Ph.D. in Physical Chemistry to Study the Dynamics of Water and Solutes in Clays Under Unsaturated Conditions (M/F)

Job Description

Thesis Subject

Understanding the dynamics of water and solutes in clays under unsaturated conditions: integration of molecular dynamics at the scale of clay platelet arrangements

Clays are important components of soil. Clay particles consist of stacked layers of aluminosilicates of varying sizes. These particles themselves form aggregates. As a result, clays have pores of various sizes, which, under undisturbed conditions, are saturated with water. In the context of underground storage of radioactive waste in a clay-rich geological formation (e.g., the Cigéo project in France), the excavation of tunnels, the radiolysis of water, and the corrosion of steel will lead to the entrapment of gases (air, H₂, etc.) within the pores of the natural formation, creating partially saturated liquid-gas conditions within the pores.
As part of the Darius project, funded by the CNRS NEEDS program (Nuclear: Energy, Environment, Waste, Society), which aims to better understand the effect of partial water saturation of the clay pore network on the diffusion of water and ions at different spatial and temporal scales, a major scientific bottleneck was identified: the diffusion coefficients measured under unsaturated conditions are much lower than those obtained from Brownian motion simulations, whereas the latter accurately reproduce the results of experiments conducted under saturated conditions. To resolve this inconsistency, this thesis proposes to explore in greater detail the distribution and dynamics of water in unsaturated clay-rich porous media, at the scale of the arrangement of clay particles.
In fact, the classical capillary approximation—used in Brownian dynamics simulations to determine the pore size below which pores are fully hydrated—and the multilayer adsorption model—used to describe the hydration of the surfaces of larger pores—may prove to be inadequate. In particular, all surfaces are treated the same way in the simulations, even though the basal surfaces and edges of the clay layers are very different from a crystallographic point of view. Furthermore, the intrinsic negative charge of the clay layers, counterbalanced by the presence of counterions at the surface, induces an electrostatic field that structures the fluid into multiple molecular layers, with a range that depends on the ionic strength.
Thus, we will first use molecular dynamics and Monte Carlo simulations to study finite layers forming dihedral angles, or pores bounded by layer edges, in the presence of water and ions. The objectives include: (i) evaluating the validity of the capillary approximation in pores formed by basal surfaces, depending on the clay type and solution composition, and (ii) studying the distribution of water in pores of varying geometries (dihedral angles, edges) as a function of the degree of desaturation and the resulting effects on species diffusion.
In a second step, the results of the molecular simulations will be incorporated into Brownian dynamics simulations at higher scales, which will then be compared with experimental data. The influence of H₂ (as a gas or in dissolved form), released during the corrosion of container steels, may also be investigated.
Molecular-scale simulations will begin with the non-polarizable ClayFF force field, which accurately describes the edges of the layers (Pouvreau et al., J. Phys. Chem. C 2017, 2019), and will then proceed to the polarizable PIM force field developed in the lab (Tesson et al., J. Phys. Chem. C 2016, 2018). Indeed, the fluid distribution over longer distances and the differences in behavior between PIM and ClayFF, as observed in Darius and previously (Lecrom S. et al., J. Phys. Chem. C 2020, Guérin et al., Clays Clay Miner 2025), could affect the wettability of different types of surfaces and induce unexpected effects on pore hydration.

Your Work Environment

The PHENIX laboratory (PHysico-chemistry of Electrolytes and Interfacial Nanosystems) is located in Paris on the Jussieu campus. The thesis will be conducted within the https://phenix.cnrs.fr/themes-de-recherche/modelisation-et-experiences-multi-echelles/en-savoir-plus-sur-lequipe-mem/ team.

Simulations using the polarizable PIM force field will be conducted in collaboration with S. Le Crom of Subatech (Nantes), who is contributing to the development of this force field. Brownian dynamics simulations at the mesoscopic scale will be conducted in collaboration with E. Ferrage of IC2MP (Poitiers).

Compensation and benefits

Compensation

2300 € gross monthly

Annual leave and RTT

44 jours

Remote Working practice and compensation

Pratique et indemnisation du TT

Transport

Prise en charge à 75% du coût et forfait mobilité durable jusqu’à 300€

About the offer

About the CNRS

The CNRS is a major player in fundamental research on a global scale. The CNRS is the only French organization active in all scientific fields. Its unique position as a multi-specialist allows it to bring together different disciplines to address the most important challenges of the contemporary world, in connection with the actors of change.

CNRS

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