Ionic liquids: history, conception, applications, and perspectives

Doféré Yeo, Issoufou Tini, Nicolas Joly, Mahasoa-Salina

Souvenir-Zafindrajaona, Jean-Pierre Mbakidi, Vincent Lequart, Ludovic

Chaveriat, Sandrine Bouquillon, Patrick Martin

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Doféré Yeo, Issoufou Tini, Nicolas Joly, Mahasoa-Salina Souvenir-Zafindrajaona, Jean-Pierre Mbakidi, et al.. Ionic liquids: history, conception, applications, and perspectives. Green chemistry letters and reviews, 2025, 10.1080/17518253.2025.2543910. hal-05233596

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# Ionic liquids: history, conception, applications, and perspectives

Doféré Yeo, Mansour Issoufou Tini, Nicolas Joly, Mahasoa-Salina Souvenir-Zafindrajaona, Jean-Pierre Mbakidi, Vincent Lequart, Ludovic Chaveriat, Sandrine Bouquillon & Patrick Martin

**To cite this article:** Doféré Yeo, Mansour Issoufou Tini, Nicolas Joly, Mahasoa-Salina Souvenir-Zafindrajaona, Jean-Pierre Mbakidi, Vincent Lequart, Ludovic Chaveriat, Sandrine Bouquillon & Patrick Martin (2025) Ionic liquids: history, conception, applications, and perspectives, *Green Chemistry Letters and Reviews*, 18:1, 2543910, DOI: 10.1080/17518253.2025.2543910

**To link to this article:** https://doi.org/10.1080/17518253.2025.2543910

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# Ionic liquids: history, conception, applications, and perspectives

Doféré Yeoª, Mansour Issoufou Tiniª,ᵇ, Nicolas Jolyª, Mahasoa-Salina Souvenir-Zafindrajaonaᵇ, Jean-Pierre Mbakidiᵇ, Vincent Lequartª, Ludovic Chaveriatª, Sandrine Bouquillonᵇ and Patrick Martinª

ªUnité Transformations & Agroressources, Univ Artois-UniLaSalle, Béthune, France; ᵇUniv de Reims Champagne Ardenne, Reims, France

**ABSTRACT**

Over the last few decades, ionic liquids (ILs) have emerged as a class of innovative solvents with unique physico-chemical properties such as low vapor pressure, high thermal stability, high solvation capacity among others making them attractive for a wide range of applications. This literature review takes an in-depth look at the physico-chemical properties of ILs, focusing on their molecular structure, viscosity, electrical conductivity, solvation power, density, and polarity. In addition, various methods for ILs syntheses are discussed, including conventional approaches and more recent synthetic methods such as green chemistry. In addition to their advantages, this review also addresses the environmental concerns associated to ILs, such as their toxicity, biodegradability, and recyclability. Finally, this review explores a large range of ILs applications in various fields, including catalytic chemistry, enzymatic synthesis, electrochemistry, extraction of bioactive molecules, depollution, and metals recovery. In summary, this review highlights the growing importance of ILs as alternative solvents and promoters to replace conventional organic solvents, while highlighting the challenges and opportunities associated with their use in a variety of applications.

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**ARTICLE HISTORY**

Received 10 March 2025
Accepted 31 July 2025

**KEYWORDS**

Alternative solvents; green solvent; eco-extraction; ecotoxicity; biodegradability; bioactive

## 1. Introduction

Solvents are liquid or supercritical chemical compounds that are used under specific physico-chemical conditions, as reaction medium for the dissolution of reactants to promote their reaction. They are also used as carriers to extract chemical compounds occurring in biodiversity matrices. They accelerate chemical reactions without modifying the reactants or products obtained and especially without being modified themselves, so they have a catalytic role. In chemistry, volatile organic compounds (VOCs), fossil derivatives in particular from petroleum, have remained used for a long time despite significant risks to human health [1–8] and heavy environmental impacts [8–10]. Some VOCs during chemical reactions release gases that pollute the air and contribute to the greenhouse effects leading to climate change. Others are dangerous because they are toxic, carcinogenic, and irritating. All this destabilizes biodiversity, ecology and leads to the emergence of diseases and the disappearance of certain species. In 1990, the scientific world began to question the chemical consequences and decided to resort to alternative concepts, eco-responsible and more respectful of man and the environment. In 1998, American chemists Paul Anastas and John

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C. Warner of the Environmental Protection Agency (EPA) launched a 'green chemistry or sustainable chemistry' program, subject to 12 principles to limit, reduce or eradicate the problems caused by chemical products or processes on the environment and human health [11,12].

Among these green solvents, we find ILs first described in 1914 by Walden [13]. They are salts melted at a temperature below 100°C, constituted of an organic cation and organic or inorganic anion. The most conventional cations are derivatives of nitrogen, sulfur, and phosphorus. In 1914, Walden [13] detected an intermediate reaction involving ethylammonium nitrate, a molten salt with a melting temperature of 12°C used in armament during the first world war. At the end of the nineteenth century, this was detected during the Friedel-Craft reaction of benzene with chloromethane catalyzed by AlCl₃, 'red oil.' Until 1980, studies combining various cationic and anionic groups were conducted, leading to the development of numerous ILs based on chloroaluminate anion, hence birth of first-generation ILs. The first-generation ILs remained used in electrochemistry for a long time until the end of the nineteenth century. However, it was at the end of the twentieth century that the scientific world evolved towards green chemistry.

Chloroaluminate anions were a common component of the first generation of ILs. They are known for releasing HCl and were gradually replaced by more stable anions such as tetrafluoroborate, hexafluorophosphate, nitrate, sulfate, or acetate. The hydrolysis stability of this second generation allowed it to expand the potential application. The second generation of ILs was born.

At the beginning of the twenty-first century, there was an exponential growth in research studies on these molten salts, with a significant number in 2021 of more than 5000 publications/year compared to about 100 publications/year until 2000. However, the second-generation ILs have one major drawback, the possible release of derivatives as fluorine. To provide a solution to the problems, the ILs from the work of Cooper and O'Sullivan [14], the third-generation ILs bearing $CF_3SO_4^-$ anion, more stable and limiting the release of fluorine than the second-generation ILs, was elaborated. However, these third-generation ILs led to water pollution essentially due to degradation problems or poor anion coordination as $PF_6$. Bonhôte and coworkers [15] therefore focused on the use of bis(trifluoromethylsulfonyl)imide anions ($Ntf_2$) and their derivatives resulting from hydrophobic characteristics and more stable than those of the $CF_3SO_3^-$ anion, thus giving rise to the fourth-generation ILs by relocating the charge of anions. However, their cost is too high and ions containing fluorine are not matching with green chemistry philosophy. Research are underway to substitute fluorinated anions without influencing ILs properties while diversifying their applications, thus leading today to fifth-generation ILs. Candidates explored are phosphinates, sulfates, sulfonates, tosylate anions, octyl sulfates, derivatives of dicyanamide or dialkyl phosphinate with long alkyl chain [15]. Similar to the fifth-generation ILs, a category of greener, bioeconomic, and multi-applicable ILs is being studied: biobased ionic liquids whose cations and anions come from biomolecules or derivatives of value-added biomolecules in eco-extraction process. These ILs are used as catalytic solvents in chemical or enzymatic reaction media and especially as solvents to dissolve biopolymers (cellulose and derivatives and/or lignin) for their transformation into value-added products in cosmetic, pharmaceutical, textile, chemical industries.

In the context of green chemistry (Figure 1A) and in relation to many standards such as Registration Evaluation Authorization and Restriction Chemicals (REACH) in Europe or Clean Air Act in the United States, solvents have appeared variously qualified as green, ecological, biodegradable/biocompatible, or sustainable. These solvents include water, solvents derived from carbohydrates or lipids, lignin solvents, deep eutectic solvents, ionic liquids, $CO_2$ mainly in the supercritical state or solvents from residual materials.

To classify these solvents, Gu and Jérôme [18] proposed 12 criteria for a solvent to be considered green: availability, price, recyclability, purity, synthesis, toxicity, biodegradability, performance, stability, flammability, storage, and renewable. Clark et al. [19] relied on the concept of life cycle assessment (LCA) and the work of Gu and Jérôme [18] to assess the durability of solvents according to six categories or stages of the life cycle (Figure 1B), namely their production, availability, use in terms of technical performance and stability, toxicity, safety, and end of life [17].

