Abstract
Recycling of metals, either rare earths for recycling magnets in windmills, lanthanides from electronics or actinides from nuclear "waste", into valuable material relies on ion specific separation, basis of the hydrometallurgy. All efficient methods known for separating ions are based on equilibria between complex fluids (concentrated acidic or basic and reverse micelles) in the Winsor II regime (w/o microemulsions). These reverse micelles have been up to now considered as spherical aggregates, even with typical volume fractions of up to 30 percent. In the diluted regime, w/o reverse micelles, that selectively up-take some ions, have been approximated as spheres containing 4 to 10 molecules of extractants, few water molecules, and complexed ions. However, it is known that in industrially relevant cases of liquid–liquid extraction (recycling of nuclear fuel, lanthanide refinement, nickel refining), extractant aggregates cannot be described as simple metal-ion complexes spherical on average, since the high conductivity observed in the oil phase proves a bicontinuity degree of the system.Realistic examples of nonionic w/o microemulsions have been simulated using the wavelet method developed by Arleth and Marčelja [1]. The thermodynamics of the interface is here determined by the Helfrich free energy [2] that depends on the mean (H), spontaneous (H0) and Gaussian (K) curvatures, and also the bending (κ) and Gaussian (κ’) elastic constants. While the spontaneous curvature (H0) corresponds to the preferred curvature of the unconstraint surfactant film, the bending and Gaussian elastic constants refer to the rigidity of the surfactant film in terms of energies. In our model, the free energy is minimized as a function of the bending and Gaussian elastic constants and the spontaneous curvature of the surfactant film. Therefore, this allows us to generate all possible microstructures thermalized of two immiscible liquids separated by a layer of surfactant, the surfactant having the desired rigidity thanks to the κ and κ’ values.Frustrated and unfrustrated bicontinuous microemulsions appear near instabilities related to the transition towards lyotropic liquid crystals. Microstructures can be distinguished via qualitative features on the scattering. Ternary phase diagrams are also calculated from the simulations.The influence of the bending κ and Gaussian κ’ elastic constants is observed on the microemulsion structures, their scattering properties, and the calculated ternary phase diagrams. Although our model points out a dependence of the microstructures and the scattering properties with the bending elastic constant κ, no influence is observed on the ternary phase diagram. On the other hand, the Gaussian elastic constant κ’ has only an influence on the ternary phase diagram.However, it is well known that efficient surfactants used for such processes are mainly ionic. Therefore, it is crucial to develop a more realistic model taking into account (i) the ionic character of the surfactant, and (ii) the presence of cations in solutions. Up to now, no predictive model of the free energies of transfer of ions between phases exists. Taking into account, in a same model, both the metal complexation and the colloidal terms [3], will provide predictive modelling of ion separation. This includes lanthanides as well as cesium and actinides. Thus, the next step of this study will be the accounting of the presence of cations in the free energy term derived from the Helfrich formalism.References[1] L. Arleth, S. Marčelja and Th. Zemb, “J. Chem. Phys.”, Vol. 115, 2001, pp. 3923 – 3936.[2] W. Helfrich, “Z. Naturforsch. C”, Vol. 28, 1973, pp. 693 – 703.[3] J.-F. Dufrêche and Th. Zemb, “J. Phys. Chem. Lett.”, 2011, submitted.