Liquid-Liquid Extraction of the Eu(III) Cation by BTP Ligands into Ionic Liquids: Interfacial Features and Extraction Mechanisms Investigated by MD Simulations

What happens at the ionic-liquid (IL)/water interface when the Eu³⁺ cation is complexed and extracted by bis(dimethyltriazinyl) pyridine "BTP" ligands has been investigated by molecular dynamics and potential of mean force simulations on the interface crossing by key species: neutral BTP, its protonated BTPH⁺ form, Eu³⁺, and the Eu(BTP)₃³⁺ complex. At both the [BMI][Tf₂N]/water and [OMI][Tf₂N]/water interfaces, neither BTP nor Eu(BTP)₃³⁺ are found to adsorb. The distribution of Eu(BTP)₂³⁺ and Eu(BTP)³⁺ precursors of Eu(BTP)₃³⁺, and of their nitrate adducts, implies the occurrence of a stepwise complexation process in the interfacial domain, however. The analysis of the ionic content of the bulk phases and of their interface before and after extraction highlights the role of charge buffering by interfacial IL cations and anions, by different amounts depending on the IL. Comparison of ILs with octanol as the oil phase reveals striking differences regarding the extraction efficiency, the affinity of Eu(BTP)₃³⁺ for the interface, the effects of added nitric acid and of counterions (NO₃^- vs Tf₂N^-), charge neutralization mechanisms, and the extent of "oil" heterogeneity. Extraction into octanol is suggested to proceed via adsorption at the surface of water pools, nanoemulsions, or droplets, with marked counterion effects.

Potential dependence of the ionic structure at the ionic liquid/water interface studied using MD simulation

The structure at the electrochemical liquid/liquid interface between water (W) and trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide, a hydrophobic ionic liquid (IL), was studied using molecular dynamics (MD) simulation in which the interfacial potential difference was controlled. On the IL side of the IL/W interface, ionic multilayers were found in the number density distribution of IL ions whereas monolayer-thick charge accumulation was found in the charge density distribution. This suggests that the potential screening is completed within the first ionic layer and the effect of overlayers on the potential is marginal. The W side of the interface showed the diffuse electric double layer as expected, and unexpectedly unveiled a density depletion layer, indicating that the IL surface is hydrophobic enough to be repelled by water. The IL ions in the first ionic layer showed anisotropic orientation even at the potential of zero charge, in which the polar moieties were oriented to the W side and the non-polar moieties preferred parallel orientation to the interface. When an electric field is applied across the interface so that the IL ions are more accumulated, the non-polar moieties changed the parallel preference to more oriented to the IL side due to the dipolar nature of the IL ions. The ionic orientations at the IL/W interface were compared with those at other two IL interfaces, the vacuum and graphene interfaces of the IL. The parallel preference of the non-polar moieties was similar at the IL/graphene interface but different from the perpendicular orientation toward the vacuum side at the IL/vacuum interface. The comparison suggests that water behaves like a wall that repels IL ions like a solid electrode.

Solvent extraction of rare earths elements from nitrate media in DMDOHEMA/ionic liquid systems: performance and mechanism studies

Extraction of La(iii), Eu(iii) and Fe(iii) was compared in n-dodecane and two ionic liquids (ILs) (1-ethyl-1-butylpiperidinium bis (trifluoromethylsulfonyl)imide [EBPip⁺] [NTf₂⁻] and 1-ethyl-1-octylpiperidinium bis (trifluoromethylsulfonyl)imide [EOPip⁺] [NTf₂⁻]). Using the extractant N,N′-dimethyl-N,N′-dioctylhexylethoxymalonamide (DMDOHEMA), the effect of pH was investigated in detail to recover extraction mechanisms. The use of ILs as the organic solvent instead of n-dodecane, greatly enhances extraction efficiency, and an ionic liquid with a shorter alkyl chain [EBPip⁺] [NTf₂⁻] provides higher extraction than [EOPip⁺] [NTf₂⁻]. The mechanistic study points out that for low nitric acid concentrations ([HNO₃] ≤ 0.01 M), metal is extracted via a cation of the ionic liquids, while for higher nitric acid concentrations ([HNO₃] ≥ 1.0 M), extraction occurs through pure solvation mechanism of DMDOHEMA as in conventional diluents. This latter case is of high interest for applications, as higher extraction can be obtained without any loss of ILs by ion exchange mechanisms.

Extraction of La(iii), Eu(iii) and Fe(iii) was compared in n-dodecane and in two ionic liquids (ILs) [EBPip⁺] [NTf₂⁻] and [EOPip⁺] [NTf₂⁻]. Extraction mechanisms have been investigated as a function of pH.

Hydrophobic polymerized ionic liquids for trace metal solid phase extraction: thallium transfer from hydrochloric acid media

To our knowledge, there are a few articles on the application of polymerized ionic liquids for metal extraction from aqueous solutions.

