1. Synthesis and preparation of  Xuansn capacitor ionic liquid

Xuansn capacitor ionic liquid have many outstanding advantages due to their unique physical, chemical and biological properties, such as excellent physical properties, easy handling, incompatibility with other co-solvents, non-volatile, and excellent solubility in various organic, inorganic, organometallic and biological substances; they have quite outstanding chemical properties, such as a wide electrochemical stability window, negligible vapor pressure, high ionic conductivity and temperature stability. At the same time, ionic liquids do not produce a large amount of pollutants in daily use, so they are classified as green solvents. Ionic liquids can meet the growing needs of resources, energy and health care, and have become one of the options for solving some major problems facing today’s society.

There are many methods for synthesizing Xuansn capacitor ionic liquid, and commonly used methods include acid-base neutralization method, halogen substitution method, microwave method, ultrasonic method, etc. This chapter mainly introduces the commonly used synthesis and characterization methods of ionic liquids. At present, there are two relatively mature methods for the synthesis of ionic liquids: direct synthesis method and two-step synthesis method. In addition, researchers have also developed new green chemical methods such as microwave heating technology, ultrasonic radiation technology and solvent thermal technology. These methods help to shorten the reaction time and improve the reaction conversion efficiency. It has been applied in inorganic and organic synthesis. Liquid phase synthesis is a feasible large-scale production method

1.1 Xuansn capacitor ionic liquid direct synthesis method

The direct synthesis method is also called the one-step synthesis method. It is a one-step synthesis method. It is a method of preparing ionic liquids through quaternization reaction or acid-base neutralization reaction. The operation is relatively simple and economical, convenient and fast, almost no by-products are produced, and the product is easy to purify. In most cases, the synthesis of Xuansn capacitor ionic liquid only requires one step. For example, nitroethylamine ionic liquid is prepared by neutralization reaction of ethylamine aqueous solution with nitric acid. The specific process is: after the neutralization reaction, the excess water is removed in vacuum. In order to ensure the purity of the ionic liquid, it is dissolved in an organic solvent (acetonitrile or tetrahydrofuran, etc.), and then treated with activated carbon. Finally, the organic solvent is removed in vacuum to obtain the ionic liquid product. A series of tetrafluoroborate ionic liquids containing different cations can be synthesized by direct synthesis. Ionic liquids such as 1-butyl-3-methylimidazolium salt can also be prepared in one step through quaternization reaction. For example, tetrabutylammonium hydroxide is used as a cation source to react with amino acids to prepare tetrabutylammonium-amino acid ionic liquids. The reaction equation is shown in Figure 3-1.

Lewis acidic AlCls-based ionic liquids can be synthesized by simple addition reactions, where Lewis acids or metal halides are added to halide salts to form the corresponding acidic ionic liquids with an additional halide species. Formula (3-1) is the reaction that occurs when the ratio between AlCl₃ and [Emim]Cl is different.

Figure 3-1 Reaction equation for direct preparation of tetrabutylammonium-amino acid ionic liquid by one-step acid-base neutralization (R is the side chain of different types of amino acids)

Based on anion replacement or anion exchange reaction with 1,3-dialkylimidazolium cations, a series of ionic liquids that are stable in air and water are easily obtained. Srour’s group used imidazole, phosphine and pyrrole alkyl ionic liquids to react with different anions [such as dicyanamide, thiocyanate, tetrafluoroborate and bis(trifluoromethylsulfonyl)imide] to prepare a series of ionic liquids that are stable at room temperature.

Heteropoly acids, as a type of traditional polymetallic acid salt-containing materials, are widely used due to their excellent properties. However, as a high proton conductor, heteropolyacids are easily deliquescent, easy to leak, and inconvenient to transport and store. These factors prevent them from being directly used in electrochemistry. In recent years, new inorganic-organic hybrid materials have emerged, which combine inorganic components (heteropolyacids) with high thermal stability and proton conductivity and organic components (ionic liquids) with highly flexible structures and flexible structural designs. This new material not only retains the advantages of ionic liquids, but can also significantly reduce the viscosity of ionic liquids. At the same time, the heteropolyanions in the system can also optimize the transport of protons in its hole structure and further improve the conductivity, making it a new electrolyte material with excellent performance. The synthetic idea of ​​this type of material is to construct sulfonic acid-functionalized ionic liquids as cations and then hybridize them with heteropolyacids. This method is not only easy to operate, but also improves the shortcomings of traditional heteropoly acids such as easy deliquescence, leakage, and poor stability, while maintaining its high conductivity and considerable activation energy.

Biopolymers have the advantages of low cost, high abundance, good renewability, and easy biodegradation, and have been widely used as raw materials for the synthesis of polymer materials. Lobregas et al. mixed equimolar amounts of epichlorohydrin and methylimidazole in 40 mL of acetonitrile (ACN) solvent to synthesize 1-glycidyl-3-methylimidazolium chloride (GMIC) ionic liquid, and then attached the ionic liquid part to starch to form starch cations. The physical appearance, solubility, morphology, and thermal behavior of the modified starch were improved.

1.2 Two-step synthesis of  Xuansn capacitor ionic liquid

When the target ionic liquid cannot be synthesized using the one-step synthesis method, a two-step synthesis method can be considered. The operation steps are as follows: first, a halide salt containing the target cation is prepared by quaternization reaction, that is, a [cation] X-type ionic liquid; then the target anion Y is used to replace X- or Lewis acid MXy is added to obtain the target ionic liquid. Among them, in the second step reaction, when the metal salt MY (commonly AgY or NH₄Y) is used, the generated AgX is neutral, and the ionic liquid is extracted with an organic solvent, and finally the organic solvent is removed in vacuo to obtain a pure ionic liquid. In the two-step synthesis method, the following points should be noted: In the process of exchanging the target anion (Y^–) with the X-anion, the reaction should be carried out as completely as possible to ensure that no X^–anion remains in the target ionic liquid, because the purity of the ionic liquid is extremely important in its application and the characterization of physicochemical properties. The synthesis of high-purity binary ionic liquids is usually carried out in an ion exchanger. In addition, the [cation] [M_n, X⁺_ny] type ionic liquid can be prepared by directly combining Lewis acid (MXy) with a halide salt. The advantages of the two-step synthesis method are good universality and high yield.

Since most halogen salts have strong water absorption, in order to avoid the influence of water vapor in the air on the product, the first step is usually carried out under the protection of inert gas: At the same time, the second step reaction cannot completely replace the halogen anion, and a small amount of by-products will remain in the ionic liquid, which can be purified to obtain high-purity ionic liquids. The typical two-step synthesis steps of imidazole ionic liquids are shown in Figure 3-2.

