Solvent Extraction: Structure of the Liquid–Liquid Interface Containing a Diamide Ligand

Abstract

Knowledge of the (supra)molecular structure of an interface that contains amphiphilic ligand molecules is necessary for af ull understanding of ion transfer during solvent extraction. Even if molecular dynamics already yield some insight in the molecular configurations in solution, hardly any experimental data giving access to distributions of both extractant molecules and ions at the liquid-liquid interface exist. Here,t he combined application of X-raya nd neutron reflectivity measurements represents ak ey milestone in the deduction of the interfacial structure and potential with respect to two different lipophilic ligands.Indeed, we show for the first time that hardtrivalent cations can be repelled or attracted by the extractant-enriched interface according to the nature of the ligand.Interfaces between two immiscible liquids are ubiquitous in chemistry and biology and are the place of numerous physicochemical processes such as sorption, [1] solvation, [2] complexation. [3] Electron, [4] electrolyte, [2] molecule, [5] or colloid [6] transfer through the liquid interfaces are relevant for numerous domains like chemistry,e lectrochemistry driven solvent extraction, [7] nanoparticle synthesis, [8] or heterogeneous catalysis. [9] Discontinuities between both solvent properties yield different conformations of the solvent molecules at the interface with abreak of the bulk symmetries.This loss of entropy is often related to as pecific orientation of the molecules when they are polarizable and induces ar egion specificity for some reactions or transfer processes.T he surface tension between both phases depends on the molec-ular interactions in this region that are different to those in the respective bulk phases and each species added to one solvent, surface active or not, will modify these interactions and then the structure as well as the dynamics of the pure liquid interface.In solvent extraction using amphiphilic ligands or extractant molecules to separate ions,the ion partitioning between both immiscible phases is significantly enhanced by the extractant/ion complex formation on one side of the interface depending on the hydrophilicity or lipophilicity of the ligand. [10] Therole of complexation (metal-ligand interaction) and supramolecular aggregation of the ligands are strongly determinant in the distribution and separation factors. [11] If the former contribution is stronger and if the complex is soluble in the phase that contains the ligand, the solute transfer from one phase to the other will be fast but its recovery will be difficult. When the equilibrium between both contributions is within af ew k B T [12] then hydrodynamic or interfacial factors become preponderant and have to be taken into account. Ion extraction can be referred to as adiffusionlimited or areaction-limited process depending on the height of the energy barrier at the liquid-liquid (LL) interface. [13] Thepresent contribution shows that determination of the extractant and ion distributions across the water-oil interface can help the un...

Reflectometry Reveals Accumulation of Surfactant Impurities at Bare Oil/Water Interfaces

“…Scattering contrast arises from a difference between the SLDs of neighboring media. Reflectometry studies with a focus on the structure of bare interfaces between water and hydrophobic media are often carried out with two types of experimental configurations offering strong contrast between the two bulk media, as schematically illustrated in Figure 1A: (i) Hydrogenous or deuterated water (H2O or D2O, respectively) or mixtures thereof contacting air or gas [33,42,43], or (ii) H2O or D2O contacting hydrogenous hydrocarbon chains in the form of oil [5,34,35,36,37,38,39,40,41] or of oil-like molecules grafted to a solid surface [44]. Contrast-optimized NR studies dealing with hydrophobized solids [45,46,47] involved considerable structural complexity, making interpretation difficult in view of the limited Qz-range [46].…”

“…For XRR experiments, MilliQ water (H2O, MilliQ® Integral ultrapure water Type 1, specific resistance ≥ 18.2 normalMΩ·cm, organic content ≤ 5 ppb) from a standard laboratory source was used. The liquid/liquid cells for XRR and NR [5,39,40] were cleaned by washing with organic solvents (chloroform, acetone, ethanol) and by plasma cleaning. Subsequently, the lower part of the liquid pool was rendered hydrophobic via covalent functionalization with octadecyltrichlorosilane (OTS) by exposure to a freshly-prepared solution of OTS in anhydrous toluene at a concentration of 1 mM for 30 min.…”

“…A large percentage of the mass of cells and tissues is organized in the form of molecular layers, such as biomembranes, whose surfaces are in contact with their aqueous surroundings and are essential for their functioning. In technology, the properties of interfaces determine, for instance, the stability of colloidal systems [2] and the transport of ions or molecules between two liquid phases [5]. Bare interfaces between water and hydrophobic media like air, oil, polymeric materials, or carbon-based materials are of special relevance for fundamental research and applications involving emulsions and foams [6,7,8].…”

“…To this end, the SLD of hydrocarbon chains can be considered a feature common to all common surfactants. Reflectometry has been used routinely to quantify the interfacial adsorption of molecules to air/water interfaces [33] and more recently also to oil/water (O/W) interfaces [5,34,35,36,37,38,39,40,41]. But so far there have been no studies evidencing the accumulation of surfactant impurities at bare water/hydrophobic interfaces.…”

Synchrotron Scattering Methods for Nanomaterials and Soft Matter Research

“…The molecular mechanism of the ion transport from an aqueous to an organic phase taking place at an aqueous-organic interface has been studied by a combination of XRR with neutron reflectivity [180]. It was shown that hard trivalent cations can be repelled or attracted by the extractant-enriched interface according to the nature of the ligand [181]. The process of metal ion extraction starts with hydration of ions in the aqueous phase, which are then transformed at the aqueous-organic interface into supramolecular complexes in the form of an inverted bilayer that dissolves in the oil phase.…”