The last few decades, ILs showed a great potential as alternatives and innovatives solvents in several application areas. In particular in catalysis [20–23], energy and electrochemistry [24], lubricants [25], design of new materials for gas absorption [26] separation and extraction [23,27–30], the design of stimuli-sensitive materials and sensors [20, 21] or biomedical and pharmaceutical applications [30–33], batteries, where they are used as electrolytes for energy storage, and medicine as drug delivery systems [23].
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<figure><img src="image_17.png" /><figcaption>**Figure 1.** (A) The 12 principles of green chemistry [16] and (B) the 6 principles of plant eco-extraction [17].</figcaption></figure>

Over the last few decades, the number of publications on ILs has increased significantly. Of few articles per year, it passed to more than 5000 per year between 1996 and 2016 [23], and then to more than 6000 between 2016 and 2024 (Scopus data on October 2024). This growing interest is due to the excellent properties of these compounds, which make them useful in a wide range of applications. This article therefore aims to provide more information on these innovative compounds.

Our objective in this review will be to discuss in more detail the synthesis pathways of ILs, the physico-chemical properties, their applications in the extraction of secondary metabolites from plants, their ability to dissolve cellulose and its derivatives, and their ecotoxicity evaluation.

## 2. Evolution of ILs and their physico-chemical properties

### 2.1. Evolution of ILs according to Scopus

ILs have emerged as a new class of alternative solvents to petroleum-based solvents in the context of green chemistry and sustainable development. Figure 2 shows the increasing number of publications concerning ionic liquids since the 2000s with their specific fields of application.

### 2.2. The physico-chemical properties of ILs

ILs, presented as green solvents, have a wide range of applications. They have a bi-ionic structure, comprising a cation and an anion. This structure gives them adjustable physico-chemical properties and a wide range of interesting functions in sustainable chemistry. However, the purity of ILs during their design remains a tedious step because impurities, unreacted reagents, remaining halides or water significantly influence their physico-chemical properties, i.e. stability, melting point, density, viscosity, ionic conductivity, polarity, and solubility in common solvents, and confuse them with their specificity.

#### 2.2.1. Thermal stability of ILs and their melting point

ILs are molten salts. They are distinguished from all other molten salts by a melting temperature strictly below 100°C. They pass from the solid state to the crystalline or vitreous state before reaching their
<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /><figcaption>**Figure 2.** Scopus data on ILs (October 2024).</figcaption></figure>

melting point (liquid state). The wide range of ILs thermal stability is due to their non-volatilization even at decomposition temperatures of up to 400°C in the liquid state. This parameter is influenced by the ion structure (radius and symmetry) and distribution of the ion charges that strongly affect forces and interactions between the atoms (hydrogen bond, HDV interactions, stacking π, inter/intramolecular Coulomb attraction) [34]. The equivalence of the ILs ion charges leads to a decrease in electrostatic interactions influenced by the increase in the ionic radius. This fact leads to the increase in the distance between the ions and decreases the charge density at the surface of the ions, and consequently to $T_m$ decrease as a function of the radius increase. Although the formation of polyatomic anions such as $BF_4^-$, $PF_6^-$, $N(CF_3SO_2)_2^{2-}$ leads to charge relocation and a weakening of interactions. Welton has demonstrated [35] that the cation also causes a $T_m$ of aromatic compounds compared to aliphatic compounds. With regard to ILs cation, the structure and nature of the cation significantly influence the ILs $T_m$. Holbrey and Seddon [36] demonstrate that ILs with imidazolium cations bearing alkyl chains ($n=2$ to 4 carbon atoms), reported by Ohno [37], are the most widely used because the length of alkyl chains induces phase separations related to interactions HDV type between charged (e.g. aromatic) and hydrophobic (alkyl) regions. The longer the alkyl chain of ILs cation is, the lower the ILs thermal stability is [38], compared to the anion, the more difficult the IL organize into a crystal lattice (poor coordination), hence the liquid state [39]. Imidazolium-based ILs have very good thermal stability, with decomposition temperatures ($T_d$) of up to 400°C [40]. However, this stability tends to decrease as a function of the hydrophilic nature of the anion, in decreasing order [41,42]:

$$NTf_2^- > BF_4^- > I^- > Br^- > Cl^-$$

Impurities or by-products influence the fidelity of the $T_d$ parameter by accelerating decomposition [24].

### 2.2.2. The density of ILs

The density of an IL is generally higher than that of water and volatile organic compounds ($1 g.cm^3 < d_{ILs} < 1.61 g.cm^3$). Decrease in density depends on the elongation of the alkyl chain of cations [43]. This density decreases differently depending on the type of cation in order.

<figure><img src="image_20.png" /></figure>

The anion influences the density of the ILs: the heavier the anion is (molar mass), the more the density of the IL is [34,37,44].
<figure><img src="image_21.png" /></figure>

### 2.2.3. The viscosity of ILs

This parameter is mainly influenced by the anion. The more the anion has the properties of interacting through the hydrogen bonds, the more the IL increases in viscosity. Chlorinated magnetic ILs are more viscous, due to the ability of chlorine to form hydrogen bonds with the hydrogen atoms of the imidazolium ring [45–47]:

$$PF_6^- > PF_4^- > BF_4^- > NTf_2^- \quad [44,46]$$

Viscosity decreases with temperature rise but above all it is strongly influenced by impurities such as water and chloride ions [48].

### 2.2.4. Conductivity of ILs

The conductivity of an electrolyte is the measure of the number of charged species available and their mobilities [49]. ILs have a high conductivity of up to 0.1 S/m [41,50]. This conductivity decreases as their viscosity increases. The more viscous it is, the less its conductivity is great, this is explained by the influence of parameters related to the size of the ions and the displacement of the charge [51] to the length of the alkyl chain of the cation [52] to the density of the IL and to impurities [53]. Macfarlane et al. [54] demonstrated a decrease in cation-dependent conductivity for ILs with the same anions in the following order:

<figure><img src="image_22.png" /><figcaption>Table 1 shows the conductivity of certain ILs.</figcaption></figure>

### 2.2.5. Polarity and solubility of ILs in gases, VOCs, and water

#### 2.2.5.1. Polarity of ILs.
ILs have a polarity between 9 and 15, they are non-coordinating polars [58]. This parameter (hydrophobic/hydrophilicity) influences ILs reactivities to dissolve compounds (reactive) in reaction medium. This polarity depends on the interactions that exist within the IL. These may be hydrogen, ionic or