Figure 3-2 Two-step synthesis of imidazole ionic liquids

The two-step method utilizes the physical properties of Xuansn capacitor ionic liquid, that is, the solubility of ionic liquids and halide salts in organic solvents is different, which can achieve extraction separation. Therefore, the product obtained by the two-step method inevitably has a small amount (about 1%~5%) of impurities (halide ions) in the ionic liquid, which affects its use effect. At the same time, the organic solvent will also produce an equimolar amount of by-products, which are not easy to remove by conventional methods. Therefore, it is necessary to explore a higher purity synthesis method.

1.3 Ultrasonic-assisted synthesis of  Xuansn capacitor ionic liquid

Ultrasonic-assisted synthesis is a method of forming a local high-temperature and high-pressure microenvironment inside a liquid by means of ultrasonic cavitation, and the vibration and stirring effect of ultrasound can greatly increase the reaction rate, especially for heterogeneous chemical reactions. The mechanical cavitation effect of ultrasound can increase the interaction between molecules and improve the reaction efficiency. If ultrasound is used to assist in the synthesis of ionic liquids [BmimlBF₄, it is beneficial to shorten the reaction time, and acetone is used as a solvent in the reaction, which can be recycled and reused, which meets the requirements of “green chemistry” and is beneficial to the purification of ionic liquids. This method has the advantages of mild reaction conditions, fast reaction speed, low energy consumption, and easy purification of products.
The [Gly]Cl ionic liquid can be successfully synthesized by using the ultrasonic-assisted heating reflux method. The suitable conditions are: reaction temperature of 60°C, reaction time of 2h, molar ratio of hydrochloric acid to glycine of 1:1.8, ultrasonic power of 125W, and the yield of the ionic liquid is 89.97%. It can be seen that ultrasound can indeed enhance the synthesis process of ionic liquids and significantly shorten the synthesis time.

1.4 Xuansn capacitor ionic liquid microwave synthesis

Microwaves refer to electromagnetic waves with a wavelength between 1mm and 1m and a frequency between 300MHz and 300GHz, which have the characteristics of transmission, reflection and absorption. Microwave heating is based on the ability of certain liquids and solids to convert electromagnetic energy into heat to induce chemical reactions. Microwave heating is superior to traditional heating methods in organic synthesis, such as oil baths, because it does not require additional power consumption to heat the container. The heating intensity increases with the increase of the frequency and field strength of the electric field, and the degree of heating can be adjusted with high precision. Microwave radiation is conducive to the polarization of molecules, so that the reaction occurs rapidly. As a means of improving chemical reactions in various synthetic processes, microwave radiation has the characteristics of rapidity, uniformity, high controllability, and unique heating methods. Ionic liquids are generally prepared by heating and reflux in organic solvents, and the reaction time ranges from several hours to tens of hours. Unlike traditional modification methods, microwave technology has many advantages, such as no need for organic solvents, fast reaction speed, high yield, and good product purity.

Xuansn capacitor ionic liquid Microwave-assisted synthesis of ionic liquids is also known as microwave two-step synthesis. First, the monomer halide RX constituting the ionic liquid cation is mixed, placed in a microwave synthesizer for magnetic stirring or mechanical stirring, and continuously or intermittently microwave irradiated for a certain period of time, and the halide or ionic liquid intermediate containing the target cation is prepared by alkylation or quaternization reaction; then the target anion is replaced by X or Lewis acid is added to prepare the target ionic liquid. When microwave heating is used, the reaction conditions must be strictly controlled. The one-step synthesis rule is to mix the synthesized raw materials in a reactor and react under microwave irradiation until the target ionic liquid is generated.
Studying new methods to optimize the polymerization process using microwave heating assistance can prepare functional polymer materials with specific properties. Room temperature ionic liquids can show low volatility and high ionic conductivity. The former property makes RTILs more environmentally friendly than volatile organic solvents, while the latter property enables them to provide a good reaction environment for absorbing microwave radiation. Therefore, combining the properties of ionic liquids with microwave heating provides a fast and environmentally friendly method for polymer synthesis.

1.5 Xuansn capacitor ionic liquid electrochemical synthesis method

The steps of preparing high-purity ionic liquids by electrochemical synthesis are to first select compounds containing target cations such as quaternary ammonium salts, quaternary phosphine salts, sulfur salts, and nitrogen-containing heterocyclic compounds, sulfur-containing heterocyclic compounds, etc., in which anions can be generated by electrolytic oxidation to generate chlorine, carbon dioxide or nitrogen, etc.; at the same time, it is also necessary to select compounds containing target anions such as acetic acid and nitric acid, in which cations can be reduced by electrolysis (such as hydrogen, ammonia or nitrogen, etc.). The gas generated by electrolysis is discharged from the electrochemical reaction cell, and the remaining target anions and cations pass through the ion exchange membrane in the electrochemical reaction cell to form the final ionic liquid. The purity of the ionic liquid synthesized by electrochemical technology can reach 99.99% after dehydration, the content of halide ions such as chloride ions can be less than 100μg·g^-1, and the content of metal ions such as sodium ions can be less than 20μg·g^-1. Electrochemical synthesis is a method for synthesizing high-purity ionic liquids, but its synthesis equipment and operation are relatively complicated.

The basic principle of electrochemical synthesis of ionic liquids is to take the electrochemical synthesis of [Emim]Ac as an example. The electrolysis device is shown in Figure 3-3. The electrolysis principle is

Figure 3-3 [Emim]Ac electrolysis device

From the reaction principle of electrochemical synthesis, the synthesis process is completed in two steps, that is, the product of the first step is used as the raw material to synthesize the ionic liquid of the second step. When preparing high-purity ionic liquids by electrochemical methods, the design of the reactor needs to consider the following factors:
(1) Selection of electrode materials: The materials are required to work stably and permanently, have high current efficiency and low overpotential, and be reasonably priced.
(2) Temperature and pressure: Except for special requirements, electrolytic cells are usually operated at normal pressure, and the temperature is often higher than room temperature to accelerate the electrode reaction rate, reduce polarization, and improve the conductivity of the electrolyte.
(3) Mass transfer method: Generally, stirring or circulating electrolyte is used to optimize mass transfer. For electrodes that release gas, the solution can be stirred by rising bubbles.
(4) Selection of ion exchange membranes, whether anion exchange membranes or cation exchange membranes are used, it is necessary to consider whether the substances in the cell generated by the product will contain impurity ions.
In the design of electrochemical reactors, material balance is performed based on the yield requirements of a single reactor, the requirements for the conversion rate of raw materials, and the current efficiency data, and the total amount of electricity passing through the reactor is calculated. Then, the total surface area of ​​the required electrodes is calculated based on the selected current density, so as to preliminarily select a reactor operation mode. Then, according to the nature of the electrochemical reaction, the physicochemical properties of the reactants and products, the temperature of the electrolyte, etc., the electrode material is selected and the overall structure of the electrolytic cell is determined. Current density is an important parameter of the electrochemical reactor, which affects the cell voltage and current efficiency. Without reducing the current efficiency, the current density is increased as much as possible to increase production capacity. However, the increase in current density will be limited by diffusion and mass transfer.
Heating reflux method

