Docusate Ionic Liquids: Effect of Cation on Water Solubility and Solvent Extraction Behavior

Ionic liquids with the docusate (dioctyl sulfosuccinate or DOSS) anion are described. Docusate is incorporated into several phosphonium ionic liquids, including tetrabutylphosphonium and phosphonium cations functionalized with an ester, carboxylic acid, or ethylene glycol group. All synthesized ionic liquids are immiscible with water except the compound with the ethylene glycol moiety in the cation, [P444E3][DOSS]. This ionic liquid exhibits a lower critical solution temperature phase behavior upon mixing with water, to yield a homogeneous phase at temperatures below 19 °C, resulting in fast kinetics for homogeneous liquid–liquid extraction. The metal extraction capabilities for both divalent and trivalent metal ions are compared for this thermomorphic ionic liquid and the other hydrophobic docusate ionic liquids in an initial screening test. [P444E3][DOSS] is selected to perform separations on the Sm/Co and La/Ni pairs, because of their relevance in the recycling of samarium cobalt magnets and nickel metal hydride batteries. The extraction mechanism is studied, and stripping of the metals is investigated.