How Acidity Rules Synergism and Antagonism in Liquid–Liquid Extraction by Lipophilic Extractants—Part I: Determination of Nanostructures and Free Energies of Transfer

ABSTRACT

Efficient formulations used in hydrometallurgy are often based on synergistic action of two oil-soluble extractants at an optimized mole fraction. In this series of two papers, we describe the N,N’-dimethyl-N,N’-dioctylhexylethoxymalonamide (DMDOHEMA)/di(2-ethylexyl) phosphoric acid (HDEHP) system, and how expression of the synergistic effect varies with acidity, using the three common representations: distribution ratios, yields, and free energies of transfer. The experimental results are detailed in this first part (Part I) with the so-called supramolecular approach, which considers the reverse micelle-like aggregates formed by the lipophilic extractant molecules. It shows that no specific aggregation state is observed at the maximum of synergism and antagonism. Besides, the critical aggregation concentration that characterizes the micellization free energy appears to follow some synergistic trends but cannot explain how acid tunes synergism to antagonism of this mixed system. Based on these results, Part II proposes a theoretical model based on the ienaic approach to interpret acid effect on synergism and antagonism.

KEYWORDS:

• extractant mixtures
• synergy
• microstructures
• the free energy of transfer
• self-assembly
• ienaics

Acknowledgments

The authors acknowledge 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. Research Agency for support through grant BIFR/CEA/16-18-002 and the Slovenian Research Agency for support through program P3-0388 are acknowledged. French ANR Agence Nationale de la Recherche grant ANR-18-CE29-0010 is acknowledged. The support of Laboratory Chemisyst ANR 11-01-05 is acknowledged.

Supplementary material

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