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Final Activity Report Summary - SMECTI-SCALE (Structure and dynamics of water and ions in smectite)

Smectite is a clay mineral ubiquitous in surface environments, both terrestrial and marine. The structural charge of the layer is compensated by inorganic cations whose interactions with water induce swelling, the intensity of which depends on the relative humidity. This project focuses on understanding the structure and dynamics of water molecules in smectite interlayer. Such a study has important implications for soil functioning (hydric transfers and elements retention) or in waste repository in which smectite is used as barrier to limit pollutant or radionuclide migration.

The objectives of the project were to (1) determine the location and structure of water molecules and (2) understand the dynamical properties of these water molecules. Water location and structure were analysed by a combination of numerous experimental techniques (High-resolution gas adsorption, Water adsorption/desorption gravimetry experiments, X-ray and Neutron diffraction) as well as numerical molecular simulation procedures (Grand Canonical Monte-Carlo simulations).

The results indicate that the location and distribution of water molecules between the interlayer and the pore spaces of the smectite medium strongly depends on both layer charge and nature of interlayer cation. The local organisation of water molecules inside the interlayer space is also strongly affected by those parameters. The dynamical properties of interlayer water molecules were investigated on the basis of molecular dynamics simulations and quasi-elastic and inelastic neutron scattering techniques to investigate jointly rotational and translational dynamics of single water molecules as well as collective motions. The obtained data reveal the influence of the layer crystal-chemistry on the diffusional and rotational properties of water molecules whereas it was shown that the collective excitations of water molecules in this confined bidimensional system strongly differ from those observed in bulk water.

Although some data treatment is still under progress, the final results will allow drawing the influence of each crystal-chemistry parameter on the structural and dynamical properties of the smectite porous medium as well as a range of expected diffusion coefficient values that are useful for predicting water mobility in natural media.

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