "top-down" method.

Description

Technical Field

[0001] This invention provides a method for synthesizing ionic liquid nanodroplets through solute-induced phase separation, belonging to the field of nanomaterials technology. Background Technology

[0002] Ionic liquids (ILs) are generally salts with melting points below 100°C, typically composed of organic cations and organic or inorganic anions. In recent decades, ILs have attracted widespread research attention due to their unique physicochemical properties, such as extremely low vapor pressure, tunable ionic conductivity, wide liquid phase temperature range, and excellent thermal stability. These properties have demonstrated significant application value in battery electrolytes, gas separation, catalytic reactions, and biocatalysis. However, the viscosity of many ionic liquids is typically in the range of 10–10,000 cP, significantly higher than that of common organic solvents. Higher viscosity reduces the diffusion rate and mass transfer efficiency of the solute in the system, thus limiting the full realization of their performance.

[0003] To optimize the mass transfer behavior of ionic liquids (ILs) and expand their application potential, transforming ionic liquids from macroscopic liquid phases into dispersed droplets, especially nanoscale droplets, has become an attractive strategy. Compared to macroscopic liquid phases, droplet systems can significantly increase specific surface area and shorten substrate diffusion distances to the micrometer or even nanometer scale, thereby potentially alleviating mass transfer limitations caused by high viscosity and improving the overall performance of ionic liquid systems.

[0004] Currently, there are two common methods for generating smaller droplets. One method involves inputting a large amount of mechanical energy into a mixture of two or more immiscible liquids, thereby breaking one of the liquid phases into small droplets and dispersing them within the continuous phase composed of the other liquids. Methods such as stirring, vortex emulsion homogenizers, and ultrasonic disruption fall into this category. Droplets generated by mechanical disruption are not small enough, and their size uniformity is far from ideal, often spanning more than two orders of magnitude in radius distribution. More importantly, volume is a cube of radius; a tenfold difference in radius translates to a tenfold difference in volume, which, under constant density, is equivalent to a tenfold difference in mass. Therefore, large-volume individuals actually account for the vast majority of the total mass of the droplet, and even if ultra-small droplets exist in the product, their mass proportion is extremely low. Another commonly used method for generating small droplets is to "squeeze" the droplets using flow control. However, this method makes it very difficult to compress the size of a single droplet to the submicron level. Moreover, liquids have high viscosity and surface tension, and their density is higher than that of water and common organic solvents, further increasing the difficulty of "squeezing" ultra-small droplets. Therefore, microfluidics and similar devices are not suitable for generating nanoscale ultra-small droplets of liquids. Summary of the Invention

[0005] Based on this, the present invention provides a method for synthesizing ionic liquid nanodroplets through solute-induced phase separation. This method involves generating a hydrophobic ionic liquid in situ through an ion exchange reaction in a homogeneous system, and then constructing nanodroplets using the liquid's nucleation and growth mechanism from scratch. This invention effectively solves the technical problems of wide droplet distribution, large size, and easy aggregation in traditional top-down methods.

[0006] The present invention is specifically implemented using the following technical solutions: A method for synthesizing ionic liquid nanodroplets through solute-induced phase separation includes the following steps: Step 1, Preparation of dispersed phase precursor solution A: Dissolve the anionic salt in ammonia solution to obtain solution A; preferably, the mass concentration of the ammonia solution is 10-15 wt%; the mass concentration of solution A is 10-20 wt%. Step 2, Preparation of dispersed phase precursor solution B: Dissolve the cationic liquid in an ethanol-water mixed solvent to obtain solution B; preferably, the mass concentration of the ethanol is 8~15 wt%; the mass concentration of solution B is 3~4 wt%. Step 3, solute-induced phase separation and nucleation: Under vortex conditions, a stabilizer solution is added to solution B, followed by rapid addition of solution A to carry out an ion exchange reaction, generating a hydrophobic ionic liquid in situ and nucleating and growing it into nanodroplets; preferably, the mass ratio of the stabilizer to the cationic liquid is 1:2-3; the volume ratio of solution A to solution B is 1:4-5; Step 4, Primary shell coating: While the system is continuously mixed, a first silane precursor is added and allowed to react statically to form a primary silica shell seed on the surface of the nanodroplets; preferably, the mass ratio of the amount of the first silane precursor added to the ionic nanodroplets is 0.3~0.6:1. Step 5, secondary seed growth: The second silane precursor is added dropwise to the system from Step 4 in batches, with each addition occurring 15 minutes apart. This allows the silica shell to be directionally deposited and thickened on the primary shell seed. After the reaction is complete, the mixture is separated and purified to obtain ionic liquid nanodroplets. Preferably, the mass ratio of the second silane precursor to the nanodroplets is 0.8~1.0:1. Preferably, the anionic salt contains bis(trifluoromethanesulfonyl)imide ([NTf2)). - ), hexafluorophosphate (PF6) - ) or tetrafluoroborate (BF4) - ) salt.

