Microemulsion as Model to Predict Free Energy of Transfer of Electrolyte in Solvent Extraction

ABSTRACT

We consider here the extraction of metals in the form of salts transferred from an aqueous to a solvent phase. Extraction is triggered by complexation and quenched by the associated necessary reorganization of the structured solvent phase. The extraction of ions changes the relative fraction of extractant molecules that is not part of the highly curved surfactant monolayer and is dispersed molecularly in the oil, and also the polar volume fraction including co-extracted water. The free energy and corresponding microstructures of the water-poor microemulsions are modelled in the frame of the Gaussian random fields (GRF) model. The curvature frustration energy significantly contributes to the free energy of extraction. A typical example of predicted isotherm using the GRF model is compared to the classically considered supramolecular complex formation, together with a minimal Langmuir model and an explicit monomer-to-film equilibrium of amphiphilic extractant. The corresponding small-angle scattering spectra and morphology changes are shown. One implication is that selectivity between a hydrated and a non-hydrated species is concentration dependent and cannot be considered as a constant as a function of the extractant concentration.

KEYWORDS:

• Microemulsion
• solvent extraction
• mesoscopic modelling
• thermodynamics

Acknowledgments

The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement no. [320915] REE-CYCLE: Rare Earth Element reCYCling with Low harmful Emissions. The authors also thank ANR MULTISEPAR (ANR-18-CE29-0010) for financial support. They are very grateful to B. Siboulet, M. Bley, and M. Špadina for fruitful discussions.

Graphical TOC Entry

Free energy of transfer of electrolytes in solvent extraction has been predicted by a water-poor microemulsion model. We demonstrated that the selectivity between hydrated and non-hydrated species is concentration dependent and cannot be considered as a constant.