1.6 Xuansn capacitor ionic liquid heating reflux method

It is one of the traditional methods for preparing ionic liquids, which generally requires organic solvents and is time-consuming. It can also be divided into a one-step synthesis method and a two-step synthesis method. The one-step method is mainly a silver salt method, which is relatively simple. The synthesis process of the two-step method is as follows: First, a halide salt containing the target cation ([cation] X-type ionic liquid) is prepared by quaternization reaction, and then Y^– or Lewis acid is added to replace the X ion with the target anion to obtain the target ionic liquid. After that, a strong protonic acid HY is added, and the reaction needs to be carried out under low-temperature stirring conditions, and then washed several times to neutrality, and the ionic liquid is extracted with an organic solvent, and finally the organic solvent is removed in a vacuum environment to obtain a pure ionic liquid. In addition, [cation] M_nX_ny+1-type ionic liquid can be prepared by directly combining Lewis acid with a halide salt. The heating reflux method is economical and simple to operate, has no by-products, and the product is easy to purify.

1.7 Xuansn capacitor ionic liquid liquid-liquid extraction method

Liquid-liquid extraction is a method for preparing room temperature ionic liquids using a water-organic solvent liquid-liquid two-phase reaction extraction method301. The basic steps are: first, select an alkali metal salt containing a target cation and a target anion, and exchange the reaction in the aqueous phase; then, based on the principle of “like dissolves like”, select an organic solvent that is insoluble in water but can dissolve the target ionic liquid, and use the target product ionic liquid to dissolve in the organic solvent, while the formed inorganic salt dissolves in the aqueous phase, thereby achieving the synthesis of high-purity ionic liquids.
Liquid-liquid extraction is an effective separation technology and is widely used in analytical chemistry. However, traditional liquid-liquid extraction often introduces organic solvents with high volatility, toxicity and flammability, which can easily cause environmental pollution and safety accidents. Therefore, it is necessary to find a new liquid-liquid extraction system that is environmentally friendly, has a high separation and enrichment multiple, and can recycle the extractant.

2. Analysis of Xuansn capacitor ionic liquid

ILs are usually composed of organic cations and weakly basic organic or inorganic anions with side chains, which in turn can be functionalized. The nature of the ions can greatly affect the physicochemical properties and basic characteristics of ionic liquids. In principle, specific properties can be obtained by matching different types of cations and anions, which is conducive to their use as “special solvents”. ILs are widely used in organic synthesis, catalysis, electrochemistry, liquid phase extraction and analytical chemistry as new environmentally friendly solvents.
At present, the structural research of Xuansn capacitor ionic liquid mainly adopts nuclear magnetic resonance, infrared spectroscopy and Raman spectroscopy to characterize the structure of ionic liquids, among which nuclear magnetic resonance and infrared spectroscopy are the most common. In addition to the above methods, electrochemical methods, mass spectrometry and ultraviolet visible spectroscopy have also been widely used. This chapter will mainly introduce the characterization methods of Xuansn capacitor ionic liquid, so as to analyze the structure of typical ionic liquids.

2.1 Xuansn capacitor ionic liquid infrared spectrum

When a beam of infrared light with a continuous wavelength passes through a substance, and the vibration frequency or rotation frequency of the group in the molecule is the same as the infrared light, the molecule can absorb energy from the original vibration (rotation) kinetic energy and reach a vibration (rotation) energy level with higher energy from the ground state level. After the molecule absorbs infrared radiation, vibration and rotation energy level transitions will occur. Therefore, infrared spectroscopy is essentially an analytical method for determining molecular structure and identifying compounds based on the relative vibration of atoms in the molecule and the rotation of the molecule. Infrared spectra can be obtained by recording the absorption of infrared light. In infrared spectroscopy, wavelength (a) or wave number (σ) is often used as the horizontal axis to indicate the position of the absorption peak, and transmittance (T%) or absorbance (a) is used as the vertical axis to indicate the absorption intensity.

Yan et al. synthesized polyvinylidene fluoride/ionic liquid functionalized silica microporous polymer electrolyte. Figure 3-4 is the spectrum obtained by infrared spectroscopy analysis. In the FTIR spectrum of IL-SiO₂, the vibration band near 960cm^-1‘ disappeared, which is the characteristic of the SiOEt group of the starting chemical. There is a strong vibration band at 1000~1200cm^-1, which is attributed to Si—O—Si, confirming the formation of the silica network. In addition, the presence of iodinated imidazole components in the silica network is determined by the peaks near 1572 cm^-1 and 1447 cm^-1, which correspond to the stretching vibration bands of the imidazole ring. In addition, the absorption peak near 3400cm^-1 indicates the presence of silanol.

Figure 3-4 Infrared spectra of TESPIm⁺I^–and IL-SiO₂

Farah et al. used polyvinyl alcohol (PVA), sodium trifluoromethanesulfonate (NaTf) and 1-butyl-3-methylimidazole bromide ([BmimlBr) as raw materials, and used activated carbon (AC), carbon black, carbon nanotubes (CNT), polyvinylidene fluoride (PVDF) and 1-methyl-2-pyrrolidone to prepare a material combining a solid polymer electrolyte and a Xuannsn capacitor ionic liquid, and conducted infrared spectrum analysis on it. The results are shown in Figure 3-5. Figure 3-5 compares the infrared spectra of pure PVA, pure NaTf, pure [Bmim]Br, PVA60 and IL-0.5SPE.