[0007] More preferably, the salt providing the anion is sodium bis(trifluoromethanesulfonyl)imide (NaNTf2).

[0008] Preferably, the cationic liquid is an imidazole cationic liquid.

[0009] More preferably, the ionic liquid is 1-octyl-3-methylimidazolium chloride or 1-butyl-3-methylimidazolium chloride.

[0010] Preferably, the stabilizer solution is an aqueous solution of polyvinyl alcohol.

[0011] Preferably, the first silane precursor is a mixture of 3-chloropropyltrimethoxysilane and tetramethoxysilane.

[0012] More preferably, the volume ratio of 3-chloropropyltrimethoxysilane to tetramethoxysilane in the first silane precursor is 1:1.

[0013] Preferably, the second silane precursor is added dropwise to the system in step 4 in four batches.

[0014] Compared with the prior art, the present invention has the following beneficial effects: (1) The “bottom-up” SIPS mechanism: abandoning the traditional mechanical crushing method, the hydrophobic ionic liquid phase is generated in situ by ion exchange reaction, which conforms to the LaMer nucleation growth model. The instantaneous nucleation of ionic liquid nanodroplets is controlled at the molecular scale, achieving excellent monodispersity and precise control of nanoscale size.

[0015] (2) Innovative “seed secondary growth” coating strategy: This solves the problems of droplet deformation, wrinkling (“shriveled fruit” shape) and mutual adhesion caused by adding silicon source in a single step. By adding a small amount of TEOS step by step, the low concentration of silicon source in the system is maintained, which effectively inhibits homogeneous nucleation in the solution, induces uniform thickening of the silicon oxide shell, and finally obtains a regular, smooth, and mechanically stable rigid sphere.

[0016] (3) Size and height controllable: Without changing the solvent ratio, only the amount of PVA added can be changed to achieve precise and continuous control of the size of nanodroplets.

[0017] (4) Strong applicability: The method of the present invention avoids the electrostatic imbalance and self-nucleation interference caused by the addition of surfactants, and shows good applicability to a variety of ionic liquids such as imidazole, pyridine, pyrrolidine and quaternary ammonium salts. Attached Figure Description

[0018] Figure 1 A schematic diagram illustrating the bottom-up method for forming ionic liquid nanodroplets and the SIPS process.

[0019] Figure 2 Transmission electron microscopy (TEM) image of droplet surface collapse and wrinkling (shriveled appearance) caused by initial coating.

[0020] Figure 3 This invention uses TEM images of the evolution of droplet morphology from shriveled to plump and regular during the process of adding TEOS (seed secondary growth method) in different batches.

[0021] Figure 4 Electron microscopy results showing the effect of different amounts of solution A and solution B on droplet size.

[0022] Figure 5 Electron microscopy results showing the effect of different PVA addition amounts on the droplet size of ionic liquids.

[0023] Figure 6 TEM images of [C8C1im][NTf2] nanodroplets under different washing methods: water washing retains the ionic liquid core; hollow silica structure is formed after alcohol washing to extract the ionic liquid.

[0024] Figure 7 The structural evolution and elemental redistribution of nanodroplets before and after electron beam irradiation are shown in the diagrams: (a) solid microspheres; (b) hollow microspheres after electron beam irradiation.

