Multiscale modelling of solvent phases for solvent extraction – MULTISEPAR

The strategy combines mainly three scales of description implemented for the extraction of lanthanides by malonamides.
- First, molecular dynamics simulations which alone can deal with the elementary act and atomic phenomena such as metal complexation.
- Secondly, Brownian dynamics modelling where the metal and its environment are considered in a similar way to reverse micelles, charged or not.
- Finally, a new approach by Gaussian random fields and the theory of microemulsion. The organic phases are thus represented as a kind of water/oil mixture made thermodynamically stable by the presence of extractants.
The important point is the link between the scales, the molecular part allowing not only to parameterize but also to justify the other scales of description. This work will also be validated thanks to the experimental studies recently carried out on these reference systems and which allow a direct comparison of many quantities.

Molecular simulations carried out on the organic phases studied speciation using a graph theory approach. We were able to determine the chemistry of the aggregates present and a thermodynamic model was proposed. The constants showed that the activity was not the predominant phenomenon. The structuring is more important in the presence of salt and indicates a micellar behavior typical of microemulsions. The calculation of the curvature parameters could thus be performed for the microemulsion model.
A Brownian dynamics code that calculates the structure of the solutions from a model potential has been developed. The equilibrium thermodynamic properties could thus be evaluated. In particular, the osmotic coefficient which allows the comparison with experimental measurements of vapor pressure has been modeled. A method is being developed to predict the transport properties. By generalizing Born's approach for organic phases, we can thus describe the conductivity of the suspension by modeling both the diffusion of charged species and the ion exchange between aggregates.
Different models of microemulsions have been compared with experiments giving the structure of the solutions. We were also able to propose a first version of a microemulsion model that recovers known behaviours: appearance of a critical aggregation concentration, extraction in the form of a Langmuir isotherm in connection with the experiments.

The continuation of this project will particularly deepen the mesoscopic Brownian and microemulsion modelling in coherence with the initial planning. The molecular simulation will calculate by umbrella sampling the interactions between aggregates in order to obtain the best possible Brownian model in continuous solvent. This one will be able to determine the importance of the deviations from the ideality by a more realistic model. It will carry out in parallel the determination of the transport properties. Finally, a code based on random Gaussian fields and a microemulsion model will be developed to predict and develop the extraction of rare earths, in connection with molecular simulations and experiments.

5 published articles

[1]
«Liquid/liquid interface in periodic boundary condition«
M. Vatin, M. Duvail, Ph. Guilbaud, J.-F. Dufrêche
Phys. Chem. Chem. Phys. 23, 1178 – 1187 (2021)

[2]
«Thermodynamics of Malonamide Aggregation Deduced from Molecular Dynamics Simulations«
M. Vatin, M. Duvail, Ph. Guilbaud, J.-F. Dufrêche
J. Phys. Chem. B 125 (13), 3409 – 3418 (2021)

[3]
«Bending: from thin interfaces to molecular films in microemulsions«
J.-F. Dufrêche and Th. Zemb
Curr. Opin. Colloid Interface Sci. 49, 133 – 147 (2020)

[4]
«How Acidity Rules Synergism and Antagonism in Liquid–Liquid Extraction by Lipophilic Extractants — Part I: Determination of Nanostructures and Free Energies of Transfer«
S. Dourdain, M. Špadina, J. Rey, K. Bohinc, S. Pellet-Rostaing, Jean-François Dufrêche, T. Zemb
Solv. Ext. Ion Exch. ASAP (2021). DOI: 10.1080/07366299.2021.1899606

[5]
«How acidity rules synergism and antagonism in liquid–liquid extraction by lipophilic extractants — Part II: application of the ienaic modelling«
M. Špadina, S. Dourdain, J. Rey, K. Bohinc, S. Pellet-Rostaing, Jean-François Dufrêche, T. Zemb
Solv. Ext. Ion Exch. ASAP (2021). DOI: 10.1080/07366299.2021.1899614

The ANR MULTISEPAR project aims to model rare earth (lanthanide) separation processes used in hydrometallurgy and for recycling. More specifically, it will focus on the solvent phase of liquid-liquid extraction processes, the modelling of which being currently at a very early stage. The multi-scale approach will be based on three complementary levels of description. First, at the atomic level, molecular dynamics simulations will calculate the structure and speciation in these solvent phases. The molecular interaction potential that we will use here has recently been validated from ab initio simulations by comparison with spectroscopy experiments. An umbrella sampling methodology will calculate the forces between solutes. The purpose of this step will be both the determination of the physico-chemical ingredients required for solvent phase modelling and also the calculation of the mesoscopic properties used by the other more macroscopic description scales. In another level of descriptions, mesoscopic Brownian simulations will be performed to calculate the effects at greater distance. Based on molecular simulation data (effective interaction potential and mobilities), either Brownian dynamics simulations or Multiparticle Collision dynamics simulations will be used to access the largest scales. The solutes activity coefficients and the stability of the solvent phase can thus be calculated. At the dynamic level, solute transport (diffusion and electrical conductivity) as well as viscosity will also be studied because they drive many industrial processes. As both experiments and molecular simulations show that in some cases solutes decompose poorly into independent particles but rather form a continuous network of hydrophilic parts in the solvent phase, we will also propose a second mesoscopic model to describe these solvent phases, this time based on a microemulsion model. Using a Gaussian random field methodology, we will propose a code representing the Gibbs energy of the solvent phase, which will make it possible to predict both the structure and the extraction properties. The fundamental quantities of this level of description will be here the properties of curvature (spontaneous curvature and rigidity) due to the extractants which will be deduced from the molecular simulations. The study of extraction as a function of the concentrations in the aqueous phase and of the extractant concentration will validate this methodology. We believe that this calculation will be a success if this microemulsion model can represent extraction equilibria with a much smaller set of parameters than traditional models based on multiple chemical equilibria between species. Thus, this project on lanthanide extraction could lead to a model that will be implemented in chemical engineering codes describing this process. We hope that through this multiscale project and the extensive use of numerical computing resources a new image of extraction mechanism will emerge from molecular modelling and that it will be able to bridge the gap to the macroscopic descriptions of this method of separation chemistry.