<table><caption>Table 1. Conductivity table of some ILs [55–57].</caption><thead><tr><th>Cation</th><th>Anion</th><th>Conductivity σ<sub>i</sub> (mS cm<sup>-1</sup>)</th><th>Temperature (°C)</th><th>Reference</th></tr></thead><tbody><tr><td rowspan="10">Emim</td><td>BF<sub>4</sub><sup>-</sup></td><td>13.60</td><td>25</td><td>[55]</td></tr><tr><td>[CH<sub>3</sub>CO<sub>2</sub>]<sup>-</sup></td><td>2.8</td><td>25</td><td>[56]</td></tr><tr><td>[CF<sub>3</sub>CO<sub>2</sub>]<sup>-</sup></td><td>9.6</td><td>25</td><td>[56]</td></tr><tr><td>[CF<sub>3</sub>SO<sub>3</sub>]<sup>-</sup></td><td>8.6</td><td>25</td><td>[56]</td></tr><tr><td>[C<sub>3</sub>H<sub>7</sub>SO<sub>3</sub>]<sup>-</sup></td><td>0.385</td><td>25</td><td>[57]</td></tr><tr><td>[C<sub>2</sub>H<sub>6</sub>SO<sub>4</sub>]<sup>-</sup></td><td>3.82</td><td>25</td><td>[57]</td></tr><tr><td>[N(CF<sub>3</sub>SO<sub>2</sub>)<sub>2</sub>]<sup>-</sup></td><td>8.8</td><td>25</td><td>[56]</td></tr><tr><td>[N(CN)<sub>2</sub>]<sup>-</sup></td><td>7.5</td><td>25</td><td>[56]</td></tr><tr><td>[Tf<sub>2</sub>N]<sup>-</sup></td><td>7.63</td><td>20</td><td>[55]</td></tr><tr><td></td><td>9.20</td><td>25</td><td>[55]</td></tr><tr><td>C<sub>4</sub>Py</td><td rowspan="3">[Tf<sub>2</sub>N]<sup>-</sup></td><td>3.3</td><td>25</td><td rowspan="3">[55]</td></tr><tr><td>Py<sub>14</sub></td><td>2.8</td><td>25</td></tr><tr><td>Pip<sub>14</sub></td><td>2.2</td><td></td></tr><tr><td rowspan="7">Bmim</td><td>BF<sub>4</sub><sup>-</sup></td><td>4.37</td><td>25</td><td>[57]</td></tr><tr><td>PF<sub>6</sub><sup>-</sup></td><td>1.8</td><td>25</td><td>[56]</td></tr><tr><td>[CF<sub>3</sub>SO<sub>3</sub>]<sup>-</sup></td><td>3.7</td><td>25</td><td>[56]</td></tr><tr><td>[CF<sub>3</sub>CO<sub>2</sub>]<sup>-</sup></td><td>3.2</td><td>25</td><td>[56]</td></tr><tr><td>[C<sub>4</sub>F<sub>9</sub>SO<sub>2</sub>]<sup>-</sup></td><td>0.45</td><td>25</td><td>[56]</td></tr><tr><td>[C<sub>3</sub>F<sub>7</sub>CO<sub>2</sub>]<sup>-</sup></td><td>1.0</td><td>25</td><td>[56]</td></tr><tr><td>[CF<sub>3</sub>SO<sub>3</sub>]<sup>-</sup></td><td>7.5</td><td>25</td><td>[56]</td></tr><tr><td rowspan="2">[EtEtIm]<sup>+</sup></td><td>[C<sub>3</sub>F<sub>7</sub>CO<sub>2</sub>]<sup>-</sup></td><td>7.4</td><td>25</td><td>[56]</td></tr></tbody></table>

[EtEtIm]<sup>+</sup>: 1-ethyl-3-ethylimidazolium.
<figure><img src="image_23.png" /></figure>

electrostatic $π$ Van der Waals (VDV) stacking interactions. These interactions vary from one IL to another depending on the variation in the nature of the ions.

The observed physico-chemical phenomenon depends on the cation and anion grafted alkyl length. Their solubility in water is favored by the intermolecular interactions between IL and water according to the following order:

$$Cl^{-} < PF_{6}^{-} < PF_{4}^{-} < BF_{4}^{-} < NTf_{2}^{-}.$$

Table 2 shows the Kamlet–Taft parameters of some ILs. These parameters are three constants: $α$, $β$, and $π^*$ which allow to describe the influent of solvent on solute and the chemical reaction. $α$ describes the acidity of the hydrogen bond (ability to donate a proton), $β$ represents the basicity of the hydrogen bond (ability to capture a proton), and $π^*$ describes the polarity and polarizability of solvent, that is, it is to stabilize charges or dipoles [59]. For each of these constants, the higher the value, the greater the associated effect.

Imidazolium-based IL has the property of being hygroscopic, the water is in aggregate form, a microdomain, acting as a co-solvent in the IL when it has been reported that the water concentration in the ILs is between 0.2 and $1 \times 10^{-3}$ mol. L$^{-1}$. Table 3 shows the miscibility of two imidazolium-base ILs in VOCs and water.

It is then necessary to measure the partition coefficient:

$$O/W = C_{\text{ste}} \text{ in the octanol/C}_{\text{ste}} \text{ phase in water;}$$

for the ionic solute the intrinsic partition coefficient is

$$C_{\text{esp not dissociated}} \text{ in the octanol} / C_{\text{esp not dissociated}} \text{ in water.}$$

The apparent coefficient of action of the Hildebrand parameter depends on the nature of the cation and the anion, due to the significant variation in the molar volume ($V_m$) of the solutes resulting in the separation of the solutes in the solvent.

$$δ = U/Vm = H-RT / Vm(\text{usually } 16.3 < 25.5 J/cm^3).$$

The length of the alkyl chain of the IL cation influences the Hildebrand parameter. An increase in the length of its alkyl chain leads to a decrease in the Hildebrand parameter. For some ILs, the enthalpy of evaporation can be determined.

<table><caption>**Table 2.** Kamlet–Taft parameters of some ILs [59].</caption><thead><tr><th>IL</th><th>α</th><th>β</th><th>π*</th></tr></thead><tbody><tr><td>BminCl</td><td>0.32</td><td>0.95</td><td>1.13</td></tr><tr><td>BminBr</td><td>0.36</td><td>0.87</td><td>1.14</td></tr><tr><td>BminNO<sub>3</sub></td><td>0.40</td><td>0.74</td><td>1.04</td></tr><tr><td>BminPF<sub>6</sub></td><td>0.54</td><td>0.44</td><td>0.90</td></tr><tr><td>BminBF<sub>4</sub></td><td>0.52</td><td>0.55</td><td>0.96</td></tr><tr><td>BminNTf<sub>2</sub></td><td>0.55</td><td>0.42</td><td>0.83</td></tr></tbody></table>

<table><caption>**Table 3.** Miscibility table of two imidazolium ILs in VOCs and water [42].</caption><thead><tr><th rowspan="2">Ionic liquids</th><th rowspan="2"></th><th colspan="5">Solvents</th></tr><tr><th>Water</th><th>Methanol</th><th>Acetonitrile</th><th>Acetone</th><th>n-Hexan</th></tr></thead><tbody><tr><td rowspan="4">Emim (éthylméthylimidazolim)</td><td>PF<sub>6</sub><sup>-</sup></td><td>M</td><td>M</td><td>-</td><td>M</td><td>I</td></tr><tr><td>CH<sub>3</sub>SO<sub>3</sub><sup>-</sup></td><td>M</td><td>-</td><td>M</td><td>M</td><td>-</td></tr><tr><td>CF<sub>3</sub>SO<sub>3</sub><sup>-</sup></td><td>I</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>NTf<sub>2</sub><sup>-</sup></td><td>I</td><td>M</td><td>M</td><td>-</td><td>-</td></tr><tr><td rowspan="7">Bmim (butylméthylimidazolium)</td><td>Cl<sup>-</sup></td><td>M</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>I<sup>-</sup></td><td>M</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>PF<sub>6</sub><sup>-</sup></td><td>I</td><td>M</td><td>-</td><td>M</td><td>I</td></tr><tr><td>BF<sub>4</sub><sup>-</sup></td><td>M</td><td>M</td><td>M</td><td>I</td><td>I</td></tr><tr><td>CH<sub>3</sub>SO<sub>3</sub><sup>-</sup></td><td>M</td><td>-</td><td>M</td><td>M</td><td>-</td></tr><tr><td>CF<sub>3</sub>SO<sub>3</sub><sup>-</sup></td><td>I</td><td>M</td><td>M</td><td>-</td><td>-</td></tr><tr><td>NTf<sub>2</sub><sup>-</sup></td><td>I</td><td>M</td><td>M</td><td>-</td><td>-</td></tr></tbody></table>

M: miscible, I: immiscible.
<figure><img src="image_24.png" /></figure>

In VOCs, the more polar a solvent is, the more it dissolves the IL. The nature of the ILs anion and the elongation of the alkyl group of less polar solvents can influence miscibility with ILs [44] and facilitate their purification [42].

### 2.2.5.2. Miscibility of ILs in gases, VOCs, and water.
The hydrophilic/hydrophobic parameter of ILs is a crucial property for their solvation ability. Their miscibility with water or VOCs depends on the nature of the anion and the length of the alkyl chains of the cation.

