Marcoule Institute for Separative Chemistry
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The Mesoscopic Modelling and Theoretical Chemistry group offers:
Post doc
One post-doctoral position is available at LMCT.
Organic phases modelling for the liquid – liquid extraction: a molecular approach (12 months)
Contact: M. Duvail, J.-F. Dufręche |
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Separation processes performed for recycling of heavy metals commonly use liquid-liquid extraction for which ions are selectively transferred from an aqueous to an organised organic phase. Modelling such processes remains quite diffcult since many phenomena occur: complexation, solvation, electrostatic interactions, polarisation forces, etc. Furthermore, these interactions occur at different length sizes.
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Recently, experiments and simulations pointed out the presence of supramolecular aggregates, similar to reverse micelles, and having characteristic sizes of several nanometers. This suggests that the extraction process may be complex since it relies on the selective formation of such compounds. In each aggregate, a polar core is surrounded by a more or less stretched interface composed of extractant molecules. Therefore, modelling such processes makes necessary the calculation of the aggregates free energies. An important term to consider is the free energy of the carbon chains which impose the spontaneous and stretched curvature of the surfactant flm. However, a lack remains concerning its role, as well as its intensity. |
We propose to determine the carbon chains free energy in small molecular aggregates with small ions in organic phase by means of molecular dynamics simulations. This project aims at proposing a model aggregate for which the size of the polar core will change.
Using a method based on thermodynamics integration, the free energy will be calculated. Simulations in implicit and explicit solvent will be performed. The most stable conformation will be deduced, as well as the free energy cost needed to deform the aggregates. Then, it will be possible to compare the results obtained using such approach with mesoscopic theories based on phenomenological packing parameters or based on the concepts of Gaussian curvatures, e.g. the Helfrich free energy model. The role of the chain lengths and sizes will be studied. Such theoretical modelling already exists in literature since 20 years for detergents and lipids. However, it has never been used for extractant molecules. These results are the base of the detergent formulation and the membrane transport. In our case, the results will be used to model the transfer of ions through phases.
A new expression for the carbon chain energy term in extraction process will be proposed. The prediction of the global phenomenon will done using different approaches:
- from a mesoscopic theory where the results obtained at the molecular scale will be used,
- from molecular modelling, taking into account explicitly the polar core, for which this work will be a frst needed step.
Beyond the publications, the calculation codes developed during the post-doc will be integrated to the simulation platform dedicated to the modelling of the liquid – liquid extraction process by evaluating the transfer energies through phases at the mesoscopic scale.
Thesis
Two PhD thesis available at LMCT.
1. Hydration forces and ionic specificity in silica surfacesThesis supervisor: J.-F. Dufręche |
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We propose to study by modelling the origin of hydration forces between silica surfaces, and their link with the specific adsorption of ions. This theoretical work will be based on a multi-scale approach. The idea consist in understanding the origin of surface phenomena who couple electrostatic force and hydration. This work is all the more significant since these phenomena drive many of the numerous applications of silicas, i.e. in separation chemistry as a nanoporous material.
More precisely, an atomic model of silica, depending on the pH and of the ions in solution will be proposed. Molecular dynamics and Monte Carlo simulations will allow the deduction of a coarse-grained model, which will be solved by density functional theory.
Together with dynamical (electrokinetic phenomena) and equilibirum (ion exchange) experiments performed in ICSM, we will determine how the various ions modify the surface and drive the behaviour and the one of the solvent. In a longer term, it should be possible to propose a theory which is quantitatively in agreement with experiments, and which takes into account molecular effects.
2. Multiscale modelling for separation chemistry: aggregates in organic phase for separation chemistryThesis supervisor: J.-F. Dufręche |
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Separation processes performed for recycling of heavy metals commonly use liquid-liquid extraction for which ions are selectively transferred from an aqueous to an organized organic phase. The description of the aqueous phase is being relatively well established, but for the organic phase nothing exists from a predictive point of view. This thesis will study the physical chemistry of liquid-liquid extraction from a theoretical approach. The main goal is the understanding of the various effects (solvation, electrostatic and Van der Waals forces, entropy), which drive the transfer from one aqueous phase to an organic organized phase. A method based on density functional theory (classical DFT) will allow the calculation of the ion distribution in the various inverse micelles. Then the various thermodynamical properties of the system will be obtained. The experimental support will be first the extraction of Europium nitrate thanks to DMDOHEMA for which experimental data have been measured. Molecular modelling will allow the checking of this mesoscopic approach and it will provide some physical parameters, specially for the solvation effects.
Internships
1. Modelling of the water/oil interface for the strategic metals extraction: a molecular approachSupervisor: M. Duvail |
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Element selective separation processes used for the recycling of strategic metals (lanthanides, actinides, etc.) rely on the liquid-liquid extraction principles. During this process, ions are selectively extracted from an aqueous phase to an organized organic phase. This is possible thanks to the presence of tensioactive molecules (surfactant molecule composed of a polar hydrophilic head, and an apolar hydrophobic tail) which are localised at the water/oil phase. Hence, a good understanding of the phenomena occuring in the aqueous phase, as well as in the organic phase is crucial to improve such process. Although, this last decade, many theoretical studies have been performed at the molecular scale to provide realistic pictures of aqueous solutions, a lack remains concerning the thermodynamics properties of ions in organic solutions and at the water/oil interface.
| In this context, we offer a training position in molecular modelling. This training aims at determining the thermodynamics properties of ions at the water/oil/surfactant interface by using molecular modelling methods (molecular dynamics). |
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First, we will focus on the elastic properties of the surfactant film (rigidity, curvature) that is localised at the water/oil interface. Different surfactant molecules (monoamides, diamides, etc.) could be simulated in order to observe the influence of the molecule's nature. Later, we may also observe how the presence of ions (lanthanides and/or uranyl) in the aqueous phase modify the surfactant film elastic and structural properties (hydration – dehydration of the polar heads of the surfactant molecule).
Modelling such systems at the molecular scale will improve our knowledge of the phenomena occuring during the liquid-liquid extraction process. At the end, this study will be used to better understand these systems (water/oil/surfactant) at the mesoscopic scale (several nanometers) already performed in the group. This training will be performed in collaboration with Dr. Philippe Guilbaud (CEA Marcoule / RadioChemistry & Processes Department).
This study, having a crucial importance, and using innovative methods may lead to publications in international scientist journals. This training may be also followed by a Ph.D. thesis funded by CEA.
2. Multiscale modelling of ions surface mobility in confined silicaSupervisor: B. Siboulet |
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The aim of the course is to model ion surface mobility in porous glass. This study is part of development of innovative methods for soil and water decontamination polluted by heavy metals.
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Porous silica used in separation chemistry are made of subnanometric channels. When driven through this porous network, an aqueous solution exchanges ions with glass surface, and is decontaminated. The process is controlled by the species surface mobility, which is the diffusion in the vicinity of surfaces. The aim of this course is to model this phenomenon at the molecular scale for a series of monovalent ions. |
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The method rests on Molecular Dynamics simulations, coupled with a thermodynamical analysis based on the density functional theory. Linking models with experiments, the student will use electrokinetics models with boundary conditions.
This course will allow an outstanding student to get to know theoretical chemistry and simulations methods. Following the results obtained, this work can be the basis for a publication in an international scientific journal, and is also the start for a Ph.D. thesis supported by CEA.