Figure 3-5 Infrared spectra of pure PVA, pure NaTf, pure [Bmim]Br, PVA60 and IL-0.5 SPE

Veisi et al. prepared a new type of sulfonic acid functionalized ionic liquid C₃N₄ C₃N₄SO₃H(IL)1, studied the catalytic activity of the ionic liquid for pyrazolo[3.4-b1pyridine and bis(indolyl)methane derivatives under neutral conditions, and used FTIR analysis to explain the functionalization principle of g-C₃N₄ (graphitic carbon nitride) by -SO₃H (Figure 3-6). The spectrum of g-C₃N₄-SO₃H shows that the original C-N network of g-C₃N₄ still exists and remains almost unchanged after functionalization. The broad peaks between 3000 and 3500 cm^-1 are attributed to the stretching vibrations of the two compounds (-NH and N-H) and the hydrogen bonding interaction of g-C3Na. The absorption peaks observed at 1305cm^-1 and 1604cm^-1 are caused by the stretching vibrations of C-N and C=N, respectively. After functionalization of g-C₃N₄ with —SO3H, some new peaks appeared, such as the absorption peaks at 1051 cm^-1 and 1152 cm^-1 corresponding to the symmetric and asymmetric stretching of S=O, respectively. The broad peak at 3000~3500 cm^-1 is caused by the —OH stretching vibration and the hydrogen bonding interaction of g-CaNa. The functionalization of g-C₃N₄ does not change its C—N network.

Figure 3-6 FTIR images of g-C₃N₄and g-C₃N₄-SO₃H

Infrared spectroscopy analysis not only plays an important role in the structural exploration of Xuansn capacitor ionic liquid, but also plays an irreplaceable role in the understanding of the system structure of ionic liquid mixtures. Xuansn capacitor ionic liquid ionic liquid mixed systems include ionic liquid-water system, ionic liquid-organic system and ionic liquid-carbon dioxide system.

Song et al. first studied the use of cholesteryl butyrate groups to synthesize a new type of ionic liquid crystal (ILC) based on hexamethylenetetramine through ionic self-assembly design, and used various techniques to study the ionic self-assembly mode, chemical structure and liquid crystal (LC) properties of ILC, and systematically studied how to promote ion migration through hexamethylenetetramine-based ionic self-assembly structure. The researchers used cholesterol and 4-bromobutyryl chloride to synthesize brominated LC cholesteryl 4-bromobutyrate (ChBA), and prepared hexamethylenetetrabromide-type ionic liquid crystal [ChBA-H1Br through the salt-forming reaction between brominated ChBA and hexamethylenetetramine. Figure 3-7 shows the FTIR spectra of the prepared [ChBA-H]Br and the precursor ChBA in the range of 500~2100cm^-1.

Figure 3-7 FTIR spectra of ionic liquid crystal [ChBA-H]Br and precursor ChBA

2.2 Xuansn capacitor ionic liquid NMR

Nuclear magnetic resonance is mainly caused by the spin motion of atomic nuclei. The study of nuclear magnetic resonance spectroscopy can reflect the changes in the chemical environment around the atomic nucleus in detail, and is one of the methods for studying molecular structure, dynamics, configuration, etc. Nuclear magnetic resonance can not only qualitatively analyze and characterize the composition and structure of various organic and inorganic substances, but also can be used for quantitative analysis. In the field of Xuansn capacitor ionic liquid, nuclear magnetic resonance technology is mainly used to confirm the types of cations and anions in the system and the interaction between anions and cations, etc. At the same time, it can also determine the purity and properties of ionic liquids, study the aggregation behavior of ionic liquids in solutions, and determine the thermodynamic behavior of ionic liquids.

Zang et al. prepared three new quaternary ammonium polyionic liquids (PILs) products, including tetradecyl dimethyl benzyl ammonium sulfonated polyvinyl benzoate, hexadecyl dimethyl benzyl ammonium chloride and stearyl dimethyl benzyl ammonium sulfonated polyvinyl benzoate (TDBAS, HDBAS and SDBAS) through three processes of polymerization, sulfonation and ion exchange, in order to improve the viscosity of liquid CO₂ (LCO₂). They also studied and discussed the characteristics of the new quaternary ammonium PILs without the addition of toxic elements such as P, B and F. Vinyl benzoate and various quaternary ammonium salts are used to provide CO₂philic groups and viscosity-enhancing groups, respectively. After polymerization, sulfonate anion groups are introduced into polyvinyl benzoate; thereafter, ion exchange is carried out with the existing quaternary ammonium salt to achieve the purpose of combining the CO₂philic groups with the viscosity-enhancing groups. These quaternary ammonium PILs can be effectively used as LCO₂ viscosity enhancers with the help of the required cosolvents. The H nuclear magnetic resonance (NMR) spectra of all the intermediates and target products (PILs) obtained are shown in Figures 3-8.

Figure 3-8’H NMR spectrum

(A) VBz (lower), PVBz (middle), SPVBz (upper); (B) TDBAS (lower), HDBAS (middle), SDBAS (upper)

Briones et al. synthesized poly(4-vinylpyridine) with different alkyl chain lengths, different molecular weights, and different hydrophobic properties by exchanging halide ions with more hydrophobic anions, which contained BF^–₄, PF^–₆(CF₃SO₂)₂N, and CF₃SO^–₃; as counterions. The PIL was synthesized from N-alkyl quaternized poly(4-vinylpyridine) P-4VP⁺-C_nBr- (n=2, 4, 5) with different alkyl chain lengths and different molecular weights (60000 and 160000). The obtained system was compared with the polyelectrolyte as a raw material, namely N-alkyl quaternized poly(4-vinylpyridine), and it was found that the system has potential application value in the separation and removal of heavy metal ions in water treatment. The obtained PIL was molecularly characterized by ¹HNMR and ¹⁹FNMR. Figure 3-9 shows the ¹HNMR spectra of P-4VP⁺-C₄Br^– and P-4VP⁺-C₄BF^–₄. In both spectra, signals corresponding to the protons of the pyridine ring were observed (7.8~8.9ppm). However, the anion exchange of Br- with BF changes the shape and chemical shift of these signals. In order to verify the presence of the counterion BE^–₄ in PIL, ¹⁹F NMR analysis was performed (Figure 3-9). Similar response characteristics were also found for the molecular characterization of the remaining systems.

Khan et al. recorded the¹H and ¹³CNMR spectra of the phosphate ester (BEHP) series of ionic liquids at resonance frequencies of 500.13 MHz and 125.78 MHz, respectively, using a Bruker-Av-III 500MHz spectrometer. The NMR spectra were collected using a cadmium trichloride solution containing 20% ​​of the sample. The FTIR spectra of the BEHP ionic liquids in the wavenumber range of 400~4000 cm-! were recorded using a dual-spectrum FTIR spectrometer (PerkinElmer). This study confirmed the presence of BEHP anions in ionic liquids.