[0025] Figure 8 The images show HAADF-STEM images and EDS elemental distribution maps of the ionic liquid microspheres after washing with ethanol.

[0026] Figure 9 The chemical structure diagrams for various ionic liquids used in the extended investigation are shown below. Pyridines: (1) [BPy] + (2) [OPy] + ; Pyrrolidines: (3) [Pyr 14 ] + (4) [Pyr 18 ] + ; Quaternary ammonium salts: (5) [N 4444 ] + (6) [N] 1118 ] + Inapplicable systems (red dashed box): (7) [C2OHMIM] + (8) [N] 11 (C2OH)2] + (9) [Choline] + (10) [N] 1111 ] + All ionic liquids share the anion [NTf2]. - .

[0027] Figure 10 TEM images of ionic liquid droplets coated with different cation structures: (a) [BPy][NTf2]; (b) [Pyr] 14 [NTf2];(c)[N4444 ][NTf2]; (d) [OPy][NTf2]; (e) [Pyr 18 [NTf2];(f)[N 1118 [NTf2].

[0028] Figure 11 TEM images of [C8C1im][NTf2] droplets coated under different alkaline catalyst conditions: (a) NaOH; (b) KOH; (c) LiOH; (d) TEA. Scale bar: 500 nm. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the preferred embodiments of this invention will be described in further detail below with reference to the examples. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0030] Example 1: Preparation of Standard Synthesis System ([C8C1im][NTf2] Ionic Liquid Nanodroplets) (1) Preparation of solution A: Weigh 29.8 mg of sodium bis(trifluoromethanesulfonyl)imide (NaNTf2) and dissolve it in 100 μL of ammonia water (mass fraction 12.5 wt%).

[0031] (2) Preparation of solution B: Weigh 30 mg of 1-octyl-3-methylimidazolium chloride ([C8C1im]Cl) and dissolve it in 20% ethanol solvent.

[0032] (3) Formation and primary coating of nanodroplets: Under intense vortex oscillation, 100 μL of PVA aqueous solution (10 wt%) was added to solution B. Subsequently, solution A was rapidly added to solution B and vortexed continuously for 30 seconds to induce solute phase separation and generate nanodroplets. Immediately afterwards, 20 μL of mixed silane (a 1:1 volume ratio of CPTMS to TMOS) was added under vortex conditions. The reaction was allowed to stand for 15 min to form a primary seed shell on the droplet surface.

[0033] (4) Seed secondary growth method coating: TEOS was added in batches: it was added in 4 batches, each batch containing 10 μL, and the time interval between two consecutive additions of TEOS was 15 min (the total amount of TEOS added was 40 μL).

[0034] (5) Purification: The above reaction system was continued to react at room temperature for 3 hours. After the reaction was completed, the product was separated by centrifugation and then purified by continuous alternating washing with anhydrous ethanol and ultrapure water to obtain well-shaped, monodisperse core-shell structured ionic liquid nanodroplets.

[0035] Comparative Example 1: Conventional wrapping without using the seed secondary growth method ( Figure 2 ) The same steps (1)-(3) as in Example 1 were used, but TEOS was not added after the addition of mixed silane. Experimental results: TEM characterization showed that although the ionic liquid coating was successfully achieved, most of the particles did not present a regular spherical shape, but showed obvious surface collapse and wrinkle characteristics (resembling "dried fruit"), and there was some adhesion between the particles, indicating that the shell was too thin and the mechanical stability was poor.

[0036] Example 2: Optimized coating using the seed secondary growth method ( Figure 3 ) A "seed growth" strategy was introduced: the initially formed ultrathin silica shell was used as a seed, and silane was added incrementally in steps. 10 μL of TEOS was added dropwise to the reaction system every 15 minutes, for a total of four additions (cumulative addition of 40 μL). Figure 3 The diagram clearly illustrates the evolution of the product's microstructure as the number of TEOS additions increases. Among other things, Figure 3 (a) No secondary seed growth was performed; (b) 10 μL TEOS was added once; (c) 10 μL TEOS was added twice; (d) 10 μL TEOS was added three times; (e) 10 μL TEOS was added four times. After the first addition of 10 μL TEOS, TEM images of the product showed no abrupt change in shell thickness, and most particles retained the characteristics of the primary product, exhibiting obvious wrinkling and depressions. With the introduction of the second and third silanes, the contrast at the shell edge gradually increased, indicating that the silica layer was continuously depositing and thickening.

