Mutual solubilities between water and non-aromatic sulfonium-, ammonium- and phosphonium-hydrophobic ionic liquids

Abstract

Although previous studies attempted to characterize the liquid-liquid phase behaviour between water and ionic liquids (ILs), the impact of non-cyclic cations on the solubilities is poorly studied and yet to be understood. In this work, the mutual solubilities between water and ILs containing the anion bis (trifluoromethylsulfonyl) . The COSMO-RS is able to correctly predict the experimental UCST behaviour and the cation impact on the immiscibility regimes observed. Natural Population Analysis (NPA) calculations were additionally performed for the isolated cations in the gas phase indicating that the differences in the water-IL mutual miscibilities might not result only from the hydrophobicity of the cation (derived from the increase of the alkyl chains length) but also from the charge distribution of the central atom and attached methylene groups. This fact explains the enhanced solubility of ammonium-based ILs in water here identified.

Hydrogen-bond acidity of ionic liquids: an extended scale

“…This charge distribution also has a significant impact on other properties, such as in the phase diagrams of binary IL-water systems, densities and viscosities, as previously shown. 60,61 On the other hand, the C-H bonds of the alkyl groups in tetraalkylphosphonium ILs are hardly polarised compared to tetraalkylammonium, which is clearly shown by the low α-value of the former IL. This behaviour can be attributed to the different charge distribution/contribution of the ammonium-and phosphonium-based IL cations as previously demonstrated by Carvalho et al 60,61 The higher acidity afforded by ammonium-based ILs also correlates well with their lower ability to form two-aqueous phase systems reflected by a higher ability to hydrogenbond with water.…”

“…60,61 On the other hand, the C-H bonds of the alkyl groups in tetraalkylphosphonium ILs are hardly polarised compared to tetraalkylammonium, which is clearly shown by the low α-value of the former IL. This behaviour can be attributed to the different charge distribution/contribution of the ammonium-and phosphonium-based IL cations as previously demonstrated by Carvalho et al 60,61 The higher acidity afforded by ammonium-based ILs also correlates well with their lower ability to form two-aqueous phase systems reflected by a higher ability to hydrogenbond with water. 62 Two sets of imidazolium-based ILs were also studied: one with 1-methyl-3-alkylimidazolium-based asymmetric ILs and the other consisting of 1,3-dialkylimidazoliumbased symmetric ILs.…”

Comprehensive study on the impact of the cation alkyl side chain length on the solubility of water in ionic liquids

“…This work represents an extension of our contribution towards the understanding of the relationship between the hydrophobic ILs chemical structures and their impact on their mutual solubilities with water [4][5][6][7][8][9][10][11][12]. Here, we focus on hydrophobic ILs, which are promising media for liquid-liquid extractions from aqueous solutions [13][14][15].…”

“…From these works [4][5][6][7][8][9][10][11][12], and in terms of IL-water interactions, the IL structural features can be summarized as follows: (i) the anion plays a dominant role, that is, the higher the hydrogen bond basicity of the anion the more intense its interaction with water [4,5,9]; and (ii) the cation head and the cation alkyl side chain length have a significant although minor contribution [4,6,8,10,12]. In addition, all these systems showed an upper critical solution temperature (UCST) behaviour, in which the UCST increases with the cation alkyl side chain length [4][5][6][7][8][9][10][11][12]. However, it should be noted that some IL-water mixtures present lower critical solution temperature (LCST) behaviour [16,17].…”

“…Here, we focus on hydrophobic ILs, which are promising media for liquid-liquid extractions from aqueous solutions [13][14][15]. The effects of various IL structural characteristics, namely the IL anion nature [4,5], cation head group [4,6,7], cation alkyl side chain length [8], structural/ positional isomers [9,10], and fluorinated moieties [11], on their phase behaviour with water have been previously reported. From these works [4][5][6][7][8][9][10][11][12], and in terms of IL-water interactions, the IL structural features can be summarized as follows: (i) the anion plays a dominant role, that is, the higher the hydrogen bond basicity of the anion the more intense its interaction with water [4,5,9]; and (ii) the cation head and the cation alkyl side chain length have a significant although minor contribution [4,6,8,10,12].…”