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Jeudi 16 mai 2013

14h - bibliothèque du CRMD

Organisational and dynamical properties of interlayer water in smectites

Eric Ferrage - IC2MP, Poitiers

Résumé :

The crystal-structure and stepwise hydration behaviour of smectite as a function of relative humidity (RH) have been extensively studied by X-ray diffraction (XRD). With increasing RH and under non-saturated conditions, smectite typically shows a stepwise hydration behaviour corresponding to the intercalation of 0, 1 or 2 discrete sheets of water molecules in its interlayer. However, heterogeneities of charge location (between octahedral and tetrahedral sheets) and/or of charge amount (from one interlayer to the other or within a given interlayer) most often lead to the coexistence of different hydration states within smectite crystals. Profile modelling of XRD patterns recorded on hydrated smectites has shown hydration heterogeneity to be systematic, thus impeding the precise structural characterization of smectite. For instance this numerical approach allows precisely determining the actual smectite layer thickness whereas the qualitative measurement of the 001 reflection position on experimental XRD patterns can lead to a dramatic overestimation of the interlayer thickness used in numerical studies.

Although powerful, XRD accounts poorly for the actual statistical positional disorder of interlayer species (H2O, cations). Computational simulations provide relevant information by supplying detailed molecular pictures of the system. Still, relatively few studies have confronted simulation results with experimental data and existing comparisons do not allow fully assessing the validity of the semi-empirical atomic interaction parameters used in theoretical simulations. In a first step, hydration behaviour of two synthetic Na-saponites with different layer charges (0.4 and 0.7/O10(OH)2) was studied by XRD from 92% RH down to 0% RH[1]. Calculated XRD patterns were then fitted to experimental profiles to determine for each sample the RH conditions to obtain almost homogeneous mono-hydrated and bi-hydrated state. Additional neutron diffraction (ND) patterns on both H2O and D2O saturated specimens were used to increase the contrast of sensitivity of the modelling approach towards water[2]. Layer thickness derived from XRD was used to constrain a simulation box for Grand Canonical Monte-Carlo simulations (GCMC-µ,V,T). The obtained density profiles of interlayer species were then introduced for calculation of XRD and ND patterns whereas water content was compared to that determined from gravimetric water sorption measurements. Depending on the choice of atomic interaction potentials, GCMC simulations were shown to allow matching experimental water contents and fitting the complete set of diffraction data[2]. Relevant information was then derived concerning the influence of layer charge on the orientational properties of interlayer water molecules.

In a second step, the dynamical properties of interlayer water were studied by means of three-axis neutron spectroscopy and molecular dynamics (MD) simulation[3,4]. MD simulations were performed on the basis of atomic interaction potentials that allowed reproducing the full set of diffraction data. The MD simulated quasi-elastic spectra were shown to be equivalent to the experimental ones. Such strategy provided classical information on the influence of clay crystal chemistry on water and ion dynamical features as well as reliable additional information on (i) dynamics associated to different types of water molecules (bonded or not to interlayer cations) and (ii) interlayer cation dynamics[4].

References :

[1] Ferrage E., Lanson B., Michot L.J. & Robert J.L. (2010) Hydration properties and interlayer organization of water and ions in synthetic Na-smectite with tetrahedral layer charge. Part 1. Results from X-ray diffraction profile modeling. Journal of Physical Chemistry C, 114(10), 4515-4526.

[2] Ferrage E., Sakharov B., Michot L.J., Delville A., Bauer A., Lanson B., Grangeon S., Frapper G., Jiménez-Ruiz M. & Cuello G. (2011) Hydration properties and interlayer organization of water and ions in synthetic Na-smectite with tetrahedral layer charge. Part 2. Towards a precise coupling between molecular simulations and diffraction data. Journal of Physical Chemistry C, 115(5), 1867-1881.

[3] Jiménez-Ruiz M., Ferrage E., Delville A. & Michot L.J. (2012) Anisotropy on the collective dynamics of water confined in swelling clay minerals. Journal of Physical Chemistry A, 116, 2379-2387.

[4] Michot L.J., Ferrage E., Jiménez-Ruiz M., Boehm M. & Delville A. (2012) Anisotropy features of water and ions dynamics in synthetic Na- and Ca-smectite with tetrahedral layer charge. A combined QENS and molecular dynamics simulations study. Journal of Physical Chemistry C, 116, 16619-16633.