Modulating the solubilities of ionic liquid components in aqueous-ionic liquid biphasic systems: A Q-NMR investigation
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2015-06-01
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Abstract Aqueous-ionic liquid (A-IL) biphasic systems have been examined in terms of deuterated water, acid, and IL cation and anion mutual solubilities in the upper (water-rich, in mole fraction) and lower phase of aqueous/IL biphasic systems at ambient temperature. The biphasic mixtures were composed of deuterated acids of various concentrations (mainly DCl, DNO3, and DClO4 from 10-2 to 10-4 M) and five ionic liquids of the imidazolium family with a hydrophobic anion (CF3SO2)2N-, that is, [C1Cnim][Tf2N], (n=2, 4, 6, 8 and 10). The analytical techniques applied were 1H NMR, 19F NMR, Karl-Fischer titration, pH potentiometry for IL cations and anions, and water and acid determination. The effects of the ionic strength (μ=0.1 M NaCl and NaNO3 as well as μ=0.1 M, 0.2 M and 0.4 M NaClO4, according to the investigated acid), the nature of the IL cation, and the nature of the mineral acid on the solubilities of the (D2O, D+, Tf2N-, C1Cnim+) entities in the lower or upper phases were determined. The addition of sodium perchlorate was found to enhance the Tf2N- solubility while inhibiting the solubility of the ionic liquid cation. Differences in IL cation and anion solubilities of up to 42 mM were evidenced. The consequences for the characterization of the aqueous biphasic system, the solvent extraction process of the metal ions, and the ecological impact of the ILs are discussed. The addition of sodium perchlorate enhances the solubility of ionic liquid (IL) anions while decreasing the solubility of the cations in a slightly acidic aqueous phase.