[0037] Example 3: Effect of Ionic Liquid Precursor Addition Amount on Droplet Morphology ( Figure 4 ) While keeping the solvent ratio, catalyst concentration, and silane addition method constant, the amount of ionic liquid precursor added was adjusted. Figure 4 To simultaneously and proportionally increase or decrease the amount of [C8Clim]Cl and NaNTf2 added while keeping the volume ratio of solution A to solution B constant. Figure 4(a)-(d) represent the dosages of [C8C1im]Cl and NaNTf2 increased to 4, 3, 2, and 1.5 times respectively under standard conditions; (e)-(h) represent the dosages of [C8C1im]Cl and NaNTf2 decreased to 0.7, 0.5, 0.35, and 0.24 times respectively under standard conditions.

[0038] This invention simultaneously scales up or reduces the amounts of [C8Clim]Cl (providing cations) and NaNTf2 (providing anions) proportionally, increasing them by 1.5, 2, 3, and 4 times, respectively, and reducing them to 0.7, 0.5, 0.35, and 0.24 times. As the concentration decreases, the nanodroplets no longer maintain a perfect spherical shape but gradually undergo anisotropic deformation, exhibiting a distinctly "eccentric" structure. Furthermore, at extremely low concentrations (0.24 times), a large number of fine, dark particles were observed. This may be because the amount of silane added remains constant; when the concentration of the ionic liquid precursor decreases sharply, the total surface area of ​​the generated droplets decreases, leading to a relative excess of silane in the system and homogeneous nucleation. Conversely, an excess of ionic liquid leads to an increase in droplet size.

[0039] Example 4: The effect of stabilizer PVA on droplet formation and particle size control ( Figure 5 ) While keeping other standard synthesis conditions constant, the reaction was investigated in the absence of PVA (0 wt%). The experiment revealed that after 3 h of static incubation, a large amount of white flocculent precipitate appeared in the system. After alternating washing with alcohol and water, TEM images showed that the product consisted of severely aggregated solid spherical particles, with no hollow structures observed. Figure 5 (ah) TEM images of the products prepared at different PVA concentrations are shown, where Figure 5 (a) 0 wt%, (b) 1.25 wt%, (c) 2.5 wt%, (d) 5.0 wt%, (e) 7.5 wt%, (f) 10 wt%, (g) 12.5 wt%, (h) 15 wt%, (i) Dependence curve of average particle size of product on PVA concentration.

[0040] This phenomenon indicates that, in the absence of PVA assistance, the silica precursor failed to successfully coat the surface of the ionic liquid droplets, instead undergoing homogeneous nucleation and aggregation. However, when a trace amount of PVA (0.125 wt%) was introduced into the system, the morphology of the product underwent a fundamental change, generating a uniform hollow nanosphere structure with an average diameter of approximately 399 ± 102 nm. This further confirms that the interfacial adsorption of PVA is a prerequisite for inducing shell growth of silica on the droplet surface.

[0041] Subsequently, the effect of PVA concentration on the size of ionic liquid nanodroplets was further investigated. As the PVA concentration gradually increased from 0.25 wt% to 1.0 wt%, the average diameter of the product exhibited a clear monotonically decreasing trend. Large-sample statistical analysis based on TEM images (number of particles N>200) showed that when the PVA concentrations were 0.25 wt%, 0.5 wt%, 0.75 wt%, and 1.0 wt%, the corresponding average droplet sizes were 386±61 nm, 353±84 nm, 288±64 nm, and 237±46 nm, respectively. Notably, when the PVA concentration was further increased beyond 1.0 wt%, the average particle size of the product no longer changed significantly, and the particle size distribution curve tended to plateau.