Figure 3-9 ¹H NMR spectra of P-4VP⁺-C₄Br^–(D2O) and P⁺4VP+-C₄BF^–₄ (acetone-d6)

The inset is the ¹⁹F NMR spectrum of P⁺-4VP+-C₄BF^–₄[poly(4-vinylpyridine)]tetrafluoroborate

2.3 UV-visible absorption spectrum

Organic compound molecules contain σ electrons that form single bonds, π electrons that form double bonds, and unbonded lone pairs of n electrons. When a molecule absorbs radiation of a certain energy, these electrons will transition to a higher energy level. The orbit occupied by the electrons at this time is called an antibonding orbital, and this electron transition is closely related to the internal structure. Ultraviolet-visible (UV-vis) absorption spectroscopy is widely used. It can not only be used to qualitatively analyze substances, but also to quantitatively analyze the physical and chemical properties of certain compounds. It can also be used in conjunction with infrared spectroscopy, nuclear magnetic resonance, etc. for qualitative, quantitative and structural analysis. For example, UV-vis absorption spectroscopy can be used to study the hydrogen bonding effect and hydrogen bond strength of ionic liquid systems.

Williams et al. conducted a series of characterizations on the ionic liquids 1-ethyl-3-methylimidazolium acetate [Emim]Ac and 1-butyl-3-methylimidazolium acetate [Bmim]Ac. UV-vis absorption spectroscopy was used to monitor the stability of mixtures of ILs with ethanol, methanol, water and aprotic solvents (DMF) at different high temperatures (100°C, 120°C and 150°C) and pressures (~35 bar and 350 bar) within 24 hours. Figure 3-10 shows the changes in the UV spectra of 1% [Bmim]Ac and 1% [Emim]Ac in ethanol solutions at 100°C, 120°C and 150°C and 350 bar within 24 hours. The shifts in these spectra all indicate that the ionic liquids have degraded. Moreover, as the degradation proceeds, the UV detector becomes saturated at lower wavelengths. The saturation of the detector at shorter wavelengths may be due to the longer path length of the high-voltage battery.

Figure 3-10 Delayed UV spectra of mixtures of 1% [Bmim] Ac (left column) and 1% [Emim] Ac (right column) in ethanol at 100℃, 120℃ and 150℃, 350 bar after 24h

In order to improve the conductivity of diesel and avoid static spark explosion/fire, Ren et al. used different quaternary ammonium salts such as dodecyltrimethylammonium chloride, tetradecyltrimethylniobium chloride, hexaalkyltrimethylammonium chloride, stearyltrimethylammonium chloride (DTAS, TTAS, HTAS and STAS) and n-butyl cyclohexanesulfonate in an ultrasonic process to synthesize a series of RTILs, and based on the reverse micelle (RM) microstructure and physicochemical properties of the synthesized ILs, the mechanism of action was elucidated and discussed. In order to detect the optical properties of the prepared samples, UV-visible absorption spectra were characterized to study the effect of ILs type on RM size. Figure 3-11 shows the UV-Vis spectra of RM at the same ILs concentration (5 ppm). Although each of the strongest absorption peaks of RM formed by these four types of ILs is concentrated near 230 nm (as shown by the arrows), there are still slight differences in the position of the strongest absorption peak. According to the analysis results of the refractive index, the molecular polarity of ILs tends to increase with the increase of cationic alkyl carbon chains, and it is difficult to form large-sized RMs due to close stacking. At the same time, according to the quantum size effect, the energy gap of the particle absorption band decreases with the decrease of the particle radius, resulting in a blue shift in the UV-visible absorption spectrum.

Figure 3-11 UV-visible spectrum of ILs RM (reverse micelle)

xuansn capacitor The unique properties of ionic liquids in physical chemistry make them play an important role in electrochemistry, separation and catalytic reaction processes. Studying the aggregation behavior of ILs helps to explain the mechanism in practical production applications. With the development of science and technology, ionic liquids have also made breakthrough progress in electrochemistry, new materials, chemical synthesis, adsorption separation technology, liquid surface treatment technology, organic catalysis and synthesis.Zha et al. selected ionic liquid surfactants CTAB, [C₁₆mim]BF₄, [C₁₂mim]BF₄, [C₁₀mim]BF₄and orange G (OG) to study the complex formation process between ionic liquids and dyes. UV-visible absorption spectrum analysis found that in the presence of OG, the UV-visible absorption spectrum changes with the increase of carbon chain length. The absorption spectrum of OG in aqueous solution with a concentration of 0.002~0.08 mmol·L^-1 is shown in Figure 3-12. Two main peaks can be observed in the figure, which increase regularly with increasing concentration. As an azo dye, hydroxyl-containing OG usually exists in aqueous solution in the form of two isomers, namely azo form (—C=N—N—C) or hydrazone form (—C=N—NH—C). There is a hydrazine-azo tautomeric equilibrium in the solution, and the proportion of different forms decreases continuously with the increase of [Cjomim]BF4 concentration. The peaks at 480 nm, 430 nm and a small peak at about 298 nm are considered to be characteristic absorption peaks of the hydrazone form [53. In summary, the UV-visible spectrum shows a strict balance of hydrazine-conjugate isomers in the solution, and azo is the main existing form.

Figure 3-12 UV-Vis absorption spectrum of OG aqueous solution

ciom(19)：0.002mmol·L^-1、0.005 mmol-L^-1、0.01 mmol-L^-1、0.02mmol-L^-1、0.03 mmol·L^-1、0.04 mmol-L^-1

0.05 mmol·L^-1‘、0.07 mmol·L^-1、0.08 mmol·L^-1
Diabate et al. found in their study of sodium saccharinate that the ultraviolet absorption spectrum of sodium saccharide was similar to the spectrum of ILS, indicating that the absorption band came from the n- and/or π→π transitions of saccharinate ions, which shows that the ionic liquid of saccharinate can be effectively separated from other divalent cations.

2.4 Xuansn capacitor ionic liquid electrospray mass spectrometry

With the development of science and technology, soft ionization mass spectrometry technology represented by electrospray mass spectrometry has flourished. Based on the analysis of cation structure by nuclear magnetic resonance, combined with mass spectrometry analysis, the molecular weight of ions and molecular fragments in ionic liquids can be obtained, including partial information of cations and anions, which helps to determine the overall structure of ionic liquids.

