THESIS BY Pauline Simonnin*


Understanding and modeling transport processes in highly confined media is a major challenge in order to innovate in the broad range of applications covered by research conducted at IFPEN: catalyst design, electricity storage and generation, sealing solutions for geological storage. In the latter area, the use of molecular dynamics made it possible to simulate the diffusion of water and gas molecules in clay nanopores.

This research benefited from the high-performance computing resources provided by GENCIa . During the simulation, a strong dependance of the diffusion coefficients obtained on the size and shape of the simulation box was demonstrated (figure). This spurious artefact is due to the hydrodynamic interactions that arise, numerically by the imposed periodic boundary conditions, equivalent to consider an infinite set of replicas of the original system. A complex analytical calculation made it possible to quantify and correct it using an explicit formula(1). The excellent agreement between theory and numerical experiments confirms the origin of
the correction, and may be use practically.

The study of clay interface diffusion highlighted the effect of ionic species, always present in the fluid, on the structure and hydrodynamic properties of these interfaces(2). Its incorporation makes calculations aimed at reproducing the diffusion phenomenon more realistic and improves their predictive capacity.

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Apparent diffusion coefficient as a function of
the shape of the simulation box: excellent correlation between the analytical formula - solid line - and simulations for different couples (H, L).

This research demonstrated the contribution of molecular dynamics tools to the acquisition of quantitative property predictions used for the simulation of transport processes in nanoporous media. The injection of these results into transport models on a larger scale opens up avenues of potential interest in the energy, chemicals and environmental sectors.

a - National high-performance computing resource.

*Thesis entitled "Fluid transport in nanopores: from molecular models to continuous models"

(1)  P. Simonnin, B. Noetinger, C. Nieto-Draghi, V. Marry & B. Rotenberg. Diffusion under Confinement: Hydrodynamic Finite-Size Effects in Simulation, J. Chem. Theory Comput., 2017, 13 (6), pp 2881–2889
>> DOI : 10.1021/acs.jctc.7b00342
(2)  P. Simonnin, B. Noetinger, C. Nieto-Draghi, V. Marry & B. Rotenberg. (2018). Mineral- and Ion-Specific Effects at Clay–Water Interfaces: Structure, Diffusion, and Hydrodynamics, J. Phys. Chem. C, 2018, 122 (32), pp 18484–18492 
>> DOI : 10.1021/acs.jpcc.8b04259


Scientific contact: Benoit Noetinger