The possibility of cadmium extraction to the ionic liquid 1-hexyl-3-methylimidazolium hexafluorophosphate in the presence of hydrochloric acid: a molecular dynamics study of the water–IL interface

Abstract

Hydrophobic ionic liquids (ILs) can form biphasic systems with aqueous media and are appropriate candidates for liquid–liquid extraction (LLE). In this study, a biphasic system composed of water and the hydrophobic IL, 1-hexyl-3-methylimidazolium hexafluorophosphate, [HMIM][PF6] was studied using molecular dynamics (MD) simulation to understand the molecular-level distribution of the heavy metal cadmium ions, Cd2+, in the water–IL biphasic system in the presence of hydrochloric acid. The experimentally observed positive effect of adding chloride to the aqueous phase on the metal extraction was studied at the molecular scale. Particularly, the effect of hydrochloric acid addition on the solubility of the IL cations and anions in the water was investigated. It was found that with adding hydrochloric acid (1 M) to the water phase, the IL cation solubility in water decreased; however, the IL anion solubility almost did not change. This can affect the extraction process of the metal ions. Moreover, it was found that during the [CdCl4]2− migration to the IL phase, a gradual breaking of the hydrogen bonds occurs between the complex and the water molecules at the interface. Therefore, quantum mechanics (QM) calculations were performed to explain the interaction energies of the cadmium complex with water and the IL. The calculated interaction energy of the ternary complex of IL–[CdCl4]2−–H2O was found to be greater than that of the binary complexes of IL–[CdCl4]2− and H2O–[CdCl4]2−. The results obtained in this work give some insights into the behaviour of the IL-based extraction systems in contact with aqueous solutions containing salts or mineral acids.