Vol.:(0123456789) 1 3 Theoretical Chemistry Accounts (2019) 138:99 https://doi.org/10.1007/s00214-019-2489-z REGULAR ARTICLE The possibility of cadmium extraction to the ionic liquid 1‑hexyl‑3‑methylimidazolium hexafuorophosphate in the presence of hydrochloric acid: a molecular dynamics study of the water–IL interface Zeinab Pouramini 1  · Ali Mohebbi 1  · Mohammad H. Kowsari 2,3 Received: 4 March 2019 / Accepted: 31 July 2019 © Springer-Verlag GmbH Germany, part of Springer Nature 2019 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-methylim- idazolium hexafuorophosphate, [HMIM][PF 6 ] was studied using molecular dynamics (MD) simulation to understand the molecular-level distribution of the heavy metal cadmium ions, Cd 2+ , in the water–IL biphasic system in the presence of hydrochloric acid. The experimentally observed positive efect of adding chloride to the aqueous phase on the metal extrac- tion was studied at the molecular scale. Particularly, the efect 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 afect the extraction process of the metal ions. Moreover, it was found that during the [CdCl 4 ] 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–[CdCl 4 ] 2− –H 2 O was found to be greater than that of the binary complexes of IL–[CdCl 4 ] 2− and H 2 O–[CdCl 4 ] 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. Keywords Ionic liquid · Liquid–liquid interface · Cadmium ions · Molecular dynamics simulation · Electrostatic potential profle · Density profle 1 Introduction Liquid–liquid extraction (LLE) or solvent extraction is one of the most common separation methods for separating sol- utes from an aqueous phase by an organic phase. During the past decades, considerable studies have been carried out on the application of ionic liquids (ILs) or deep eutectic sol- vents (DESs) as their versatile alternatives in LLE processes. ILs are neoteric solvents consisting of organic cations and organic or inorganic anions, with special characteristics including unique dissolution capacity, low vapour pressure, high fash point, and good (electro) chemical stability. The physical properties of DESs are similar to ILs. Therefore, both ILs and DESs have been recently employed in LLE as appropriate replacements for traditional molecular solvents [1–10]. From an environmental point of view, ILs can be Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00214-019-2489-z) contains supplementary material, which is available to authorized users. * Ali Mohebbi amohebbi@uk.ac.ir; amohebbi2002@yahoo.com 1 Department of Chemical Engineering, Faculty of Engineering, Shahid Bahonar University of Kerman, Kerman, Iran 2 Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan 45137-66731, Iran 3 Center for Research in Climate Change and Global Warming (CRCC), Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan 45137-66731, Iran