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Séminaire 19.05.2010 - 14h

par Benjamin Rotenberg - 26 août 2010

Eric Ferrage (laboratoire Hydrasa, Poitiers) présentera un séminaire le 19 mai 2010 à 14h dans la bibliothèque du PECSA (7e étage, batiment F, porte 754) intitulé :

Water organization in synthetic Na-smectite with tetrahedral layer charge. An experimental validation of numerical data


Smectites are clay minerals ubiquitous in surface environments, both terrestrial and marine, where they often represent one of the main mineral components. As a function of relative humidity (RH) and under non-saturated conditions, smectite typically shows a stepwise hydration behaviour corresponding to the intercalation discrete sheets of water molecules in its interlayer. However, structural heterogeneities of charge location and/or of charge amount most often lead to the coexistence of different hydration states within smectite crystals. These heterogeneities are best revealed by comparing X-ray diffraction (XRD) patterns recorded on the same smectite sample under contrasting RH conditions. Calculated XRD patterns can then be fitted to experimental profiles using a trial-and-error procedure. The obtained structural models allow describing the hydration heterogeneity and the evolution of swelling smectite layer thickness with RH. The obtained water content can also be compared with gravimetric isotherm water desorption data. This comparison allows discriminating the relative contributions of H2O molecules from hydrated interlayers (crystalline water) and from the pore space network.

By taking into account smectite hydration heterogeneity it is possible to further refine the interlayer structure of hydrated smectite layers. However, common crystallographic approaches account poorly for the actual statistical positional disorder of confined water and therefore do not provide a comprehensive characterisation of the structure and organisation of interlayer species. Computational simulations can supply detailed molecular pictures of the system, notably by discriminating water molecules forming the cation hydration shell from those filling the interlayer space but the basic comparison between theoretical and experimental data performed so far do not allow assessing the validity of the semi-empirical atomic interaction parameters used in these theoretical simulations. A specific quantitative collation procedure between computational simulations and experimental diffraction data has been developed in order to tackle this problem. Following this methodology swelling clay samples with contrasted layer charge are equilibrated in homogeneous mono- and bi-hydration states. The associated layer thickness and relative humidity are used to constrain the simulation box for Monte-Carlo calculations in the Grand-Canonical (µ, V, T) ensemble (GCMC). The resulting water contents are then compared to those determined by water vapour desorption gravimetry experiments while the obtained GCMC interlayer species density distributions along the c*-axis are used to generate 00ℓ reflection profiles which are then compared to XRD and neutron diffraction patterns recorded on both hydrogenated and deuterated specimens. It will be shown that such collation approach allows a critical assessment of existing potentials and leads to a more realistic description of interlayer water in expandable clay minerals.

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