Determination of pK aIL Values of Three Chelating Extractants in ILs: Consequences for the Extraction of 4f Elements

Abstract

Aqueous–ionic liquid biphasic systems have been examined in terms of water and acid solubilities in the IL-rich phase at ambient temperature. The biphasic mixtures were comprised of acids of various concentrations (H+; Cl−, H+; $$ {\text{NO}}_{3}^{ - } $$ , H+; $$ {\text{ClO}}_{4}^{ - } $$ , mainly from 1 × 10−2 to 1 × 10−4 mol·dm−3) and four ionic liquids of the imidazolium family [C1C n im][Tf2N] (n = 4, 6, 8 and 10). The effects of ionic medium (μ = 0.1 mol·dm−3, by use of Na+; Cl−, Na+; $$ {\text{NO}}_{3}^{ - } $$ or Na+; $$ {\text{ClO}}_{4}^{ - } $$ , according to the acid investigated), the nature of the IL cation as well as the nature of the acid on the mutual solubilities of (H2O, H+, [C1C n im]+ and [Tf2N]−) entities were determined. Then, three chelating compounds (HL), which belong to the β-diketone family (thenoyltrifluoroacetone (HTTA), 4-benzoyl-3-methyl-1-phenyl-2-pyrazolin-5-one (HP) and 4-benzoyl-3-phenyl-5-isoxazolone (HPBI)), were added to [C1C4im][Tf2N] and subsequent determination of the H+ distribution between the two phases allowed the determination of their dissociation constants (pK aIL) in the water-saturated ionic liquid phase. A very strong effect of the IL cation on the HTTA pK aIL value was observed from n = 4 to n = 10. The influence of this phenomenon on the lanthanide extraction process is discussed.