Synergism as a Phenomenon in Solvent Extraction of 4f-Elements with Calixarenes

The main objective of the present review is to provide an overview on the synergistic solvent extraction of lanthanoids, in particularly on the use of β-diketone/calixarene i.e. acidic/neutral mixture, as well as the analysis of data hitherto reported in this scientific topic. Based on the previously presented results, the extraction efficiency has been careful examined and discussed mainly in terms of the corresponding equilibrium constants among other outlined extraction parameters. Furthermore, the evaluation of the chelating ligand structures and pKa values are emphasized. Major conclusions on the role of calixarene's ligating groups towards the mechanism, the improved extraction enhancement and selectivity are additionally provided. Consequently, it is very important that the obtained remarkable architecture of the formed complexes with very high coordination numbers (10/12) including a few cyclic molecules is in accordance with the well-established principles of coordination 4f-ion chemistry. Finally, a critical analysis of the lanthanoid intra-group selectivity (light/middle, heavy/middle and light/heavy 4f-ion pairs) versus the extraction performance of the acidic/neutral combinations and, on the other hand, the nature of the water-immiscible molecular or ionic diluent used have also been carried out and are here presented and critically addressed.