Liquid–Liquid Equilibrium Data for [Mmim][DMP] + Salt Aqueous Biphasic Systems and Their Application for [Mmim][DMP] Recovery

This work focuses on a new aqueous biphasic system (ABS) composed of hydrophilic ionic liquids (1,3-dimethylimidazole phosphate dimethyl, [Mmim][DMP]), salts (K3PO4·3H2O or K2CO3), and water. Liquid–liquid equilibrium (LLE) data for the ABS have been determined experimentally at T = 298.15 K. The binodal curves were fitted to the Merchuk, Pirdashti, and Jayapal equations. The order K3PO4·3H2O > K2CO3 of recovery efficiency may be related to the Gibbs energy of hydration of the anions, the molar entropy of hydration of the anions, and the Hofmeister series for the strength of the kosmotropic salts, and the strength of [Mmim][DMP]–H2O interactions and salt–H2O interactions determined the phase ratio of the ABS. The effects of the anions structure on the phase equilibrium were investigated by Gaussian 09 software. It was found that the salting-out ability of different salts could be promoted by the increase of the anion valence of salt which can hydrate more water molecules, and the greater the binding energy of the salt with a single water molecule is, the easier it is for the salt to combine with water to form an ABS. To sum up, [Mmim][DMP] could be recovered from aqueous solution using a new ABS, and the recovery efficiency could reach 98.56%. This proposed method can be used as an effective way to recycle [Mmim][DMP] from aqueous solutions.