The operation steps of electrospray mass spectrometry are to put the sample solution into the electrospray chamber through a very thin injection tube. Under the action of a strong field, the sample solution is separated by charge and electrostatic attraction, and decomposed into many charged micro-droplets at the outlet. Under the influence of the electric field, the charged droplets move against the direction of the dry gas flow and drift toward the mass spectrometer port. The dry gas here makes the droplets evaporate rapidly and increases the charge concentration on the droplet surface. If the limit value of Coulomb repulsion and the limit value of the droplet surface tension are equal, then the droplets will break into smaller droplets at this moment, and then become extremely small droplets, because the curvature radius of the droplets is very small and its surface electric density is very large, resulting in a very strong electric field on the droplet surface. This strong electric field is sufficient to parse ions from the liquid, allowing collision-activated cleavage to occur when the ions enter the first vacuum zone with a pressure of several Torr (17=133.322Pa) through the glass capillary, so that the fragments of the sample molecules can be obtained, and further structural information can be obtained. Electrospray mass spectrometry has high sensitivity and can make ionized molecules carry multiple charges. The generation of such multi-charged ions greatly expands the mass range that can only be analyzed by ordinary mass spectrometry, allowing the mass spectrometer to analyze protein molecules with molecular weights of hundreds of thousands of mass units.

2.5 Raman spectroscopy

When light is irradiated on a substance, two types of scattering will occur: one is elastic scattering, in which the scattered light has the same component as the wavelength of the excitation light; the other is inelastic scattering, in which the scattered light has a different component from the wavelength of the excitation light, collectively referred to as the Raman effect. Compared with infrared spectroscopy, Raman spectroscopy can measure vibrations that are symmetrically related to the symmetry center.
When monochromatic light is used to irradiate gas, liquid or transparent samples, most of the light will be transmitted in the original direction because they have the characteristic of a wavelength much smaller than the particle size of the sample, and the remaining small part will be scattered at different angles to produce scattered light. In addition to Rayleigh scattering, these scattered lights with the same frequency as the original incident light also have a series of Raman spectral lines in the vertical direction. Several very weak Raman spectral lines are symmetrically distributed and shifted from the frequency of the incident light. This phenomenon is called the Raman effect. The length of the spectral line is directly related to the vibration or rotation energy level of the sample molecules. Therefore, Raman spectroscopy can obtain a series of information on molecular vibration or rotation. Both infrared spectroscopy and Raman spectroscopy obtain information about functional groups by measuring molecular vibration spectra, but the two have different selectivities. If they can be measured at the same time, more complete information can be obtained. In terms of identifying inorganic salts, compared with infrared spectroscopy, Raman spectrometers are more convenient for obtaining spectral information below 400 cm!. Raman spectroscopy and infrared spectroscopy can complement and corroborate each other. The main advantage of Raman spectroscopy is that it can provide fingerprint vibration information, thereby reflecting the subtle changes in the structure of ionic liquids and ionic liquid-like liquids. At the same time, Raman spectroscopy can measure the vibration spectra of many different state systems, especially reflecting the local structural changes in ionic liquids and ionic liquid-like liquids. At present, the analysis technology of Raman spectroscopy has been recognized and widely used in the field of substance identification and the spectral characteristics of molecular structure research.

Kausteklis et al. used Raman spectroscopy to study the aggregation process of water molecules called “water bags” in the imidazolyl room temperature ionic liquid 1-butyl-3-methylimidazolium nitrate ([C₄mim][NO₃]). When the content of heavy water (D₂O) in the RTIL-D₂O system continued to increase, changes in the Raman spectrum could be observed in four different spectral regions. The study found that different parameters of the Raman bands were sensitive to the amount of D₂O added. The formation process of the “water bag” was determined by the concentration dependence of the monitored shifts and the discontinuity of the integrated intensity of the Raman bands.
Figure 3-13 shows the Raman spectra of pure [C₄mim][NO₃] and pure D₂O in the entire spectral range between 550cm-! and 3300cm^-1.

Tsuchida et al. used Raman spectroscopy to study the solvation structures of divalent iron, cobalt and nickel complexes in the ionic liquid triethyl n-pentylbis(trifluoromethylsulfonyl)imide ([P₂₂₂₅][NTf₂]). Figure 3-14 shows the concentration dependence of the deconvoluted Raman spectra of the iron (II), cobalt (II) and nickel (II) samples in [P₂₂₂₅][NTf₂] in the frequency range of 720~780 cm^-1. The Raman bands can be attributed to the combination of the stretching v(SNS) and bending vibrations a_s;(CF₃) of the [NT6]- anion. The two bands at 740cm-‘ and 751cm-1 come from the free INTfF anion bound to the metal ion and the non-free NTfz anion, respectively.

Figure 3-13 Raman spectra of pure [C₄mim][NO₃ and pure D2₂, distributed in the front of the rational beans with added D
The illustration is the structure of C₄mimNO₃

Figure 3-14 Raman spectra of [P222s][NTfz] at 373K, containing different concentrations of (a) Fe(Ⅱ), (b) Co(Ⅱ) and (c) Ni(Ⅱ)

Red line: 0.23 mol·k^-1; orange line: 0.30 mol·kg^-1; yellow-green line: 0.38 mol·kg^-1; green line: 0.45 mol·kg^-1; blue line: 0.53 mol·kg^-1; purple line: 0.59 mol·kg^-1

2.6 Photoelectron spectroscopy

The basic principle used in photoelectron spectroscopy is Einstein’s law of photoelectric effect. If you want to observe the emission of electrons, the material needs to be exposed to electromagnetic waves with a sufficiently short wavelength (high photon energy). This is because the electrons in the material are bound to different quantized energy levels. When a certain wavelength of light quanta is irradiated to the sample, the valence electrons or core electrons in the atom absorb a photon and make a dipole transition from the initial state to a highly excited state and leave the atom. In the early days, because there was a photocurrent that was easier to observe, this phenomenon was called the photoelectric effect; now the more commonly used terms are photoionization or photoemission. If the sample is irradiated with monochromatic (i.e., fixed frequency) photons, the energy of this process can be defined by the Einstein relationship:

hv=E_k+E_b

Where: hv is the incident photon energy;E_k is the energy of the electron ejected by the incident photon; E_b is the ionization energy of the electron, or binding energy. Photoionization requires a certain minimum photon energy, called the threshold photon energy hv0. For solid samples, the term work function is often used, denoted as φ.

