12.3.1 Solubility

There is a lack of investigations on VOCs affinity towards ILs. Some results are available dealing with the partition coefficient between an ionic liquid and water [18]:

Polar organic compounds, such as dichloromethane and trichlorobenzene, are soluble in ILs. Huddleston [18] determined the partition coefficient between a given ionic liquid and water, specific to these compounds (log D) (Eq. 12.1):

For instance, a value of 1.8 and 2.4 were found for toluene — water [18] and [C6Mim][PF6] — water [26]. Huddleston [18] showed that ILs can solubilize apolar or lowly polar compounds. The literature dealing with the solubility of organic compounds in ionic liquids remains limited [27—31]. However, a trend seems to emerge showing that polar compounds are more easily soluble than apolar ones. Huddleston [18] also explored the replacement of usual solvents by ionic liquids in the liquid-liquid extraction of VOCs; they found that the affinity of a solute (the VOC) for ILs increases with the augmentation of the log POW value of the considered VOC.

Regarding the Henry’s constants, the scarce available results concern toluene and sulfur VOC, dimethyl sulfide (DMS), dimethyl disulfide (DMDS); they were measured for two ILs, [C4Mim][PF6] and [C4Mim][NTf2] and are compared in Fig. 12.3 to some other solvents [4, 32]. For instance, the Henry’s constants of DMDS and toluene in water are 123 and 615 Pa. m3.mol_1, respectively; they were found to be significantly lower in ILs, with ratio of the partition coefficients in water and the considered IL, [C4Mim][PF6] and [C4mim][NTf2], of 284 and 448 for toluene and 33 and 37 for DMDS, respectively [4]. If compared to the often used silicone oil (polydimethylsiloxane, PDMS), partition coefficients were similar for DMDS, while toluene showed a higher affinity for [C4Mim][NTf2] if compared to [C4Mim] [PF6] and PDMS [4].

In conclusion, compared to the most often used non-aqueous phase liquid (NAPL), namely silicone oils, especially polydimethylsiloxane, for hydrophobic VOCs removal [10], which have been often implemented in TPPB [11—13], ionic liquids appear especially promising owing to their solvent capacities.