#### 2.2.5.2.1. Miscibility of ILs and gases.
The high emission of greenhouse gases into the atmosphere during the synthesis of industrial or laboratory products via VOCs generates a heavy ecological balance. Harmful gases released during synthesis are generally CO₂, SO₂, H₂S, and NH₃ [18]. It is therefore wise to find green processes and alternative methods that substitute VOCs or to confine these types of gases during chemical transformations. ILs appear as alternatives for the imprisonment of these gases by dissolution. For example, [Bmim][BF₄] has a absorption potential (dissolve) of CO₂, it is the same as the absorption of SO₂ by [C₄Py][SCN] [60] or with dual-based Lewis ILs. Unlike CO₂, SO₂ are strongly absorbed in magnetic ILs compared to CO₂ that is miscible to hydrophobic protic ILs and ILs based on substituted benzoate [61]. It has also been observed that the increase in the length of the cation chain significantly impacts the absorption of these gases while the nature of the anion is favorable to electrostatic interactions, HDV and hydrogen bonds. NH₃ is miscible in cation ILs carrying hydroxyl (acid) function such as [EtOHmim][BF₄], [choline][NTf₂] [62].

There are methods of chemoinformatics prediction of miscibility of ILs with gases, the COSMO-RS [63].

#### 2.2.5.2.2. Miscibility of ILs to water.
Water is considered as a contaminant that affects the physico-chemical properties of ILs. The water content of an IL is determined by the Karl-Fischer method. Nevertheless, the solubility of ILs in water is influenced by the hydrophobicity of its anion of and this in ascending order:

$$BF_4^- < CH_3(C_2H_4O)_2SO_4^- < C(CN)_3^- < PF_6^- < N(SO_2CF_3)^2^-$$

and also the length of the alkyl chains of the cation [64]. The hydrophobicity of an IL is determined by the coefficient of sharing of the IL between octanol and water (Kow) and adversely influenced, like viscosity, under the influence of the nature of the IL anion to coordinate with the cation [65]. Mizuuchi et al. [66] have shown that it is possible to improve the water solubility of a poorly water-soluble ionic liquid by combining it with another ionic liquid that is more miscible with water, thus facilitating its use in aqueous media. Jacquemin has reported that imidazolium-based ILs with CF₃CO₂⁻ as counter-anion (third-generation ILs) although are more stable than their predecessors and limit the release of fluoride, the latter are highly miscible to water, which limits their applications [15]. The solubility of ILs in water also increases with the size of the anion. In their study, Klähn et al. showed that, for the same cation, solubility follows an increasing order (BF₄⁻ < PF₆⁻ < NTf₂⁻) according to the size of the anion [67]. This trend can be explained by the size of the anion. The larger the anion, the more its charge is distributed over a large surface area. This reduces electrostatic interactions with water molecules.

#### 2.2.5.2.3. Miscibility of ILs to conventional solvents.
The miscibility of ILs in traditional organic solvents is influenced by the choice of cation and anion they contain. In general, they are immiscible with apolar solvents such as alkanes, toluene, and diethyl ether [40]. On the other hand, they are miscible with more polar solvents such as short-chain alcohols, halogenated solvents such as dichloromethane (DCM) and chloroform, and other organic solvents such as acetonitrile (ACN), tetrahydrofuran (THF), dimethylsulphoxide (DMSO), and ketones [40,68]. Nevertheless, ILs can be immiscible to alkanes, ether, and toluene by forming a biphasic system [68].

### 3. ILs design

ILs are salts with melting points below 100°C consisting of an organic cation and an organic or inorganic anion [25]. The most classical cations constituting ILs are derived from nitrogen-, sulfur- and phosphorus-containing compounds (Figure 3).

They are bioeconomic and easy to prepare [30]. For anions, inorganic anions such as halides (Cl⁻, Br⁻) or hexafluorophosphate (PF₆⁻), tetrafluoroborate (BF₄⁻), nitrates (NO₃⁻) [69] can be used, thus giving to the formation of second-generation ILs. The synthesis of this type of solvent is based on two main essential
<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /><figcaption>**Figure 3.** Some classic cations and anions that make up ILs: (a) ammonium, (b) imidazolium, (c) pyridinium, (d) phosphonium, (e) sulfonium and anions: (a') bromide, (b') chloride, (c') hexafluorophosphate, (d') tetrafluoroborate, (e') acetate, (f') trifluoroacetate.</figcaption></figure>

<figure><img src="image_27.png" /><figcaption>**Figure 4.** Quaternization step.</figcaption></figure>

steps: (i) the formation of the cation desired by a quaternization reaction of the heteroatom and (ii) protonation in an acidic medium or alkylation *via* an $S_N2$ reaction (Figure 4).

If the substrate is a secondary or primary amine, the ionic liquid obtained will be protic, while if the substrate is a tertiary amine, the IL will be aprotic. It has been reported in the literature [14,37], in the case of an acid-base quaternization reaction of a tertiary amine with H-X, there is sometimes a risk of obtaining inappropriate and corrosive ILs. It will then be necessary to continue the synthesis by carrying out anionic metathesis reaction (Figure 5) [70]. This step allows the anion X to be replaced by a more stable anion X' to obtain stable end product.

If the latter is an R-COOH, hydrophilic IL will be obtained, while the use of $MeSO_2/ROH$ or $EtOH/RNH_2$ leads to a hydrophobic one. A second reaction with a Lewis acid is also possible, leading to an IL (exothermic magnetic ionic liquid, manufactured in the anhydrous state) [71] (Figure 6) having catalytic properties to optimize yields in chemical synthesis [72].

It has also been reported in literature [73] that ILs, as well as phosphonium cations with tosylate alkyl anions (Figure 7), allow for purer products and an almost complete recovery of the organometallic catalyst used in solid molten salts (ILs).

<figure><img src="image_28.png" /><figcaption>**Figure 5.** Anionic metathesis.</figcaption></figure>
<figure><img src="image_29.png" /></figure>

<figure><img src="image_30.png" /><figcaption>**Figure 6.** Synthesis of hydrophilic, hydrophobic and magnetic ionic liquids [42].</figcaption></figure>
<figure><img src="image_31.png" /></figure>

<figure><img src="image_32.png" /><figcaption>**Figure 7.** Quaternization step of trialkyl phosphate with tosylate [55].</figcaption></figure>

<figure><img src="image_33.png" /><figcaption>**Figure 8.** Synthesis of IL from amino acid derivatives and anionic metathesis for obtaining chiral ionic liquids with molten rings [74].</figcaption></figure>

In studies, imidazolium tosylate has been used as a precursor to obtain chiral ILs condensed cycle (Figure 8). These ILs with PF₆ and NTf₂ as anions are derived from alpha amino acids [70,74].

Alpha amino acids are used to produce biobased ILs (Figure 9), and the esterification allows to reduce their melting temperature [75].

Thus new organic anions have been developed, including biobased anions such as prolinate, lactate, succinate (Figure 10) [76,77].

<figure><img src="image_34.png" /><figcaption>**Figure 9.** Synthesis of alpha amino acid-based ILs [75].</figcaption></figure>

<figure><img src="image_35.png" /><figcaption>**Figure 10.** ILs with tetrabutylammonium cation and anions such as (1) L-lactate, (2) L-tartrate, (3) L-Malonate, (4) L-Succinate, (5) L-malate, (6) L-Pyruvate, (7) D-glucuronate, (8) D-galacturonate [76].</figcaption></figure>
<figure><img src="image_36.png" /></figure>

The synthesis of this type of solvent is based on two main steps: the first consists in the formation of the desired cation by quaternarization of the heteroatom followed either by protonation in an acidic medium or by alkylation *via* an S<sub>N</sub>2 reaction [76,77]. The development of these molten salts by anionic metathesis or acid-base neutralization with biobased anions has led to the development of the fifth-generation ILs. Some of these ILs have been designed by replacing sulfonium, phosphonium or imidazolium cations with cations from natural sources such as choline and betaine (Figure 11), making ILs greener and more eco-responsible [78].

**4. Methods of ILs synthesis and their purification**

Many protocols have been set up for the design of ILs. In this review, we will focus on the main synthetic pathways used and mastered by all, namely.

**4.1. Design of ILs by classical synthesis**

This path usually requires a two-step design of the IL. The first step consists in the formation of a cation by quaternization, especially by alkylation, of a tertiary amine, or phosphine, or dialkyl sulfide compounds with a halogenoalkane derivative (RX) thus giving a halide anion ILs. This step generally requires the use of an excess of RX, leading to a highly contaminated IL. The longer alkyl chain of the compound to be quaternized, the longer is the reaction time. Similarly, the nature of the halogen bonded to the halogenalkane has a significant influence on its reaction time. For example, for the identical alkyl chain, the halogenalkane with the most voluminous halogen will have the shortest reaction time ($I^{-} > Br^{-} > Cl^{-}$) [79].

