Method for quantitatively characterizing ionic liquid structure at interface

Abstract

The invention discloses a method for quantitatively characterizing an ionic liquid structure at an interface, which comprises the following steps of: regulating a self-assembly structure of ionic liquid on the surface of an electrode through voltage, measuring data such as Young modulus, film thickness, indentation depth and effective contact area of an ionic liquid film under different voltage conditions, and combining the data with reciprocals (1 / F0) of the interaction of quantitative molecules to obtain the ionic liquid structure at the interface. According to the method, an ionic liquid structure model (EMSF) is constructed, the rigidity and the stability of different ionic liquid membranes can be quantitatively analyzed according to the EMSF model by measuring interaction, and a new theory and method support is provided for design of an ionic liquid-electrode interface. The method has a wide application prospect, and an optimized interface design scheme can be provided for the fields of electrochemical batteries, lubrication and the like.

Description

Technical Field

[0001] The present invention relates to the field of ionic liquids and interfaces under voltage regulation, and relates to a method for quantitatively characterizing the structure of ionic liquids at interfaces. Background Art

[0002] Ionic liquids are salts composed of organic cations and inorganic or organic anions. They remain liquid at or near room temperature, with equal amounts of cations and anions, resulting in overall electrical neutrality. They are therefore also known as room-temperature ionic liquids or low-temperature molten salts. Ionic liquids possess numerous unique physicochemical properties, such as very low vapor pressure, non-flammability, excellent thermal stability, and high electrical conductivity, which have attracted widespread attention in various fields. Their low melting point and viscosity, particularly their advantages in thermal stability and chemical inertness, as well as their ability to rapidly spread on the surfaces of various materials and form regular layered structures, demonstrate their great potential as lubricants.

[0003] In order to obtain ionic liquids with excellent performance, in-depth analysis of their structural characteristics at the interface is of great significance. Currently commonly used analytical methods include spectroscopic techniques such as infrared spectroscopy, Raman spectroscopy, and nuclear magnetic resonance, imaging techniques such as scanning probe microscopy and transmission electron microscopy, and diffraction and scattering techniques such as X-ray diffraction and neutron scattering. However, existing spectroscopic methods generally have difficulty in accurately characterizing the microstructural properties of ionic liquids at the interface (Interfacial Structure of Room-Temperature Ionic Liquids at the Solid–Liquid Interface as Probed by Sum Frequency Generation Spectroscopy), which limits their in-depth application in interface design and performance regulation. Summary of the Invention

[0004] The present invention provides a method for quantitatively characterizing the structure of ionic liquids at interfaces. The method introduces the ionic liquid structural model (EMSF). This factor quantifies the structural properties of ionic liquid membranes under different voltage conditions by combining parameters such as Young's modulus, membrane thickness, indentation depth, and effective contact area of ​​the ionic liquid membrane at the interface under voltage regulation. It further describes the quantitative relationship between the ionic liquid membrane structure and molecular interactions under voltage regulation.

[0005] The technical solutions for achieving the purpose of the present invention are as follows:

[0006] In a first aspect, the present invention provides a method for quantitatively characterizing the structure of an ionic liquid at an interface, comprising the following steps:

[0007] (1) [P 6,6,6,14][MEEA] ionic liquid was dissolved in ethanol solvent to prepare a concentration volume ratio of 10 -3 :1 ionic liquid solution;

[0008] (2) drop-coating the ionic liquid solution on the mica surface and vacuum drying to form an ionic liquid film;

[0009] (3) Using AFM to measure the thickness t of the ionic liquid film, as well as the indentation depth δ at different voltages, quantitative molecular interaction F0 and elastic modulus E;

[0010] (4) Using a quartz crystal microbalance to measure the adsorption capacity of the ionic liquid solution at different voltages, the effective contact area A was obtained;

[0011] (5) Based on the thickness t, effective contact area A, indentation depth δ and elastic modulus E at different voltages, the definition of the ionic liquid structure model EMSF at voltages from -4 V to +4 V is given:

[0012]

[0013] (6) Based on EMSF, the structural characteristics of the ionic liquid membrane at the interface were analyzed.

[0014] Furthermore, the solvent can be any one of acetonitrile, dimethylsulfamide, ethanol, and carbon tetrafluoride, preferably ethanol.

