Strontium Nitrate Extraction to Ionic Liquids by a Crown Ether: A Molecular Dynamics Study of Aqueous Interfaces with C₄mim⁺- vs C₈mim⁺-Based Ionic Liquids

Abstract

In order to gain microscopic insights into the extraction mechanism of strontium cations by 18-crown-6 (18C6) to room temperature ionic liquids (ILs), we simulated by molecular dynamics (MD) strontium complexes in neat ionic liquids and at their interfaces with water. We compared two ILs, based on the PF(6)(-) anion and either 1-butyl-3-methylimidazolium (C(4)mim(+)) or 1-octyl-3-methylimidazolium (C(8)mim(+)) cations. Regarding the complexes, two states were considered: charged [Sr⊂18C6](2+) vs neutral [Sr⊂18C6,(NO(3))(2)], where the nitrates are either fully dissociated or coordinated to Sr. In "dry" or "humid" [C(4)mim][PF(6)] and in "dry" [C(8)mim][PF(6)] IL, the neutral complex is found to be the most stable one. In the binary IL/water solutions, the charged complexes mostly partition to the aqueous phase, whereas the neutral [Sr⊂18C6,(NO(3))(2)] complexes are more concentrated in the interfacial domain. The aqueous solutions in contact with the ionic liquids contain C(4)mim(+), but almost no C(8)mim(+) ions, supporting a classical extraction mechanism to [C(8)mim][PF(6)] and an ion exchange mechanism to [C(4)mim][PF(6)]. Furthermore, remarkable events occurred during the dynamics, where complexes were extracted to the IL phases. When compared to the interfacial landscapes obtained with the same solutes at a classical organic liquid (chloroform)/water interface, those with ILs allow us to better understand specific features of liquid-liquid extraction to ILs.

Formation of Aqueous Biphasic Systems with an Ionic Liquid Induced by Metallic Salts: Nanoscopic Views from Molecular Dynamics Simulations

“…Generally, for a given M x X y salt, ABS formation is enhanced by the size and hydrophobicity (or reduced hydrophilicity) of ionic components of the IL , and by the hydrophilic and salting-out character of the dissolved M x X y salt, − following the Hofmeister ion series whose interpretation in terms of thermodynamic and structural features remains controversial. ,, The hydrophilic/hydrophobic character , is often related to the Gibbs free energy of ion hydration and to concepts like structure breaking (“chaotropic”)/making (“cosmotropic”) features in water. ,− Recently, Shahriari et al correlated the amount of IL necessary to form an ABS and the entropy of hydration of the salt ions, drawing the implication of a dominant entropic driving force for ABS formation, as for hydrophobic ILs ,, or polymer-based ABSs, , presumably due to the formation of “water–ion complexes”. Insights into ion solvation and into their mutual interactions have been gained from NMR spectroscopy and simulations in aqueous salt solutions containing IL ions, ,,, but these may not suffice to fully describe the “organization” of the solution, the distribution of all ions, and finally the formation of ABSs in conditions when it occurs. Likewise, the nature of the aqueous–aqueous interface of ABSs has neither been characterized so far nor compared, for instance to aqueous interfaces with hydrophobic (water-immiscible) ILs. − …”

Evidence for Indirect Action of Ionizing Radiation in 18-Crown-6 Complexes with Halogenous Salts of Strontium: Simulation of Radiation-Induced Transformations in Ionic Liquid/Crown Ether Compositions

“…The crown ether molecules are partly bound in macrocyclic complexes with metal cations under agitation of the organic RTIL/CE phase with the aqueous metal-loaded phase. The composition of the macrocyclic complex has been demonstrated to vary strongly depending on hydrophobicity of RTIL cations. , In particular, an anionic component of the complex may originate from either metal salt dissolved in aqueous phase or RTIL itself. The latter case occurs, for example, upon metal cation extraction with RTIL through a cation exchange mechanism.…”

Selective separation of strontium by multitopic ion‐pair receptor: A DFT exploration

“…In addition to experimental studies, use of computational methods like density functional theory (DFT) or molecular dynamics (MD) are very popular to address and understand the selectivity of various ligands towards radionuclide of interest at molecular level. These are extremely useful to design new host molecules and predict their behavior prior to experimentation.…”