The second step is the anionic metathesis to substitute the anion of the IL to make it purer and according to the specificity of the desired IL. However, it is difficult to remove halide impurities even if the use of an anionic resin or neutralization by an acid can improve their purity in this way. Nevertheless, the synthesis remains very expensive and time-consuming.

**4.2. Design of ILs by continuous flow**

This design pathway of ILs has more advantages than conventional synthesis. Continuous flow allows the reduction of the reaction time of alkylation by quaternization, optimizes the yield and above all leads to ILs with a high degree of purity [80,81].

Despite these advantages, it remains contrary to the concepts of green chemistry, since the synthesis is particularly endothermic and the increase in yields depends on the increase in temperatures. For example, Figure 12 shows how bromobutane reacts with methylimidazole to form 1-butyl-3-methylimidazolium in continuous flow. This clearly shows the correlation between increasing temperature and the rate of conversion [80].

**4.3. Design of ILs by sonochemistry**

Sonochemistry-mediated ILs alkylation of compounds carrying either a tertiary amine, a phosphine, or an alkylsulfonate, makes it possible to optimize the yield and the reaction time.

It is an inexpensive synthesis pathway that promotes the reactivity of the reagents [82] by mechanical action, bringing them into an excited state, without requiring a catalytic solvent [83,84]. It takes place in an ultrasonic bath. This pathway allows the homogenization of the reaction medium, whatever its viscosity.

<figure><img src="image_37.png" /><figcaption>**Figure 11.** Structure of choline and betaine cations.</figcaption></figure>
<figure><img src="image_38.png" /></figure>

<figure><img src="image_39.png" /><figcaption>**Figure 12.** Effect of temperature when designing IL by continuous flow [80].</figcaption></figure>

<figure><img src="image_40.png" /><figcaption>**Figure 13.** Ultrasound-assisted synthesis of ionic liquids based on 1-butyl-3-butylimidazolium [84] and 1-alkyl-3-methylimidazolium [83].</figcaption></figure>

Figure 13 presents the synthesis of ionic liquids by sonochemistry. This pathway also leads to the formation of ILs in a one pot by coupling quaternization and metathesis steps [85].

### 4.4. Design of ILs by microwave irradiation

This ILs design by microwave technology is similar to that by sonochemistry (mechanical). The microwave irradiation leads to the excitation of the reactants (activation of reactivity). Unlike sonochemistry, microwave irradiation is more efficient and reduces quaternization reaction time [86,87] and improves efficiency and yields compared to other synthetic pathways [88]. Figure 14 shows the one-pot synthesis of ILs by microwave irradiation. However, this method requires a lot of optimizations depending in particular on the nature of the ILs considered, both in terms of irradiation power and time (Table 4) [89].
<figure><img src="image_41.png" /></figure>

<figure><img src="image_42.png" /><figcaption>**Figure 14.** One-pot synthesis of ILs by microwave irradiation [87,89].</figcaption></figure>

<table><caption>**Table 4.** Preparation of ILs based on alkylimidazolium halides [89].</caption><thead><tr><th>Entry</th><th>Alkylhalide (RX)</th><th>RX-mmol</th><th>MIM-mol</th><th>MW-Time/s</th><th>Yield<sup>a</sup> (%)</th><th>Yield<sup>b</sup> (%)</th></tr></thead><tbody><tr><td>1</td><td>1-chlorobutane</td><td>4</td><td>2</td><td>30 + 15 + 15 + 15 + 15</td><td>76</td><td>50 (5)</td></tr><tr><td>2</td><td>1-bromobutane</td><td>2.2</td><td>2</td><td>30 + 15 + 15 + 15</td><td>86</td><td>76 (5)</td></tr><tr><td>3</td><td>1-iodobutane</td><td>2.2</td><td>2</td><td>30 + 10 + 10 + 10</td><td>92</td><td>93 (3)</td></tr><tr><td>4</td><td>2-bromobutane</td><td>4</td><td>2</td><td>30 + 15 + 15 + 15 + 15</td><td>71</td><td>61 (5)</td></tr><tr><td>5</td><td>1-chlorohexane</td><td>2.2</td><td>2</td><td>30 + 15 + 15 + 15 + 15</td><td>81</td><td>53 (5)</td></tr><tr><td>6</td><td>1-bromohexane</td><td>2.2</td><td>2</td><td>30 + 15 + 15 + 15</td><td>89</td><td>78 (5)</td></tr><tr><td>7</td><td>1-iodohexane</td><td>22</td><td>2</td><td>30 + 10 + 10 + 10</td><td>93</td><td>89 (3)</td></tr><tr><td>9</td><td>1-iodoheptane</td><td>2.2</td><td>2</td><td>30 + 10 + 10 + 10</td><td>94</td><td>95 (3)</td></tr><tr><td>10</td><td>1-bromooctane</td><td>2.2</td><td>2</td><td>30 + 15 + 15 + 15</td><td>91</td><td>73 (5)</td></tr><tr><td>11</td><td>1,4-dibromobutane</td><td>1</td><td>22</td><td>30 + 15 + 15 + 15</td><td>81</td><td>76 (5)</td></tr><tr><td>12</td><td>1,4-diiodobutane</td><td>1</td><td>2.2</td><td>30 + 15 + 15 + 15 + 15</td><td>91</td><td>89 (3)</td></tr><tr><td>13</td><td>1,6-dichlorohexane</td><td>1</td><td>2.2</td><td>30 + 10 + 10 + 10</td><td>82</td><td>56 (5)</td></tr><tr><td>14</td><td>1,6-dibromohexane</td><td>1</td><td>2.2</td><td>30 + 10 + 10 + 10</td><td>92</td><td>72 (5)</td></tr><tr><td>15</td><td>1,6-diiodohexane</td><td>1</td><td>2.2</td><td>15 + 15 + 10 + 10</td><td>85</td><td>97 (3)</td></tr><tr><td>16</td><td>1,8-dichlorooctane</td><td>1</td><td>2.2</td><td>30 + 15 + 15 + 15 + 15</td><td>78</td><td>72 (5)</td></tr><tr><td>17</td><td>1,8-dibromooctane</td><td>1</td><td>2.2</td><td>30 + 15 + 15 + 15</td><td>92</td><td>76 (5)</td></tr><tr><td>18</td><td>1,8-diiodooctane</td><td>1</td><td>2.2</td><td>15 + 15 + 10 + 10</td><td>94</td><td>93 (3)</td></tr></tbody></table>

## 5. Purification of ILs

Purification of IL is a tedious process with conventional methods of separation and purification, such as mechanical, diffusion, thermal separation, or ion chromatography. Considering that ILs are non-volatile molten salts, it is not easy to purify them by distillation, nevertheless it has been shown in literature that ILs with bis(triflimide) anion can be distilled due to their high thermal stability. For example, Earle et al. have demonstrated that some ILs can be purified by distillation between 200°C and 300°C [90].

Some reagents generating the ILs cationic group, such as the 1-methylimidazolium cation, do not react completely during the design of ILs and also require huge amounts of halogenalkyl [91] at the quaternization stage. Finally, the unreacted reagents are impurities that are difficult to separate from the desired product and have a significant impact on the physico-chemical quality of the ILs [92]. Regarding the ILs anionic group, the anions are generally heteroatoms or transition metals. Traces of metals may appear in hydrophilic ILs during the quaternization and metathesis stages. The presence of these metals poses problems for applications in medicinal chemistry. This is the case with certain ILs produced from biological supports, but containing traces of silver. These ionic liquids have antibacterial properties, but can also cause undesirable effects [71]. The use of

<footnote><sup>a</sup>Using MW at power 240 W. <sup>b</sup>Using alternative heating method (oil bath at 80°C). Entry, mp; 11, 67–69°C, 12, 110–112°C; 13, 85–87°C; 14, 112–116°C. MW: microwave; MIM: 1-methylimidazole; MW-Time/s: microwave time in seconds.</footnote>
<figure><img src="image_43.png" /></figure>

chromatography on activated carbon or silica gel [65] or alumina [77,89,93] is already used to remove stained impurities [94]. The presence of traces of metals, even in very small quantities, can distort experimental results, particularly in electrochemistry or NMR spectroscopy [92]. In the case of hydrophobic ILs design, halogen ionic impurities or transition metals can be removed by simple washing with water [95].