[0015] Furthermore, the ionic liquid can be [P 6,6,6,14 ][MEEA](trichlorohexadimethoxydimethylethanolamine), [P 6,6,6,14 ][TFSI](Trifluoromethanesulfonyl imide salt of trihexadimethoxadiazole), [P 6,6,6,14 ][DOSS](dioctyl succinate of trihexadimethoxane), [P 6,6,6,14 ][PF6](trihexadimethoxadiazole hexafluorophosphate), [P 6,6,6,14 ][BF4] (trihexammonium tetrafluoroborate), [BMIM][PF6] (1-butyl-3-methylimidazolium hexafluorophosphate), [BMIM][TFSI] (1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl imide), [EMIM][PF6] (1-ethyl-3-methylimidazolium hexafluorophosphate), [EMIM][TFSI] (1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl imide), etc., preferably [P 6,6,6,14 ][MEEA].

[0016] Furthermore, the substrate can be made of mica, graphite, titanium, or aluminum alloy, preferably mica.

[0017] Furthermore, the volume ratio of ionic liquid to solvent is 10 -7 :1~10-2 :1, preferably 10 -3 :1.

[0018] Furthermore, the ionic liquid is dissolved in a solvent and then drop-coated on the substrate, with a volume of 1 cm 2 2 to 6 μL, preferably 2 μL, is drop-coated on the substrate.

[0019] Furthermore, the voltage control range is -5V to +5V, -4V to +4V, -3V to +3V or -2V to +2V, preferably -4V to +4V.

[0020] Furthermore, the vacuum drying temperature is 20±°C, and the drying time is 12±4h.

[0021] Furthermore, the thickness t of the ionic liquid film, as well as the indentation depth δ, quantitative molecular interaction F0 and elastic modulus E at different voltages are measured by any one of atomic force microscopy, scanning electron microscopy, white light interferometry, thin film reflection spectroscopy, and laser interferometry, preferably atomic force microscopy.

[0022] Furthermore, the adsorption amount of the ionic liquid solution at different voltages is measured by any one of a quartz crystal microbalance, gravimetric method, ultraviolet-visible spectroscopy, and fluorescence spectroscopy, preferably a quartz crystal microbalance. Compared with the prior art, the present invention has the following advantages:

[0023] This paper proposes the ionic liquid structure model (EMSF) for the first time, which quantifies the structural characteristics of ionic liquid membranes under different voltage conditions by using parameters such as Young's modulus, membrane thickness, indentation depth and effective contact area of ​​the ionic liquid membrane at the interface. Furthermore, experiments show that the reciprocal of the EMSF (1 / EMSF) and the reciprocal of the molecular interaction force (1 / F0) show a high linear correlation (R 2 >0.99), accurately describing the quantitative relationship between ionic liquid membrane structure and molecular interactions under voltage regulation. This invention provides a direct quantitative characterization method for membrane structural properties, a reliable predictive tool for performance optimization, and a new theoretical basis for optimizing the design of ionic liquid-solid lubrication interfaces. This approach holds significant application value in future ionic liquid-based electrochemical energy storage and electronically controlled lubrication systems.

[0024] It should be understood that all combinations of the aforementioned concepts and the additional concepts described in more detail below, as long as such concepts are not mutually inconsistent, can be considered as part of the inventive subject matter of the present application. In addition, all combinations of the claimed subject matter are considered as part of the inventive subject matter of the present application.

[0025] The foregoing and other aspects, embodiments, and features of the present invention will be more fully understood from the following description in conjunction with the accompanying drawings. Other additional aspects of the present invention, such as features and / or beneficial effects of the exemplary embodiments, will become apparent from the following description or through practice of specific embodiments according to the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is the thickness change of ionic liquid membrane under different voltages.

[0027] Figure 2 Quantify the changes in molecular interaction values ​​for ionic liquid membranes at different voltages.

[0028] Figure 3 A quartz crystal microbalance (QCM) was used to measure the equilibrium adsorption amount of ionic liquids per unit area on the gold surface at different voltages.

[0029] Figure 4 is the Young's modulus of the ionic liquid membrane at different voltages.

[0030] Figure 5 Figure 3 is the relationship between the lateral friction force and the normal load in the friction test of the ionic liquid membrane.

[0031] Figure 6 It is the linear fitting relationship between the reciprocal of the molecular interaction force (1 / F0) and the reciprocal of the EMSF (1 / EMSF). DETAILED DESCRIPTION

[0032] In order to better understand the technical content of the present invention, specific embodiments are given and described below with reference to the accompanying drawings. In this invention, various aspects of the present invention are described with reference to the accompanying drawings, which show many illustrative embodiments.

