| The physicochemical nature of the metal-extractant species that form during the liquid-liquid extraction of metals ions in representative alkanic and aromatic solvents has been investigated by molecular modelling. In particular, it was determined how the metal complexes are organized, the type of forces holding them together, and the orientations and locations of water molecules in macromolecular metal-extractant species. In addition, the effect of extractant types (HDEHP (bis(2-ethylhexyl) phosphoric acid), HOEHEHP ((2-ethylhexylphosphonic acid mono-2-ethylhexyl ester), HDEHPI (bis(2-ethylhexyl) phosphinic acid), HDHP (bis(hexyl) phosphoric acid) and HDEHDTP (bis(2-ethylhexyl) dithiophosphoric acid)) on the nature of nickel-extractant species have been examined. The molecular modelling findings were correlated with the experimental results of other investigations.;When HDEHP molecules interact with each other in n-heptane, the HDEHP molecules form dimeric species. On the other hand, when the metal salt is present in n-heptane at high concentration, the metal-extractant complexes assemble into open (nickel-HDEHP), semi-open (nickel-HOEHEHP), semi-enclosed (nickel-HDEHPI), expanded (nickel-HDHP), and semi-expanded (nickel-HDEHDTP) channel, rodlike reverse micelles. Cobalt-HDEHP species form rodlike reverse miceles, but does not show the formation of any open water channel. Sodium-HDEHP species exist as a deformed-oblate ellipsoidal reverse micelles. The addition of water tends to break up the NaDEHP reverse micelles into smaller micelles. At "dilute" conditions, the nickel-HDEHP species exist as monomers and dimers. The main forces holding the metal-extractant complexes together are van der Waals and electrostatic forces except in the case of nickel-HDEHP, nickel-HOEHEHP and nickel-HDHP species, where an additional force (hydrogen bonds) is present. This type of force enhances the interactions between the complexes and provides additional stability that maintains a water channel between the hydrocarbon tails of the complexes where water molecules are located.;The molecular modelling findings have provided more detailed information about the microstructure of the metal-extractant species in solvent extraction systems. Also, more insight was obtained on structure-extractability-selectivity correlations. Molecular modelling has illustrated that cobalt-HDEHP reverse micelles are more stable than nickel-HDEHP reverse micelles, which can explain the selectivity of cobalt over nickel. In addition, molecular modelling has shown that the open water-channel model proposed by Neuman and co-workers is valid for nickel-HDEHP reverse micelles and, with some modifications, for other nickel-extractant reverse micelles as well. |
