Mutual Solubility of Water and Structural/Positional Isomers of N-Alkylpyridinium-Based Ionic Liquids

Abstract

Despite many previous important contributions to the characterization of the liquid-liquid phase behavior of ionic liquids (ILs) plus water systems, a gap still exists as far as the effect of isomers (of ILs) is concerned. Therefore, in this work, a comprehensive study of the liquid-liquid equilibria between water and isomeric pyridinium-based ionic liquids has been performed. Atmospheric pressure mutual solubilities between water and pyridinium-based ionic liquids combined with the common anion bis[(trifluoromethyl)sulfonyl]imide were experimentally determined between (288.15 and 318.15) K. The main goal of this work is to study the isomeric effects on the pyridinium-based cation, namely, the structural and positional isomerism, as well as the alkyl side chain length. To the best of our knowledge, the influence of both structural and positional isomerism on the liquid-liquid behavior in ionic-liquid-water-containing systems is an unexplored field and is here assessed for the first time. Moreover, from the experimental solubility data, several infinite dilution molar thermodynamic functions of solution, namely, the Gibbs energy, the enthalpy, and the entropy, were estimated and discussed. In addition, aiming at gathering a broader picture of the underlying thermodynamic solvation phenomenon, molecular dynamics simulations were also carried out for the same experimental systems.

Understanding the impact of the central atom on the ionic liquid behavior: Phosphonium vs ammonium cations

“…Specific heuristics have been previously proposed covering the ILs' structural characteristics, namely the alkyl chain length and isomerism and the presence/absence of the aromaticity effect in a large range of physical, chemical, and biological properties. [4][5][6][7] It aims at understanding the impact in the ILs thermodynamic, thermophysical, and biological properties promoted by the simple substitution of a phosphorous by a nitrogen atom on the cation's core. In contrast to the ion substitution studies, the atom substitution, of the constituent ions, has been object of a limited number of studies.…”

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

“…All experiments to determine the solubility of water in the diverse ILs were conducted using the analytical method reported in our previous studies [4][5][6][8][9][10][11][12]. Briefly, samples of ILwater binary mixtures are placed in tightly-closed glass vials with a septum cap.…”

“…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].…”

“…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].…”

“…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].…”