[0042] Example 5: Morphology characterization of ionic liquid nanodroplets ( Figure 6-8 ) from Figure 6 You can watch ( Figure 6 a is a TEM image of IL@silica after water washing only; Figure 6 (b is a TEM image of IL@silica washed with alcohol). The product was gently washed with deionized water; the ionic liquid was not washed away here. Due to the strong hydrophobicity of [C8C1im][NTf2], water washing only removed residual impurities on the surface without damaging the internal droplet nucleus. Under the high vacuum environment of TEM, the coexistence of solid and hollow spheres can be observed. Because the ionic liquid is relatively sensitive to electron beam irradiation, the ionic liquid in the droplet nucleus migrated under the action of the electron beam. TEM observation shows that some IL@silica underwent an evolution from a solid sphere to a hollow structure under electron beam irradiation. Notably, even under long-term irradiation, some microspheres maintained a solid morphology, which may be attributed to these particles having a thicker silica shell or a more stable internal structure, thus effectively resisting electron beam damage.

[0043] To further visualize the spatial composition distribution within the ionic liquid droplets, we used energy-dispersive X-ray spectroscopy (EDS) to perform surface scanning imaging analysis of the elemental distribution of the samples. For example... Figure 7 As shown ( Figure 7 a shows the HAADF-STEM image and EDS elemental distribution of IL@silica solid microspheres after water washing; Figure 7(b represents a hollow sphere after electron beam irradiation). For intact solid microspheres that have not undergone hollowing, the Si element, representing the silica framework, exhibits a continuous and uniform distribution throughout the entire particle. Notably, its signal intensity gradually increases from the center to the edge, clearly outlining the spherical contour and shell boundary of the microsphere. Meanwhile, F and S, characteristic elements of ionic liquids, are mainly confined to the central core of the microsphere, and their distribution range closely matches the morphological contour of the microsphere. This "core-shell" spatial distribution characteristic of Si encapsulating F and S elements confirms that the ionic liquid droplet has been successfully and completely encapsulated within a silica shell.

[0044] In stark contrast, when we perform EDS analysis on microspheres that have become "hollowed out" due to electron beam bombardment within the same field of view, such as... Figure 7 As shown in b, a strikingly different elemental distribution pattern was observed. After the formation of the hollow structure, the distribution of F and S elements changed: they were no longer confined to the central region of the microspheres, but instead exhibited a clear tendency to diffuse outwards, eventually accumulating in the silica shell region. This phenomenon indicates that under the high-energy action of the electron beam, the originally coated ionic liquid migrated.

[0045] Based on this, we utilized the property that ionic liquids are readily soluble in ethanol, and used ethanol to perform three centrifugal washings on the sample. TEM images after ethanol extraction showed that the obtained product exhibited a well-defined, well-dispersed hollow spherical structure, with the shell remaining continuous and intact, without obvious breakage or collapse. These hollow silica nanoparticles had an average diameter of 237±46 nm, and the corresponding EDS further showed ( Figure 8 The F and S element signals, representing ionic liquids, no longer have a specific spatial distribution and only appear as weak and scattered background signals; while the Si element is uniformly distributed in the shell; this result confirms that silicon dioxide has formed a continuous and dense coating layer on the droplet surface.

[0046] Example 6: Exploring the universality of the synthesis of ionic liquid nanodroplet structures ( Figure 9-10 ) We selected commonly used pyridinium, pyrrolidinium, and quaternary ammonium ionic liquids as subjects for further investigation. The specific cationic chemical structures provided in this section are shown in the figure. The specific synthesis process is consistent with the standard procedure. Figure 9 ).

[0047] When cations were provided by ionic liquids 1-6 mentioned above, numerous and clear hollow spherical structures were observed in all systems, indicating that the formed ionic liquid droplets were successfully coated. When the cation side chain was butyl (C4), the resulting droplet morphology exhibited significant irregularity, with some structures collapsing or flattening. In contrast, when the side chain length increased to octyl (C8), the droplets in all systems showed a significant improvement in morphological regularity, exhibiting a more uniform spherical structure. Figure 10 ).

