For conventional solvents, molecules solubilization is generally expressed in terms of dielectric constant of solvent. However, these criteria could not be applied to ionic liquids. Several studies have been conducted to establish a specific ranking based on interactions between ILs and solutes [43]. This model considered inter­actions involving hydrogen bond and polarizability. According to these authors, ionic liquids are polar solvents but they have completely different behaviors from conventional solvents, so it is impossible to compare these two kinds of solvents only on their polarity.

Most of the time, the n interactions between aromatic cations and solutes are prevailing. Imidazolium ring is a good electron acceptor and due to the strong electron delocalization, the nitrogen does not form H bonds easily. On the contrary, a pyridinium cation is a good electron donor [44].

Some ionic liquids have been functionalized (TSILs: Task Specific Ionic Liq­uids) for a specific purpose. For example, ureas or thioureas groups were introduced in few ionic liquids in order to capture CO2, H2S or heavy metals [45]. RTILs (Room Temperature Ionic Liquids) have also been developed to perform extraction of metals [46] or elements such as Uranium [47]. Bifunctionalized ionic liquids can be used to optimize the extraction of Europium [48]. The [C4Mim] [NTf2] was proved to be effective in extraction of inorganic acids (HNO3, HCl, HReO4, HClO4) from aqueous phases [49].

Several authors have studied the distribution of organic compounds, such as aniline, benzene derivatives and organic acids or organic anions (phenolates) between an ionic liquid phase (usually [CxMim] [PF6, BF4 or NTf2]) and an aqueous phase. As expected, the most lipophilic compounds are more soluble in the ionic liquid phase [50—61]. In addition, it has been shown that aromatic compounds were more soluble in ionic liquids than their aliphatic counterparts, probably due to п-stacking interactions [43, 62—64].

Some studies have reported that the physical and chemical properties of ionic liquids are due to intra-molecular hydrogen interactions and Van Der Waals interactions. These studies explain the particular geometries adopted by ILs [65, 66]. Hydrogen bonds define the cation position versus the anion but also the distance between the two poles of an ionic liquid [67]. The structural organization of an IL can be explained by a combination of both types of interaction in the liquid phase, Coulomb and Van der Waals interactions [68, 69]. We observe that these interactions have a direct influence on the melting point, viscosity and ionic liquids enthalpy of vaporization. These inter­actions between the anion and the cation can also explain the variable hydro — phobicity observed. It is essentially depending on the nature of the anions. The self-organization of ionic liquids may also influence their potential extraction and gas absorption [65, 66, 70, 71].

The [CxMim] [PF6] were applied to the extraction of anionic dyes [72], to the identification or the extraction of organic pollutants in soil or in water [54, 73, 74] or to extract and separate bioactive molecules from plants [75—77]. They are known to solubilize some natural polymers (cellulose, BSA, etc.), sugars or amino acids [46, 78—82]. They constitute the overwhelming majority of the ionic liquids used for the extraction of organic molecules.

N-methylimidazoliums functionalized by carboxylic chains (CH2CO2H) and associated with fluorinated anions such as BF4~ or PF6~ were synthesized in order to trap organic compounds (chlorophenyl, amines) [83]. Some ionic liquids (mainly alkylimidazoliums) are described for the absorption of volatile organic compounds such as benzene, toluene, phenols, anilines or of sulphur heterocycles [84, 85]. Some studies report the solubility of several hydrocarbons (benzene, toluene, xylene, heptane, hexadecane, methanol, acetonitrile or chlo­roform) in ionic liquids such as [CxMim] [PF6] [86]. This work has shown that aromatic hydrocarbons, methanol and acetonitrile are soluble or partially soluble in [CxMim] [PF6], while aliphatic hydrocarbons are immiscible. It has been shown that for a fixed cation, the anion could affect the interaction between ILs and VOCs. Hard anions (NO3~,MeSO3~, etc.) are worse hydrogen bonds acceptor than softer anions such as B(CN)4~ and offer low affinities with VOCs [87, 88].

Some studies have demonstrated that the ability of the anion to accept hydrogen bonds was related to the distribution of organic compounds between the ionic liquid phase and an aqueous phase [89]. A study was published by

Milota, and describes a process for absorption of gaseous VOCs (MeOH, formaldehyde, phenol, acrolein, acetaldehyde and propionaldehyde) in an ionic liquid absorption column (Tetradecyl (trihexyl) phosphonium dicyanamide) [90, 91].

Predictive models have been developed by Chen, to improve understanding of interactions between ionic liquids and organic compounds [92]. Oliferenko, conducted a study based on 48 ionic liquids and 23 industrial gases (alkanes, alkenes, fluoroalkanes…). The most soluble compounds described in this study are butadiene and butene (high polarizable molecules) [93]. The absorption value of gas was measured for several ILs. The CO2 absorption is the most widely described [94], as also discussed above (see Sect. 12.3.2). Thus, Jalili, found that H2S was better absorbed than CO2 in IL [95]. Henry constants of few gases absorbed by [C4Mim] [PF6] were published by Safamirzaei, and the solubility of gases (CO2, ethylene, ethane, CH4, Ar, O2, CO, H2, N2) has been reported [96, 97]. It seems that it is the nature of the anion which is crucial for the absorption of gas by ILs [98]. Thus, supported imidazoliums are well known to have a good absorption capacity for CO2 and this characteristic is exacerbated when the cation is functionalized with an amino acid or an amine [99].

It also appears that the solubility of CO2 increase with the molecular weight of the ILs [94]. These authors have studied the selectivity of absorption of different gas versus CO2 (H2, N2, O2, CH4, H2S, etc.). By reducing the size and molecular mass of ionic liquids, it is possible to trap the most volatile gases. Increasing the pressure also improves the absorption of gases in ILs [96]. Many analyzes can be conducted to study the interaction between VOCs and ionic liquids but the most common is the FTIR spectroscopy to visualize the characteristic peaks of the functions present on VOCs (alcohols, aldehydes, etc.) and potential shifts induced by the ionic liquid/VOC interactions [100].