Foelske-Schmitz et al. studied different substrates of the ionic liquid [Emim]TFSI] (such as Au, Ag, glassy carbon with different pretreatments, and n-type doped silicon) by X-ray photoelectron spectroscopy (XPS). The researchers used XPS spectra with high signal-to-noise ratio to detect element signals other than C, O, F, S or N [marked by blue bars in Figure 3-15(a)]. Figure 3-15(a) is [Emim][TFSI] data 5 deposited on silver and n-type silicon carriers recorded at an average scanning angle of 51°. None of the quantitative analysis results for the studied samples showed significant differences from the expected stoichiometry. Figure 3-15(b) shows the C1s signal of [Emim][TFSI] deposited on argon ion n-type Si. Changing the sensitivity has no obvious effect on the C1s line shape, as can be seen from the C1s spectrum recorded from [Emim][TFSI] deposited on n-type silicon [Figure 3-15(b), top]. The C1s lines are almost identical at 27°±6° (red curve) and 75°±6° (blue curve), further ruling out severe contamination by hydrocarbons.
There are pictures

Figure 3-15 (a) Spectra of [Emim][TFSI] deposited on n-type silicon (top) and Ag substrate (bottom). (b) C 1s spectrum of [Emim][TFSI] deposited on n-type Si. Top: Overlay of C1s spectra recorded at a take-off angle of 12.7°, 27°±6° (red curve) and a take-off angle of 75°±6° (blue curve); Bottom: Deconvolution of the integrated C 1s line

Foelske et al. deposited droplets of the ionic liquid 1-ethyl-3-methylimidazole bis(trifluoromethylsulfonyl)imide ([Emim][Tf₂N)] on silicon and silicon with different doping states. On the germanium semiconductor surface, how the electronic properties of the semiconductor/ionic liquid interface affects the charging effect was explored based on the XPS analysis results. Figure 3-16 is the XPS spectrum of [Emim][Tf₂N] deposited on a silicon substrate with unknown conductivity. All detected core energy levels were attributed to the ionic liquid, and no contaminants such as Si or C1 were detected in the measurement scans. The quantitative analysis results of these spectra are shown in Table 3-1 and can be compared with the expected nominal values. After completing the spray gun experiment, a quantitative analysis of the elements measured and scanned showed that the composition of the ionic liquid did not change significantly, which may be caused by the long-term use of low-energy X-rays and electrons for irradiation.

Figure 3-16 Normalized XPS measurement spectra of [Emim][Tf₂N] on an n-type semiconductor substrate with unknown resistivity before and after long-term X-ray irradiation. The blue area shows the area used for quantification (Table 3-1)

Table 3-1 Quantitative results of XPS measurement spectra (Figure 3-16) of [Emim][Tf₂N] on all substrates before (black) and after (gray) long-term X-ray irradiation

2.7 X-ray diffraction

When X-rays with a certain wavelength are irradiated onto a crystalline material, the X-rays are scattered by encountering regularly arranged atoms or ions in the crystals. The phase of the scattered X-rays is enhanced in certain directions, thus showing a unique diffraction phenomenon corresponding to the crystal structure. X-ray diffraction is a structural and phase analysis method with extremely wide applications. The phase structure and composition of ionic liquids can be determined based on the X-ray diffraction pattern.

Sasi et al. synthesized bio-based ionic liquid crystal derived polymer electrolyte membrane (BILC-SPE) by changing the composition ratio of cardanol-derived 3-(4-(3-pentadecanylphenoxy)butyl)-1-methylimidazole ammonium hexafluorophosphate (PmimP) and polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP). The XRD pattern of the activated electrolyte membrane and the XRD pattern of the dry electrolyte membrane are shown in Figure 3-17. From the XRD characterization diagram, it can be clearly seen that the crystallinity of the electrolyte membrane decreases after activation, and the synthetic peak at a lower diffraction angle slightly shifts to a higher interplanar spacing, thereby confirming the presence of the target product.

Figure 3-17 XRD patterns of solid electrolyte membrane before and after activation

Yan et al. synthesized polyvinylidene fluoride/ionic liquid functionalized silica microporous polymer electrolyte.

The XRD characterization results of pure IL-SiO₂ and PVDF/IL-SiO₂ polymer films are shown in Figure 3-18. No crystallization peak corresponding to IL-SiO₂ was observed in the figure (see Figure 3-18a), which is consistent with the results of electron diffraction analysis. As the concentration of IL-SiO₂ increases, the relative intensity of the diffraction peak corresponding to the polymer film decreases (see Figure 3-18b~g). This shows that the addition of IL-SiO₂ reduces the crystallinity of PVDF. The hydrogen bond formation of the remaining silanol and hydrolyzed silane prevents the recrystallization of the polymer chain. The addition of IL-SiO₂ nanofillers may be beneficial to the formation of complexes with the polymer. It can then act as a cross-linking center for the polymer by reducing the self-organization tension of the polymer and increasing the hardness.

Figure 3-18 XRD patterns of pure IⅡ-SiO₂ and PVDF/IL-SiO₂ polymer films
a. Pure IL-SiO₂; b. PVDF/IL-SiO₂, (9:1 mass ratio); c. PVDF/IL-SiO₂, (8:2 mass ratio); d. PVDFIL-SiO₂, (7:3 mass ratio); e. PVDF/IL-SiO₂, (6:4 mass ratio); f. PVDF/IL-SiO₂, (5:5 mass ratio); g. PVDF/IL-SiO₂, (4:6 mass ratio)

Farah et al. synthesized a material combining a solid polymer electrolyte of polyvinyl alcohol with a sodium salt and an ionic liquid, and characterized it by X-ray diffraction, as shown in Figure 3-19. The XRD spectrum shows that the addition of ionic liquid increases the number of useful free ions, which helps to improve ionic conductivity.

Figure 3-19 XRD patterns of PVA, NaTf, [Bmim]Br, PVA60 and IL-0.5 SPE

2.8 Synchrotron Radiation

When high-energy electrons move at a speed close to the speed of light in a magnetic field, if the direction of movement is perpendicular to the magnetic field, the electrons will be deflected by the Lorentz force perpendicular to their direction of movement. According to the theory of electrodynamics, charged particles will generate electromagnetic radiation when they are accelerated, so these high-energy electrons will generate electromagnetic radiation in the tangent direction of their orbits. This electromagnetic radiation was first observed on a synchrotron, so it is called synchrotron radiation, abbreviated as synchrotron radiation or synchrotron light.
The light source of synchrotron radiation is bright, stable, directional, and parallel, making it an ideal light source for liquid scattering experiments. Yoshimura et al. used synchrotron radiation ultraviolet-visible electron spectroscopy to study the electronic structure of ionic liquids, used molecular orbitals to calculate the valence state of isolated ions, and obtained the state density through spectral calculations.

Synchrotron radiation X-ray absorption fine structure spectroscopy (XAFS) is one of the effective technical means to study the local microstructure of matter, and it has atomic selectivity. From the spectrum, we can obtain the structural information of several shells adjacent to a central atom, such as the electronic structure of the central atom/the type and number of coordinated atoms/the bond length of the coordination bond/the disorder of the system, etc. Since XAFS does not rely on long-range ordered crystal structures, it is particularly suitable for the study of complex amorphous structures such as ionic liquids and the reduction process of metal ions. Zou et al. used XAFS to observe the structure of ionic liquid zinc chloride-choline chloride at different molar ratios. The analysis showed that when the molar fraction of zinc chloride in the ionic liquid was high, ion pairs and chlorine-zinc-chlorine formed the main ion complexes.