However, water is a real impurity in the design of certain identity documents. It is therefore necessary to eliminate traces or the presence of water within a certain range of liquid density [15] revealed by the Karl-Fisher volumetric titration [15,95–98].

## 6. Toxicity and biodegradability of ILs

Toxicity and biodegradability are key factors in assessing the environmental impact of chemicals [99,100]. According to the criteria defined by Gu and Jérôme [18], from an environmental point of view, a green solvent should be non-toxic, biodegradable, derived from renewable resources, recyclable, and obtained from environmentally friendly chemical synthesis processes. However, the toxicity and biodegradability of ILs have not been sufficiently evaluated [101,102]. Phytotoxicity and cytotoxicity have been observed for many traditional ILs [103]. This has led to the development of new ILs with negligible toxicity, biocompatibility, simple preparation, and good durability. These are biocompatible ionic liquids prepared specifically from compounds derived from natural resources [103]. In their study, Wang et al. synthesized a series of biocompatible ILs from choline and organic carboxylic acids [104]. These ionic liquids facilitate improved pretreatment of lignocellulosic biomass [104,105] and the production of biobased platform compounds using biochemical, chemical, and thermochemical processes [106].

### 6.1. Toxicity of ILs

The toxicity of a chemical substance is a crucial factor in various contexts, particularly in human health and the environment. Under global environmental regulations, precise knowledge of the toxicity of a chemical substance is required before it can be applied and marketed. Knowledge of the relationship between the chemical structure of ILs and their toxicity is essential for the design of more biocompatible and environmentally friendly solvents [107]. Several tests can be used to assess the toxicity of ILs such as tests on enzymes, mammalian cells, microalgae, bacteria, vertebrates, and invertebrates [108,109]. However, the tests carried out on freshwater and marine species are considered to be the most accurate and reliable [110]. Indeed, on the basis of current information on inter-laboratory and intra-laboratory comparisons, the toxicity results obtained using these tests are the least variable for toxicity tests carried out under similar experimental conditions [111]. These toxicity tests are carried out with the marine bacterium *Aliivibrio fischeri* and the microalgae *Raphidocelis subcapitata* using the Microtox® toxicity kit [107,112,113]. According to standard UN labeling guidelines [112], ILs can be labelled as non-toxic if and only if their EC50 > 100 mg/L. According to these guidelines, bacteria are not considered as indicators of ILs toxicity, but only tests on aquatic microalgae such as *Raphidocelis subcapitata*, which are much more sensitive [107]. Generally speaking, cations have a higher toxicity impact than anions because they have a high lipophilicity and affinities with the phospholipid membrane of microorganisms via the electrostatic attraction of the positive charge [114]. Cations with longer alkyl chains are normally more toxic [115,116] due to the effect of the side chain, which increases the lipophilicity of the IL and therefore promotes reactivity with biological membranes [117]. However, the toxicity of ILs reaches a threshold point. Beyond this point, the number of carbon atoms added to the alkyl chain no longer increases the toxicity of IL [116]. The effects of structure on the toxicity of ionic liquids are summarized in Table 5.

## 7. Applications and recyclability

In 1914, Walden [13] identified a type of solvent. This was ethylammonium nitrate, derived from the combination of an organic ammonium cation and a polyatomic anion. This compound melts at 12°C and remains liquid at room temperature. It is the first IL identified as a solvent of interest. Since then, the scientific community has developed multitudes of ILs by combinations of organic cations and various and varied organic or inorganic anions, thus giving rise to successive generations of ILs, with the aim of solving the problems linked to the use of VOCs harmful to human health and the environment. The aim of ILs structural
<figure><img src="image_44.png" /><figcaption>**Table 5.** Summary of structural effects of IL on toxicity [114].</figcaption></figure>

<table><thead><tr><th>Low toxicity</th><th>High toxicity</th></tr></thead><tbody><tr><td>Cation:</td><td>Imidazolium, Phosphonium,</td></tr><tr><td>Cholinium</td><td>Ammonium, Pyridinium, Quinolinium</td></tr><tr><td>Guanidium</td><td>Hydrophobic functionalization (ex: aryl)</td></tr><tr><td>Sulfonium, Morpholinium</td><td>Long alkyl chain</td></tr><tr><td>Dicationic fraction, Protic fraction</td><td>More substituents (example: peralkylated)</td></tr><tr><td>Short alkyl chain</td><td>Bioactive molecules (example: herbicide, menthol, ampicillin, theophylline)</td></tr><tr><td>Fewer substituents</td><td></td></tr><tr><td>Hydrophobic functionalization (example. ester, hydroxyl)</td><td></td></tr><tr><td>Anion:</td><td>Hydrophobic structures</td></tr><tr><td>Halides</td><td>Metal-containing molecules (e.g. GdCl<sub>4</sub>, MnCl<sub>4</sub>, CoCl<sub>4</sub>)</td></tr><tr><td>Hydrophilic anions</td><td>Unstable in the aquatic environment (e.g. BF<sub>4</sub>, PF<sub>6</sub>, SbF<sub>6</sub>)</td></tr><tr><td>Small molecular volume</td><td>Bioactive molecules</td></tr><tr><td>Bronsted high acidity anion</td><td></td></tr><tr><td>Amino acid-based anions</td><td></td></tr></tbody></table>

modifications was to improve the physico-chemical properties of ILs, such as increasing their miscibility to certain VOCs or aqueous solutions, or the improvement of their ability to dissolve compounds. From a synthetic chemistry point of view, it was also necessary to customize ILs to effectively catalyze chemical and enzymatic reactions during the production of active ingredients with added values (for health, transformation, or environmental sanitation). ILs have physico-chemical properties, namely a melting point below 100°C, thermal and chemical stability up to 400°C. They are non-volatile, non-inflammatory, and biodegradable. They have almost zero vapor pressure, and some of these ILs interfere with life. Thus ILs are usable in a wide range of applications in chemistry, including chemical synthesis, enzymatic synthesis, electrochemistry, eco-extraction of bioactive compounds or extraction of pollutants and metals, gas absorbents, storage and development of energy, vectorization in biological environment or having biological potentials.

## 7.1. Applications of ILs in chemical/enzymatic synthesis

Green chemistry requires the use of green processes and in particular green catalysts. ILs are thus presented as potential catalytic alternatives to VOCs solvents, as reaction medium, during chemical reactions. It has been reported by Chenlin et al. [118] an acidolysis reaction assisted by an acid IL medium to produce biofuel on a large scale from maize by chemical destruction of lignocelluloses. The use of ILs as reaction medium also makes it possible to carry out, without enzyme or catalyst, the hydrolysis of sugars (e.g. fructose, glucose, and galactose) or disaccharides (e.g. sucrose, maltose, and lactose) to 5-hydroxymethylfurfural (HMF), for the production of biofuels and chemicals [119] and for the conversion of biomass [120]. A similar work was performed by using ILs for the pretreatment of the walls of certain organisms to improve the enzymatic hydrolysis of cellulose and glucose derivatives [121–127]. ILs are also used as reaction medium in the transesterification reactions of vegetable oils [128,129], in the cross amino acid aldolization reaction (hisdidine) [130], for the Diels–Alder intramolecular reaction [131] or the heteroatom exchange reaction [132]. ILs are also used as an asymmetric catalyst [133], to synthesize chalcogenides by desulfurization [134], in epoxidation of olefins [135], cycloaddition to epoxides [22], cyclic carbamates [136], Wittig reaction [137] and Michaelian reactions such as Aza-Michael, phospha-Michael, Michael addition, nitration, thio-Michael, reduction, condensation, Clauson–Kaaas reaction [20,21], polymer preparation [137].

## 7.2. Applications of ILs as dissolution/extraction solvents

### 7.2.1. Biopolymers

ILs have also found many applications in the dissolution of polymers. Their particular properties give them a great ability to solubilize biopolymers such as cellulose or lignin [133,138–141] under very mild conditions. Biobased ILs (cations and anions of natural origin) have also demonstrated their effectiveness in extracting biopolymers in a more eco-extractive way [142].