[0033] The embodiments of the present invention are not necessarily intended to encompass all aspects of the present invention. It should be understood that the various concepts and embodiments described above, as well as those described in more detail below, can be implemented in any of a variety of ways, as the concepts and embodiments disclosed herein are not limited to any particular implementation. In addition, some aspects disclosed herein can be used alone or in any appropriate combination with other aspects disclosed herein.

[0034] Example 1:

[0035] ① Phosphate ammonium salt based ionic liquid [P 6,6,6,14 ][MEEA] was dissolved in ethanol to prepare a concentration of 10 -3 [P 6,6,6,14 ][MEEA] ionic liquid solution in ethanol.

[0036] ② Apply 2 μL of the prepared ionic liquid solution to the freshly cut mica surface with a surface area of ​​1 × 1 cm 2 The coated mica surface was then dried in a vacuum environment at 20°C for 12 hours until the solvent was completely evaporated to form a stable ionic liquid film.

[0037] ③ Use an atomic force microscope (AFM, Bruker Dimension Icon) to scan the membrane surface and obtain the surface morphology of the membrane. The membrane thickness is determined by measuring the height difference between the membrane surface and the substrate. Based on the membrane surface undulation and membrane deformation, the ion membrane thickness t is calculated. The membrane thickness changes under different voltages are as follows: Figure 1 shown.

[0038] ④ Using an AFM (Dimension Icon, Bruker) in contact mode, different voltages (-4, -2, 0, +2, +4 V) were applied to the gold colloidal probe under ambient conditions for force measurement. The prepared gold colloidal probe was used to capture the first approach and retraction force-distance curves during the "continuous approach" process to obtain the indentation depth δ of the gold colloidal probe on the IL surface and the quantitative molecular interaction F0. The quantitative molecular interaction value changes under different voltages are shown in Figure 2. Figure 2 shown.

[0039] ⑤ Use quartz crystal microbalance (QCM) to measure [P 6,6,6,14 The adsorption amount of ][MEEA] ionic liquid ethanol solution on the gold surface. The change in quartz crystal frequency at adsorption equilibrium was measured in ionic liquid ethanol solutions of different concentrations. There is a certain relationship between the adsorption amount and the effective contact area. The frequency change is converted into mass change, and then the effective contact area A is calculated. The equilibrium adsorption amount per unit area at different voltages is as follows: Figure 3 shown.

[0040] ⑥ Use AFM tapping mode (240AC-NA probe) to obtain morphological and phase images of ionic liquid films on gold surfaces at different voltages. Combined with PeakForce quantitative nanomechanical measurement (QNM) method, the Young's modulus E of ionic liquid films at different voltages is measured. Figure 4 shown.

[0041] ⑦ Friction force test was performed using AFM in contact mode with a scan range of 1 μm × 1 μm (scan rate of 1 Hz, scan angle of 90°). Voltages of -4 V, -2 V, 0 V, +2 V, and +4 V were applied to the ionic liquid / gold surface, and the torsional resonance frequency was recorded simultaneously to measure the lateral friction force F. N The relationship with the normal load is as follows Figure 5 The friction force is calibrated using the Liu method.

[0042] ⑧Combining the Young's modulus (E), indentation depth (δ), film thickness (t), effective contact area (A) and other data obtained from AFM test, the ionic liquid structure model (EMSF) is used: Characterizes the structure of ionic liquid membranes under voltage regulation. In this expression, Young's modulus captures the rigidity of the ionic liquid membrane, indentation depth represents the deformation under load, thickness squared reflects the compactness of the structure, and effective contact area ensures normalization of the actual molecular interaction area. Figure 1-4 As shown in Figure 3, at higher voltages (such as +4 V, -4 V), the EMSF factor is smaller, its structure is more compact, the ionic liquid membrane becomes more ordered, and its thickness is reduced due to the enhanced electrostatic attraction, resulting in a tighter and more rigid ion arrangement. Figure 6 It shows that the inverse of the molecular interaction force (1 / F0) and the inverse of the EMSF (1 / EMSF) show a high linear correlation, which is consistent with the Hertzian-like behavior of the ionic liquid membrane. Under low (2V, -2V) or neutral voltage (0V) conditions, the EMSF factor is larger, and the ionic liquid membrane exhibits higher ion mobility and a softer structure, which leads to deeper depressions (larger δ) and larger effective contact areas, which are related to lower F0, and the ionic liquid structure is easily destroyed. This conclusion can be found in Figure 5 It has been verified in the literature that the friction coefficient of the ionic liquid membrane at higher voltage is higher, corresponding to lower ion mobility, which is related to the harder ionic liquid membrane structure at higher voltage; at lower voltage or neutral conditions, the ionic liquid membrane is softer, corresponding to faster ion mobility and lower friction.