[0048] Example 7: The effect of alkaline catalysts on the droplet formation process ( Figure 11 ) When the ammonia in the system was replaced with strong inorganic bases—sodium hydroxide (NaOH), potassium hydroxide (KOH), lithium hydroxide (LiOH), and a bulky organic base—triethylamine (TEA), silica still managed to encapsulate the ionic liquid droplets, exhibiting a clear hollow structure. Specifically, when using strong bases KOH and NaOH, which have significantly different ionic radii from ammonia, the resulting droplet morphology was highly consistent with the standard ammonia system, exhibiting perfectly spherical shapes with excellent monodispersity, extremely high sphericity, and uniform shell thickness. This indicates that KOH… + with Na + Ions did not disrupt the ordered assembly at the ionic liquid interface and promoted the formation of a dense shell. However, while triethylamine could also induce silica nucleation at the interface, resulting in larger droplet sizes and some droplets sticking together, we speculate that the weak basicity of triethylamine might slow down the hydrolysis and condensation of silica. This could mean that some droplets collided and fused before the shell could completely encapsulate them. This fusion resulted in a larger final droplet size than the initial droplets.

[0049] In summary, the experimental results under different alkaline catalysis conditions demonstrate that this droplet coating strategy has high applicability to the external environment.

[0050] The embodiments described above are some, but not all, embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for synthesizing ionic liquid nanodroplets through solute-induced phase separation, characterized in that, Includes the following steps: Step 1, Preparation of dispersed phase precursor solution A: Dissolve the anionic salt in ammonia water solvent to obtain solution A; Step 2, Preparation of dispersed phase precursor solution B: Dissolve the cationic liquid in a mixed solvent of ethanol to obtain solution B; Step 3, solute-induced phase separation and nucleation: Under vortex conditions, a stabilizer solution is added to solution B, followed by rapid addition of solution A to carry out an ion exchange reaction, generating a hydrophobic ionic liquid in situ and nucleating and growing it into nanodroplets; Step 4, Primary shell coating: While the system is continuously mixed, the first silane precursor is added and allowed to react statically to form primary silica shell seeds on the surface of the ionic liquid nanodroplets. Step 5, secondary seed growth: The second silane precursor is added dropwise to the system in step 4 in batches, with an interval of 15 min between each addition, so that the silica shell is directionally deposited and thickened on the primary shell seed. After the reaction is completed, the ionic liquid nanodroplets are obtained by separation and purification.

2. The method for synthesizing ionic liquid nanodroplets by solute-induced phase separation as described in claim 1, characterized in that, The anionic salt contains bis(trifluoromethanesulfonyl)imide ([NTf2)). - ), hexafluorophosphate (PF6) - ) or tetrafluoroborate (BF4) - ) salt.

3. The method for synthesizing ionic liquid nanodroplets by solute-induced phase separation as described in claim 2, characterized in that, The salt that provides the anion is sodium bis(trifluoromethanesulfonyl)imide (NaNTf2).

4. The method for synthesizing ionic liquid nanodroplets by solute-induced phase separation as described in claim 1, characterized in that, The cationic liquid is an imidazole cationic liquid.

5. The method for synthesizing ionic liquid nanodroplets by solute-induced phase separation as described in claim 4, characterized in that, The ionic liquid is 1-octyl-3-methylimidazolium chloride or 1-butyl-3-methylimidazolium chloride.

6. The method for synthesizing ionic liquid nanodroplets by solute-induced phase separation as described in claim 1, characterized in that, The stabilizer solution is an aqueous solution of polyvinyl alcohol.

7. The method for synthesizing ionic liquid nanodroplets by solute-induced phase separation as described in claim 1, characterized in that, The first silane precursor is a mixture of 3-chloropropyltrimethoxysilane and tetramethoxysilane.

8. The method for synthesizing ionic liquid nanodroplets by solute-induced phase separation as described in claim 1, characterized in that, The volume ratio of 3-chloropropyltrimethoxysilane to tetramethoxysilane in the first silane precursor is 1:

1.

9. The method for synthesizing ionic liquid nanodroplets by solute-induced phase separation as described in claim 1, characterized in that, The second silane precursor was added dropwise to the system in step 4 in four batches.

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