To better understand the mechanisms and parameters influencing the dissolution of polymers in ILs, Novoselov et al. [143] studied the effect of the molecular structure of ILs on their ability to dissolve natural polymers (cellulose, dextran, fibroin). In their work, they show that IL anion plays a major role in the solubilization of these biopolymers by forming hydrogen bonds with the protons of the active groups of the polymer. It has also been
<figure><img src="image_45.png" /></figure>

reported by Lu et al. that when the cation carries a hydrogen atom on its heterocyclic aromatic ring, the latter interacts with hydroxyl groups and esters then favoring the dissolution of cellulose and the extraction of polar compounds [26,140]. Garron et al. [133] and Hou et al. [21] reported in their respective studies that imidazolium with heterocyclic aromatic cations and $Cl^-$ anions significantly dissolve cellulose and hemicelluloses for the separation of phenolic compounds. 1,3-dimethylimidazolium phosphonate and 1-ethyl-3-methylimidazolium thiocyanate [140] allow the dissolution of lignin to extract phenolic compounds. It has been reported by Diop [144] that the introduction of an allyl chain into the cationic group improves the dissolution of lignin favored by a strong interaction between lignin and the allyl group.

The use of ILs as cosolvents can also be considered: 1-ethyl-3-methylimidazolium alkylbenzene sulfonate in solution in THF and water, in adequate proportion, thus allows the dissolution of lignin and the extract of phenolic compounds [42].

In 2012, the effect of additives was studied by Zhang et al. [145] who showed that the dissolution of cellulose in choline acetate ([Ch]OAc) was not effective to a large extent.

They demonstrated that adding 5%–15% by weight of tributylmethylammonium chloride ([TBMA]Cl) to cholinium chloride allowed 6% by weight of cellulose to dissolve in just 10 min at 110°C. In contrast, under the same conditions, only 4% by mass of cellulose was dissolved in 1-butyl-3-methylimidazolium chloride, an IL conventionally used for cellulose dissolution even after 8 h of reaction [145].

Many studies have focused on extracting the active components of traditional plants using ILs as alternative solvents, to solve the problem caused by traditional methods of extracting natural products with organic chemical reagents. The 2018 review written by Xiao et al. exhaustively summarized the work done on the extraction of natural products using ionic liquids [146]. Among the natural products, flavonoids, alkaloids, terpenoids, and phenylpropanoids can be extracted using ILs. Extractions always use imidazoliums coupled to halogenated or polyhalogenated anions, as well as microwave or ultrasound activation techniques. Yields are often between 2% and 7% by mass. This is the yield traditionally obtained for the conventional extraction of this type of compound, but ILs do not use basic or extremely acidic conditions.

Other studies [142,147] show that cellulose dissolution is favored by the swelling properties of IL. At this level, the ability of the IL anion to establish stronger hydrogen and electrostatic interactions than those within cellulose between the glucoses making up its structure enables cellulose to be dissolved.

### 7.2.2. Essential oils

The extraction of essential oils from plants is mainly carried out by hydrodistillation, hydrodiffusion, or using supercritical carbon dioxide (scCO₂) [148]. If the processes using water are energy-consuming, extraction with scCO₂ although generally expensive (pressurized material), allows to obtain good yields of essential oil but not on a large scale.

Percolation can also be used but since it extracts many aromatic substances, the harvested essential oils are often rich in non-volatile compounds. Extraction by 'cold pressing' does not involve any heat. Most citrus essential oils are extracted by this process, which involves subjecting the material to great mechanical pressure. Fine et al. reported the extraction of essential oils such as alpha-pinene and limonene from pine and orange peels, as well as vegetable oils such as methyl esters, glycerol and glycerol carbonate from sunflower and rapeseed [149].

Finally, extraction can also be carried out with solvents such as petroleum ether, methanol, ethanol, or hexane. This is for example the case of jasmine with maceration or enfleurage methods. The fear of traces of residual solvent in this case is major and does not apply to cosmetic uses.

The addition of ILs (substituted imidazoliums and associated halogenated anions) to water during hydrodistillation increases the yields of this operation [150] and extracts large amounts of bioactive compounds [27]. However, the operating conditions remain energy-intensive and energy-consuming, and it is not excluded to detect traces of ILs or derivatives in essential oils [151]. In addition, in this work, the recycling of ILs is not mentioned.

### 7.2.3. Aroma extraction

Aromatic molecules are low molecular weight organic compounds whose vapor pressure at atmospheric pressure is at room temperature. These volatile compounds belong to the different classes of organic chemistry: hydrocarbons, aldehydes, acids, alcohols, and esters. They are present in food matrices in very small
<figure><img src="image_46.png" /></figure>

amounts, or even in trace, except in the case of spices and aromatics where their concentration can sometimes be very high. For example, clove essential oil contains 74% eugenol, an aromatic compound with numerous properties, including anti-inflammatory, insecticidal, antimicrobial, and antioxidant properties [152].

Their extraction can be carried out using the same techniques used for essential oils, the use of ILs is not yet common due to potential residues, their toxicity and the lack of evidence regarding their effect on an increase in extraction yields [153,154].

### 7.3. Applications of ILs in chitin deacetylation

Because of their unique properties, the use of ILs as solvents has been explored in various applications, notably in the deacetylation of chitin [145,155]. The ability of ILs to dissolve a wide range of compounds makes them ideal solvents for a variety of applications, including deacetylation reactions. In their study, Ma et al. showed that [TBA][OH] can effectively dissolve and deacetylate chitin at room temperature. They also showed that pre-treating chitin with a small amount of [TBA][OH] improved the activity of chitin deacetylase from *Rhodococcus equi* CGMCC14861 (ReCDA) and therefore increased the rate of conversion of chitin to chitosan. Aspras et al. in 2017 also used [Bmim][Br] to increase chitin deacetylase activity. 1-Butyl-3-methylimidazolium acetate can also be used as a reaction medium to increase the degree of deacetylation of chitosan [156].

### 7.4. Applications of ILs in electrochemistry and energy conservation

The combination of cations and anions gives ILs specific properties such as high thermal stability, non-volatilization, and high conductivity. These properties make them useful in applications such as electrolytic materials, energy conservation, and conversion devices in batteries and supercapacitors. It is reported by Watanabe et al. [24] that the type of IL (protic or aprotic) defines the properties of their uses and makes the ILs a good conductor and stores energy according to the nature of the metal compounds constituting the battery or the supercapacitor, because protic ILs release $H_3O^+$ ions corrosive to certain metals. The nature of the anion and cation also plays an essential role in melting point, viscosity, their ability to resolve metal ions and the oxidative stability of anions.

### 7.5. Applications of ILs in the recovery and depollution of certain metals

Ionic liquids have been studied for their use in the decontamination of certain metals, particularly as corrosion inhibitors [157] and extracting solvents or chelating agents [158,159]. It has been reported in the literature that ILs can be protect metallic materials against corrosion by forming a protective surface on their surface [157]. In their work, Ikeuba et al. demonstrated the IL ethyl dimethyl propylammonium bis(triifluo-methyl sulfonyl)imide ([EDMPA][TFSI]) effectively inhibited the corrosion of the aluminum alloy AA3003 in acidic solution. The inhibition efficiency reached 93.2% at 303 K and 94.7% at 333 K [157].

In their study, Aslam et al. in 2021 used the IL L-alanine methyl ester nitrate (L-AlaC<sub>1</sub>NO<sub>3</sub>) to inhibit corrosion on steel plates. They showed that corrosion inhibition increased with temperature and IL concentration. In their study, Raj et al. extracted cobalt and nickel efficiently from an aqueous solution using low concentrations of the ILs methyltrioctylammonium oleate and methyltrioctylammonium linoleate. In their study, Wang et al. in 2020 demonstrated that the use of acetylcholine-based IL [Ach][Tf<sub>2</sub>N] enabled gold to be recovered from an Au (III) chloride solution with an efficiency of 98% [160]. Huang et al. in 2014 found that 1-butyl-3-methyl-imidazolium hydrogen sulfate IL [Bmin][HSO<sub>4</sub>] was effective in extracting copper from printed circuit board scrap [161]. Chen et al. conducted a comprehensive study on copper extraction from PCB scrap using a series of six acidic ILs such as [BSO<sub>4</sub>HPy]HSO<sub>4</sub>, [BSO<sub>3</sub>HMIm]HSO<sub>4</sub>, [BSO<sub>3</sub>-HPy]OTf, [MIm]HSO<sub>4</sub>, [BSO<sub>3</sub>HMIm]OTf, and [Bmim]HSO<sub>4</sub>. They proved that all the ILs tested could extract almost 100% of the copper from waste printed circuit boards [162].