[0043] In summary, the present invention provides a method (EMSF) for quantitatively characterizing the structure of ionic liquids at interfaces, which is used to describe the quantitative relationship between the structure of ionic liquid membranes and molecular interactions under voltage regulation. By combining parameters such as the Young's modulus, membrane thickness, indentation depth and effective contact area of ​​the membrane, EMSF successfully quantitatively correlates the relationship between the structural changes of ionic liquid membranes and molecular interactions, filling the gap in the prior art for the quantitative description of ionic liquid structural changes under voltage regulation (Nanoscale Perturbations of Room Temperature Ionic Liquid Structure at Charged and Uncharged Interfaces.). The experimental results show that under negative voltage, the stability of the ionic liquid membrane is optimal, the friction coefficient is significantly reduced, while the friction force is higher under positive voltage. In addition, this method provides a new idea for the design and optimization of ionic liquid membranes, making the ionic liquid structure model EMSF expected to be applied to the fields of electrochemical energy storage and electronically controlled lubrication in the future, providing important theoretical support for the regulation of electrochemical device performance and lubrication system interfaces.

Claims

1. A method for quantitatively characterizing the structure of ionic liquids at an interface, characterized in that: The following steps are involved: (1) dissolving the ionic liquid in a solvent to prepare an ionic liquid solution; (2) drop-coating the ionic liquid solution on the substrate surface and vacuum drying to form an ionic liquid film; (3) Measure the thickness t of the ionic liquid film, as well as the indentation depth δ at different voltages, quantitative molecular interaction F0 and elastic modulus E; (4) Measure the adsorption amount of the ionic liquid solution at different voltages to obtain its effective contact area A; (5) Based on the thickness t, effective contact area A, indentation depth δ and elastic modulus E at different voltages, the ionic liquid structure model EMSF is constructed: (6) Based on EMSF, the structural characteristics of the ionic liquid membrane at the interface were analyzed.

2. The method according to claim 1, wherein The solvent is selected from any one of acetonitrile, dimethylsulfamide, ethanol or carbon tetrafluoride, preferably ethanol.

3. The method according to claim 1, wherein The ionic liquid is selected from any one of the following: 6,6,6,14 ][MEEA]、[P 6,6,6,14 ][TFSI]、[P 6,6,6,14 ][DOSS]、[P 6,6,6,14 ][PF6]、[P 6,6,6,14 ][BF4], [BMIM][PF6], [BMIM][TFSI], [EMIM][PF6], [EMIM][TFSI], preferred [P6,6,6,14][MEEA].

4. The method according to claim 1, wherein The substrate is made of any one of mica, graphite, titanium and aluminum alloy, preferably mica.

5. The method according to claim 1, wherein The ionic liquid is dissolved in a solvent, wherein the volume ratio of the solvent to the ionic liquid is 10 -7 :1~10 -2 :1, preferably 10 -3 :

1.

6. The method according to claim 1, wherein The ionic liquid solution was dropped onto the substrate surface, with a drop size of 1 cm 2 2 to 6 μL, preferably 2 μL, is drop-coated on the substrate surface.

7. The method according to claim 1, wherein The vacuum drying temperature is 20℃±5℃, and the drying time is 12±4 hours.

8. The method according to claim 1, wherein In step (3) and step (4), the voltage control range is any range of -5V to +5V, -4V to +4V, -3V to +3V or -2V to +2V, preferably -4V to +4V.

9. The method according to claim 1, wherein The thickness t of the ionic liquid film, as well as the indentation depth δ and elastic modulus E at different voltages are measured by any one of atomic force microscopy, scanning electron microscopy, white light interferometry, thin film reflection spectroscopy, and laser interferometry, preferably by atomic force microscopy.

10. The method according to claim 1, wherein The adsorption amount of the ionic liquid solution at different voltages is measured by any one of quartz crystal microbalance, weight method, ultraviolet-visible spectroscopy, and fluorescence spectroscopy, preferably by quartz crystal microbalance.

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