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.
The synthesis, structure and electrochemical properties of several silver‐containing liquid metal salts have been investigated. These ionic liquids comprise of a silver‐containing cation with a silver centre coordinated by two or more alkylamine ligands and a bis(trifluoromethylsulfonyl)imide (Tf2N) anion. With the monodentate amines tert‐butylamine (tBuAm), iso‐butylamine (iso‐BuAm), sec‐butylamine (sec‐BuAm), 2‐ethylhexylamine (2‐EtHexAm), di(2‐ethylhexyl)amine and piperidine (pip), compounds with the formula [Ag(L)2][Tf2N] are formed, several of which are room‐temperature ionic liquids and all melt at or below 100 °C. In the case of [Ag(L)2][Tf2N] (L=tBuAm, iso‐BuAm and pip), single‐crystal X‐ray diffraction shows that, in the solid state, the silver centres are two‐fold coordinated by two amine ligands and the anion and cation exist as separated ion pairs. With ethylenediamine (en), two different compounds were formed, depending on the silver‐to‐ligand ratio and their structures were elucidated by X‐ray diffraction. [Ag(en)][Tf2N] has a very high melting point and is polymeric in the solid state, with en ligands coordinated to different silver(I) centres, creating one‐dimensional chains. [Ag(en)2][Tf2N], on the other hand, is a room‐temperature ionic liquid, with four‐coordinate silver(I) centres, but is actually polymeric in the solid state. The electrodeposition behaviour of [Ag(2‐EtHexAm)2][Tf2N] and [Ag(en)2][Tf2N] was investigated both at room temperature and at 90 °C and it was possible to achieve very high current densities in unstirred solutions and to electrodeposit closed, crack‐free, silver coatings. A crystal of [Ag2(en)Cl2] was obtained from a solution of [Ag(en)][Tf2N] in deuterochloroform and its structure is described.
Ionic liquids with an ether-functionalised cation and the bis(2-ethylhexyl)phosphate anion show thermomorphic behaviour in water, with a lower critical solution temperature. These ionic liquids are useful for homogeneous liquid-liquid extraction of first-row (3d) transition metals.
The use of ionic liquids (ILs) as solvents for extraction of metals is a promising development in separation science and technology; yet, the viscosities of ionic liquids (ILs) can be so high that long reaction times are required to reach the equilibrium state. An aqueous biphasic system (ABS) consisting of the nonfluorinated carboxyl-functionalized phosphonium IL [P444C1COOH]Cl and a 16 wt % NaCl solution is described. The IL-rich phase of the aqueous biphasic system has a very low viscosity, in comparison to the pure IL [P444C1COOH]Cl. This system has excellent extraction properties for scandium. Different extraction parameters were investigated, including contact time and metal loading. The influence of the pH on the solubility of the IL cation in the water-rich phase was determined via quantitative 1H NMR. The stripping of scandium with oxalic acid from the IL phase was also investigated. A plausible extraction mechanism is proposed where three IL cations are deprotonated to form zwitterionic compounds that can coordinate scandium(III) ions.
The synthesis, structural, thermal and electrochemical properties of fluorine-free silver-containing ionic liquids are presented. The ionic liquid cations are formed by a silver(i) ion surrounded by two 1-alkylimidazole ligands, with the counter anions being nitrate ions. Depending on the alkyl chain length, the complexes were found to be liquids at room temperature or melting slightly above this. For the solid compounds it was possible to elucidate the structure by single crystal X-ray analysis. The ionic liquids are electroactive, have good mass transport properties and can be used for the electrodeposition of silver at high current densities. The thermal properties and stability of these compounds were tested by differential scanning calorimetry and thermogravimetric analysis. The viscosity of the ionic liquids follows a Vogel-Tamman-Fulcher relationship as a function of temperature. The electrochemical properties of the complexes were tested by cyclic voltammetry and the resulting electrodeposits were examined using scanning electron microscopy and atomic force microscopy.
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