### 7.6. Applications of ILs in analytical chemistry

ILs have interesting physico-chemical properties such as their non-volatilization, high viscosity, thermal and chemical stability, polarity, and existing interactions within structuring groups. These physico-chemical
<figure><img src="image_47.png" /></figure>

characteristics make it possible to use ILs as additives (chiral ILs), grafting agents for stationary phases in analytical and separative sciences of complex analytes.

The use of ILs to modify solid columns has improved the efficiency and selectivity of GC columns. In particular, chemical grafting of ILs, specifically of the dicationic or bolaform type, gives good results. This type of column has been used for the separation of alcohols [163–166], phthalates and PAHs at temperatures of up to 400°C [167]. ILs can also be used as diluents in complex mixtures to separate and identify residual chiral compounds in pharmaceutical products [168]. The use of ILs as additives in the mobile phase or grafted-on the stationary phase makes it possible to improve the column, in HPLC, in other words to solve the effects of silanol or alkyl groups (reversed phase columns) affecting in numbers the retention coefficient and better separating analytes. ILs are also used in mass spectrometry (ESI or MALDI) to improve the sensitivity of the detector separation of ions, of polysaccharide compounds for example [169], or as matrices for the analysis of peptides and proteins in MALDI type mass spectrometry [170].

## 7.7. ILs for lubricants

Thanks to their low volatility, good thermal stability, high heat capacity, and fluidity at low temperatures, ILs can be used as lubricants for various materials, such as steel-steel, steel-copper, and steel-silicon dioxide systems [171]. For example, alkylimidazolium tetrafluoroborates and hexafluorophosphates are used as lubricants in steel-steel systems. The IL [Emim][BF₄] is suitable for steel-aluminum systems, while [Bmim][PF₆] is used for ceramic materials [171]. In their study, Song et al. showed that the use of ILs 1-butyl-3-ethylimidazolium bis((trifluoromethyl)sulphonyl)imide ([C₄C₁im][NTf₂]) and 1-decyl-3-methylimidazolium bis((trifluoromethyl)sulphonyl)imide ([C₁₀C₁im][NTf₂]) as lubricants offer several advantages over traditional engine oils. Their use reduces the coefficient of friction, reduces wear on parts, and limits oil losses during use [172].

The list of applications for ILs is not exhaustive. There are still a number of applications in which ILs have shown great potential as an alternative to traditional organic solvents. For example, they are used as absorbents for air and water purification, as materials for battery electrolytes, as drug delivery systems [23], and as plasticizers for certain polymer materials such as polyvinyl chloride (PVC) and methyl methacrylate (PMMA) [171].

## 7.8. Recycling of ILs

ILs are stable at high temperatures, which complicates their recycling. Conventional solvent recovery methods are therefore not very effective, as detailed in the section on purification.

In general, their recycling is done to reach their decomposition temperature and then recover them by recondensation at low temperature. The same methods of purification of ILs can be used to recycle them according to the type and their miscibility to water and usual solvents. However, for magnetic ILs metal anion losses occur. For this case, it would be necessary for example to proceed to liquid-liquid extraction and then recover the metals by precipitation, centrifugation and filtration on micro-membrane [173]. However, this process remains long and costly. In addition, the recovery of metal anions for IL redesign remains difficult, which can affect the physico-chemical properties of IL [174].

Another method is to put the magnetic ILs in aqueous solution with the presence of EDTA as a powerful complexing agent [175] risk of pollution. LCA techniques are essential especially for the ‘waste’ stage given their constitution in atoms of Fluor, Sulfur, or Phosphorus and their chemical structure [176].

## 8. Conclusion

ILs have interesting physico-chemical properties, non-flammability, negligible vapor pressure. These properties give ILs green solvent qualities comparable to VOCs solvents whose toxicities are known to human health and the environment; it comes to question their biodegradability. If toxicological studies [177] have been reported on the ability of alkyl chains to establish interactions with cell membranes, including
<figure><img src="image_48.png" /></figure>

phospholipid bilayers, ILs are however incompatible with the enzymatic mechanisms of certain organisms. Even if biobased ILs with cholinium and betainium cations and alcoholate or carboxylate anions are developed [178], without prejudice, ILs cannot be qualified as totally green solvents because of too small amounts of literature dealing with their ecotoxicity and biodegradability, especially since some ILs have biological potential activity against certain microorganisms with pathogenic properties to humans. Design standards for ILs [179] are established.

Jastorff et al. in 2003 [180] challenged the scientific community on the toxicity of ILs. After a study in 2006 [181] on the recycling or purification of conventional imidazolium ILs with NTf₂ anion by distilling them at their decomposition temperature and recondensing them at low temperatures, scientists began to question their dangerousness especially in industrial environments. It has been found [182,183] that some ILs, despite their lower flammability compared to DMSO but comparable to polypropylene, pyrrolidium ILs at their decomposition temperature emit toxic compounds such as CO, SO₂, and HF [184]. The question of biodegradability of hydrophobic ILs with NTf₂ anion, insoluble or very poorly soluble in aqueous media, can interact with salts (Na⁺, Cl⁻) present in rivers and this is not negligible in case of leakage of ILs channels in industrial environments.

Nevertheless, ILs have controllable physico-chemical properties, such as their non-volatility, their stable thermal and chemical stability up to high temperatures, their recyclability compared to VOC solvents and their ability to solubilize large molecules and polymers. They currently remain better appreciated, are multi-functional and more efficient than some VOCs.

## 9. Outlook

Despite their excellent properties and their proven effectiveness in several application areas, ILs are confronted at some challenges like toxicity problems, regulation, and resource issues. Indeed, the majority of ILs available on the market come from petrochemical resources. To meet these challenges, it is important to focus much more research on the synthesis of ILs from natural and renewable resources. Their toxicity, biodegradability, and renewable nature also need to be better studied. These efforts will enable us to be better aligned with current requirements and the principles of green chemistry.

## Author contributions

CRediT: **Doféré Yeo**: Conceptualization, Formal analysis, Investigation, Writing – original draft; **Mansour Issoufou Tini**: Data curation, Investigation, Visualization, Writing – original draft; **Nicolas Joly**: Conceptualization, Formal analysis, Investigation, Project administration, Resources, Supervision, Validation, Writing – review & editing; **Mahasoa-Salina Souvenir-Zafindrajaona**: Conceptualization, Formal analysis, Writing – review & editing; **Jean-Pierre Mbakidi**: Data curation, Formal analysis, Writing – review & editing; **Jean-Pierre Mbakidi**: Data curation, Formal analysis, Writing – review & editing; **Jean-Pierre Mbakidi**: Data curation, Formal analysis, Writing – review & editing; **Jean-Pierre Mbakidi**: Data curation, Formal analysis, Writing – review & editing; **Vincent Lequart**: Conceptualization, Investigation, Supervision, Writing – review & editing; **Ludovic Chaveriat**: Formal analysis, Resources, Writing – review & editing; **Sandrine Bouquillon**: Conceptualization, Investigation, Supervision, Writing – review & editing; **Patrick Martin**: Conceptualization, Funding acquisition, Project administration, Resources, Supervision, Validation, Writing – review & editing.

## Disclosure statement

No potential conflict of interest was reported by the author(s).

## Funding

All the researchers would like to thank the Hauts-de-France Region, the University of Artois, and the University of Reims Champagne Ardenne. This work is funded by the University d'Artois and the Hauts-de-France region as part of the doctoral contract on the design of Biosourced Ionic Liquids for Extractive and Reactive Applications (LIBIOS: Biosourced Ionic Liquids).
<figure><img src="image_49.png" /></figure>

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