Mutual Solubilities of Water and Hydrophobic Ionic Liquids

Abstract

The ionic nature of ionic liquids (ILs) results in a unique combination of intrinsic properties that produces increasing interest in the research of these fluids as environmentally friendly "neoteric" solvents. One of the main research fields is their exploitation as solvents for liquid-liquid extractions, but although ILs cannot vaporize leading to air pollution, they present non-negligible miscibility with water that may be the cause of some environmental aquatic risks. It is thus important to know the mutual solubilities between ILs and water before their industrial applications. In this work, the mutual solubilities of hydrophobic yet hygroscopic imidazolium-, pyridinium-, pyrrolidinium-, and piperidinium-based ILs in combination with the anions bis-(trifluoromethylsulfonyl)imide, hexafluorophosphate, and tricyanomethane with water were measured between 288.15 and 318.15 K. The effect of the ILs structural combinations, as well as the influence of several factors, namely cation side alkyl chain length, the number of cation substitutions, the cation family, and the anion identity in these mutual solubilities are analyzed and discussed. The hydrophobicity of the anions increases in the order [C(CN) 3 ] < [PF 6 ] < [Tf 2 N] while the hydrophobicity of the cations increases from [C n mim] < [C n mpy] e [C n mpyr] < [C n mpip] and with the alkyl chain length increase. From experimental measurements of the temperature dependence of ionic liquid solubilities in water, the thermodynamic molar functions of solution, such as Gibbs energy, enthalpy, and entropy at infinite dilution were determined, showing that the solubility of these ILs in water is entropically driven and that the anion solvation at the IL-rich phase controls their solubilities in water. The COSMO-RS, a predictive method based on unimolecular quantum chemistry calculations, was also evaluated for the description of the water-IL binary systems studied, where it showed to be capable of providing an acceptable qualitative agreement with the experimental data.

Hydrogen-bond acidity of ionic liquids: an extended scale

“…Thus, the acidity of cyclic ILs derives mainly from the cation aromaticity and also from the number of atoms forming the ring. This higher acidity provided by aromatic and 5-member ring cations correlates well with their ability to be solvated by water by enhanced hydrogen-bonding, either in binary 58 or ternary aqueous systems. 59 On the other hand, for the non- , as experimentally observed.…”

Aging Effects on the Properties of Imidazolium-, Quaternary Ammonium-, Pyridinium-, and Pyrrolidinium-Based Ionic Liquids Used in Fuel and Energy Production

“…Thus, ILs containing free halides or anions that could decompose to form free halides, such as BF 4 -or PF 6 -, were avoided 8,21 . The anions studied, bis(trifluoromethylsulfonyl)imide effects from water in the IL will be negligible [22][23] , but the effects of oxygen on degradation of the IL could have significant impact 24 . Further studies will be completed to compare the effects of an oxygen free environment with these results.…”

Extraction of vanillin using ionic-liquid-based aqueous two-phase systems

“…For the cation improvement study, the chloride anion was kept, while combined with the following cations: [ [33], and thus decreasing the energy of cavity formation to accommodate a vanillin molecule. However, the increase of the alkyl side chain length leads to a decrease on the strength of the coulombic and polar interactions, while increasing the dispersive interactions, between the IL ions [15,34]. Therefore, these two contributions will act in different directions leading to the observed maximum value on the partition coefficients at about [C 6 mim]Cl.…”

“…Indeed, few years ago, Gutowski et al [12] showed that aqueous solutions of imidazolium-based ILs can form ATPS under the addition of appropriate inorganic salts, such as K 3 PO 4 . Since then, the equilibrium properties of systems comprising ILs, for the development of specific extraction and isolation procedures, have been inves- tigated at our laboratory [9,[13][14][15][16][17][18][19]. It was already shown that ATPS formed by K 3 PO 4 and ILs are extremely advantageous for the partition of several biomolecules yielding larger partition coefficients than those conventionally obtained with polymer-inorganic salts or polymer-polymer ATPS [6,10,13].…”