Overview of the Effect of Salts on Biphasic Ionic Liquid/Water Solvent Extraction Systems: Anion Exchange, Mutual Solubility, and Thermomorphic Properties

Abstract

Hydrophobic (water-immiscible) ionic liquids (ILs) are frequently used as organic phase in solvent extraction studies. These biphasic IL/water extraction systems often also contain metal salts or mineral acids, which can significantly affect the IL trough (un)wanted anion exchange and changes in the solubility of IL in the aqueous phase. In the case of thermomorphic systems, variations in the cloud point temperature are also observed. All these effects have important repercussions on the choice of IL, suitable for a certain extraction system. In this paper, a complete overview of the implications of metal salts on biphasic IL/water systems is given. Using the Hofmeister series as a starting point, a range of intuitive prediction models are introduced, supported by experimental evidence for several hydrophobic ILs, relevant to solvent extraction. Particular emphasis is placed on the IL betainium bis(trifluoromethylsulfonyl)imide [Hbet][Tf2N]. The aim of this work is to provide a comprehensive interpretation of the observed effects of metal salts, so that it can be used to predict the effect on any given biphasic IL/water system instead of relying on case-by-case reports. These prediction tools for the impact of metal salts can be useful to optimize IL synthesis procedures, extraction systems and thermomorphic properties. Some new insights are also provided for the rational design of ILs with UCST or LCST behavior based on the choice of IL anion.

Selective leaching of rare earth elements from bauxite residue (red mud), using a functionalized hydrophobic ionic liquid

“…Ions that promote the dissolution of a hydrophobic substance in an aqueous solution are called salting-in ions, while ions that exclude a hydrophobic substance from an aqueous solution are called salting-out ions. It is reported (Dupont et al, 2015) that HbetTf2N solubility in water was found to be 14 wt% yet it is greatly affected in the presence of ions. The addition of sulfate anions that promote salting out (lowering IL solubility) of the IL, is an effective way to minimize IL losses (Dupont et al, 2015;Freire et al, 2009) and can be applied also in contaminated IL aqueous streams for IL recovery (Neves et al, 2012).…”

Experimental and theoretical assessing the salts effect on the equilibrium of water + acetone + HMIMPF6 ionic liquid system

“…This reaction, in turn, increases the IL solubility in aqueous phase and further leads the acetone extraction to diminish [8,27,28].…”

“…It has to be noted that results presented in these figures are based on salt-free composition data. Dupont et al [8] proposed a ''liquid-precipitate" model to rationalize the effect of inorganic salts on the solubility of any hydrophobic IL. Based on this model, charge-neutralizing ion pairs can be formed between the IL ions and salt counterions.…”

“…Currently, the LLE of ternary systems, containing ILs and organic molecules [2,[5][6][7], as well as the electrolytes in their role the shifting the equilibrium tie-lines is being studied [2,8]. LLE is the result of molecular interactions, which can significantly change due to presence of salts, which causes ionic forces, affecting the thermodynamic equilibrium [9].…”

“…This matter can be explained by hydrating of salt ions where some of the water molecules surrounding electrolyte molecules become unavailable; therefore, the solute tendency is favored to transfer more from aqueous phase to organic phase [8][9][10][11].…”