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

The formation of aqueous biphasic systems (ABSs) based on aqueous ionic liquid (IL)/salt mixtures has been investigated via molecular dynamics simulations (with IL butyl-methyl-imidazolium triflate; salts NaCl, CsCl, SrCl₂, and EuCl₃). The analysis of ion distributions, solvation, and mutual interactions during the dynamics reveals the heterogeneity of all solutions due to ion segregation into mutually exclusive IL and salt domains, even in monophasic solutions ("ionic sociology"). Ion segregation and ABS formation are found to increase with (i) the salt content and (ii) the IL content, (iii) in the order Na⁺ < Sr²⁺ < Eu³⁺, and (iv) when the IL ion "polarity" is diminished, following experimental trends. The structuration of the solution is rationalized as a synergistic water transfer from the best donating ion pair (first hydration shell of hydrophobic moieties of IL ions) to the best accepting pair (M ⁿ⁺ and Cl^- ions, beyond their first shell). In ABSs, the IL- and salt-containing phases are linked by a well-defined "interface" that decreases in width when MCl _n becomes more hydrophilic and/or more concentrated. In the IL-rich phase of ABSs, the hydration of IL ions and their mutual interactions are shown to be similar to those displayed at aqueous interfaces.

Solvent extraction of Th(IV) from aqueous solution with methylimidazole in ionic liquid

An extraction of Th(IV) was performed using 1- methylimidazole (1-MIM) or 2-methylimidazole (2-MIM) as the extractant in imidazolium type ionic liquids (ILs) or n-pentanol. The extractability for Th4+ in ILs was by far higher than that obtained in n-pentanol. The extraction mechanism was determined by slope analysis and ESI-MS. The transfer of Th4+ with MIM into ILs proceeded through both a cationic exchange and a neutral solvation mechanism, whereas the partitioning of Th4+ with MIM into n-pentanol only underwent a neutral solvation mechanism. The thermodynamic parameters values (ΔH, ΔS and ΔG) for extraction of Th4+ with 1-MIM in IL were calculated and the results indicated the extraction reaction was spontaneous and went through endothermic process. Separation of Th4+ from the solution of lanthanides (III) and uranium was also carried out by 1-MIM in ILs and n-pentanol.

Thallium Transfer from Hydrochloric Acid Media into Pure Ionic Liquids

Pure hydrophobic ionic liquids are known to extract metallic species from aqueous solutions. In this work we have systematically investigated thallium (Tl) extraction from aqueous hydrochloric acid (HCl) solutions into six pure fluorinated ionic liquids, namely imidazolium- and pyrrolidinium-based ionic liquids with bis(trifluoromethanesulfonyl)imide and bis(fluorosulfonyl)-imide anions. The dependence of the Tl extraction efficiency on the structure and composition of the ionic liquid ions, metal oxidation state, and initial metal and aqueous acid concentrations have been studied. Tl concentrations were on the order of picomolar (analyzed using radioactive tracers) and millimolar (analyzed using inductively coupled plasma mass spectrometry). The extraction of the cationic thallium species Tl(+) is higher for ionic liquids with more hydrophilic cations, while for the TlX(z)(3-z) anionic species (where X = Cl(-) and/or Br(-)), the extraction efficiency is greater for ionic liquids with more hydrophobic cations. The highest distribution value of Tl(III) was approximately 2000. An improved mathematical model based on ion exchange and ion pair formation mechanisms has been developed to describe the coextraction of two different anionic species, and the relative contributions of each mechanism have been determined.

Mutual solubility of water and hydrophobic ionic liquids in the presence of hydrochloric acid

Possible mechanism for ionic liquid dissolution in HCl solution.

Ionic liquid-based uranium(vi) extraction with malonamide extractant: cation exchange vs. neutral extraction

The U(VI) extraction with DMDBMA is performed in the [C₄mim][Tf₂N] ionic liquid (IL) either as a cationic or a neutral species, without the help of IL ions. However, this does not prevent a noticeable pollution of the aqueous phase with the IL.

Extraction of actinides by tertiary amines in room temperature ionic liquids: evidence for anion exchange as a major process at high acidity and impact of acid nature

The extraction of U(vi) and Pu(iv) by tri-alkyl amines in room temperature ionic liquids, [C_nmim][Tf₂N] (where, n = 4, 6 or 8) follow anion exchange mechanism.

Comparing extraction, synergism and separation of lanthanoids using acidic and neutral compounds in chloroform and one ionic liquid: is the latter always “better”?

Five lanthanoid ions were extracted with 3-methyl-1-phenyl-4-(4-trifluoromethylbenzoyl)-pyrazol-5-one alone and in combination with partially substituted calix[4]arenes as synergistic agents using CHCl₃ and one IL.

Homogeneous liquid-liquid extraction of rare earths with the betaine-betainium bis(trifluoromethylsulfonyl)imide ionic liquid system

Several fundamental extraction parameters such as the kinetics and loading were studied for a new type of metal solvent extraction system with ionic liquids. The binary mixture of the ionic liquid betainium bis(trifluoromethylsulfonyl)imide and water shows thermomorphic behavior with an upper critical solution temperature (UCST), which can be used to avoid the slower mass transfer due to the generally higher viscosity of ionic liquids. A less viscous homogeneous phase and mixing on a molecular scale are obtained when the mixture is heated up above 55 °C. The influence of the temperature, the heating and cooling times, were studied for the extraction of neodymium(III) with betaine. A plausible and equal extraction mechanism is proposed in bis(trifluoromethylsulfonyl)imide, nitrate, and chloride media. After stripping of the metals from the ionic liquid phase, a higher recovery of the ionic liquid was obtained by salting-out of the ionic liquid fraction lost by dissolution in the aqueous phase. The change of the upper critical solution temperature by the addition of HCl or betaine was investigated. In addition, the viscosity was measured below and above the UCST as a function of the temperature.