2.9 Electrochemical method

The electrochemical method is a method that uses the potential, current and resistance (or conductivity) of the sample to be measured to analyze the composition and concentration of the analyte in the sample. The measurement results of the electrochemical analysis method come from the local sample in contact with the electrode surface, so the analysis results have significant heterogeneity, and any factors that affect the electrode surface of the object to be measured will be reflected in the measurement results. In electrochemistry-related theories, the performance of a substance to be tested is related to its activity rather than its concentration, but in practice, it often must be expressed in terms of concentration. When the ion concentration is high, the difference between activity and concentration is more obvious, so when the ionic strength of the sample solution is high, necessary control measures need to be taken.

Li et al. studied the highly reversible oxidation process of sulfur in AlCl₃/urea ionic liquid. Sulfur was electrochemically oxidized by AICI^–₄ to form AISC1₇, and the sulfur oxidation efficiency could reach 94%. AI-S battery based on sulfur oxidation can cycle stably at around 1.8V.

Finger et al. prepared a halogen-free, metal-free and water-free methyl bicarbonate precursor by decarboxylation under acidic conditions, and compounded the ionic liquid based on hydrogenated sulfide element HE- (E=S, Se, Te) anion and the prepared organic salt. The obtained product has a high purity. The researchers used cyclic voltammetry (CV) to study the redox behavior of polysulfide salts and hydrosulfides. Using dimethyl sulfoxide (DMSO) containing 50mmol·L^-1 tetrabutylammonium hexafluorophosphate as the electrolyte, the cyclic voltammogram of [BMPyr][HS1 (Figure 3-20) shows an irreversible redox process, with obvious oxidation at 0.78V, smaller oxidation peaks at 0.73V and 0.55V, and reduction at 1.64V. According to HS^–→H⁺+S, the electrochemical oxidation of the hydrosulfide aqueous solution at the electrode proceeds in a two-electron process.

Figure 3-20 Cyclic voltammogram of [BMPyr][HS] GC electrode, scan rate 100mV·s^-1, arrows indicate scan direction and starting potential

Vélez et al. experimentally studied the synthesis and characterization of a new electrolyte based on double-bonded bimetallic pyrrolidinium and piperidine bis(trifluoromethylsulfonyl)imide ionic liquids, in which two cations are connected by a variable length of ethylene oxide chain interval. The electrolyte was assembled with LiMn₂O₄ (LMO) spinel in a lithium half-cell, and the results showed that both the cation structure and the length of the ethylene oxide chain spacer group have a significant impact on the electrochemical performance of the LMO battery. Batteries based on pyrrolidinium electrolytes with a cation structure connected by two ethylene oxide units have superior rate capabilities. The difference between the cyclic voltammograms in Figure 3-21 (a) shows that the battery performance depends on the ionic liquid solvent used in the electrolyte. The charge/discharge curve at the lowest test rate of 0.2C is shown in Figure 3-21 (b).

Figure 3-21 (a) Cyclic voltammogram (third cycle, scan rate 0.12 mV·s^-1) and (b) charge-discharge curve (third cycle, 0.2C) of LMO/electrolyte/Li battery recorded at 60°C using bimetallic pyrrolidinium and piperidine IL electrolyte with 1 mol·L^-1LiTFSI

2.10 Thermogravimetric analysis

Thermogravimetric analysis (TGA) refers to a thermal analysis technique that determines the relationship between the mass and temperature change of the sample to be tested under program-controlled temperature. There are two most commonly used measurement principles, namely the displacement method and the zero-position method. The so-called displacement method is based on the relationship between the inclination of the balance beam and the proportional change in mass, and the inclination is detected by a differential transformer and the like, and automatically recorded. The zero-position method uses a differential transformer method and an optical method to measure the inclination of the balance beam, and then adjusts the current of the coil installed in the balance system and the magnetic field to rotate the coil to restore the inclination of the balance beam. Since the force applied by the rotation of the coil is proportional to the mass change, and this force is proportional to the current in the coil, it is only necessary to measure and record the change in current to obtain the mass change curve.
Briones et al. found from TGA measurements that the thermal degradation curve of ionic liquids changes with the change in the length of the alkyl chain, which in turn affects its glass transition temperature (T) value, which is attributed to the larger or smaller segmental movement of the alkyl chain. The results were analyzed according to the alkyl chain length, molecular weight and negative ion effect, indicating that the removal efficiency of hexavalent chromium by ionic liquid was 72.2%.

2.11 Solution calorimetry

There are two thermodynamic methods for quantifying the interactions between organic molecules in Xuansn capacitor ionic liquid.
The first method is to use multi-parameter correlations to describe the thermodynamic functions of solutions or solvations. This method is applicable to two types of solute-solvent systems: (1) one solute dissolved in a series of solvents (Kamlet-Taft-Abbod); (2) a series of solutes dissolved in a solvent (Abraham). These two types of multi-parameter correlations can be used to study the molecular interactions and solvation properties of various solute-ionic liquid systems.

The second approach is based on the separation of solvation thermodynamic functions from contributions of non-specific solvation (van der Waals interactions) and specific solvation (hydrogen bonding, donor-acceptor interactions). This approach can be applied to the study of ionic liquid solutions. It has been observed that the dipole moment of the solute molecule does not affect the solvation enthalpy in ionic liquids. In recent years, the group contribution method has been successfully applied to determine the solvation enthalpy of aromatic compounds in molecular solvents and later extended to ionic liquid solutions. The van der Waals interaction scaling of ionic liquids is evaluated based on the enthalpy of alkanes in their medium.
However, the application of these two approaches to the analysis of intermolecular interactions in ionic liquids can produce inconsistent results in some cases. According to the data obtained by multiparameter correlation, the acid values ​​of N-alkyl substituted ionic liquids related to the hydrogen bonding proton donor ability are very close to those of aliphatic alcohols. However, the solvation approach shows the opposite results, where the specific interaction enthalpies of N-alkyl substituted ionic liquids with weak (nitriles), medium (ketones, ethers) and strong (amide) proton acceptors are almost zero.
In the case of multiparameter correlation, the disagreement may be due to the difference between the values ​​obtained for specific parameters. Regarding the solvation method, one can use solution calorimetry to analyze the relationship between hydrogen bond breaking and formation upon dissolution (solvent reorganization).

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