Anisotropy on the Collective Dynamics of Water Confined in Swelling Clay Minerals

International audienceCollective excitations of water confined in the interlayer space of swelling clay minerals were studied by means of inelastic neutron scattering. The effect of bidimensional confinement on the dynamics of the interlayer water was investigated by using a synthetic Na-saponite sample with a general formula of Si7.3Al0.7Mg6O20(OH)(4)Na-0.7 in a bilayer hydration state. Experimental results reveal two inelastic signals, different from those described for bulk water with a clear anisotropy on the low-energy excitation of the collective dynamics of interlayer water, this difference being stronger in the perpendicular direction. Results obtained for the parallel direction follow the same trend as bulk water, and the effect of the confinement is mainly manifested from the fact that clay interlayer water is more structured than bulk water. Data obtained in the perpendicular direction display a nondispersive behavior below a cutoff wavenumber value, Q(c), indicating a nonpropagative excitation below that value. Molecular dynamics simulations results agree qualitatively with the experimental result

Diffusion of Water in a Synthetic Clay with Tetrahedral Charges by Combined Neutron Time-of-Flight Measurements and Molecular Dynamics Simulations

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Influence of the ionic strength and solid/solution ratio on Ca(II)-for-Na+ exchange on montmorillonite. Part 2: Understanding the effect of the m/V ratio. Implications for pore water composition and element transport in natural media

International audienceThe aim of the present paper is to clarify previous results [1] showing that selectivity coefficients determined for the exchange of Na+ for Ca2+ in montmorillonite were dependent on the solid/solution ratio. The organization of montmorillonite suspensions upon Na+/Ca(II) exchange was analyzed by combining optical microscopy, small-angle X-ray scattering and X-ray diffraction. All samples displayed flocculated characteristics, eliminating the possibility of contrasting accessibility of sorption sites with the solid/ solution ratio. Modeling of experimental X-ray diffraction patterns was used to quantify the relative proportions of interlayer Ca2+ and Na+ cations along the exchange isotherm. The results further confirmed the influence of the solid/solution ratio on the degree of interlayer Ca(II)-for-Na+ exchange, and specific selectivity coefficients for interlayer sites were determined. The effect of the solid/solution ratio was finally interpreted by the resulting local changes in the solution chemistry. We demonstrated that by accounting for the Donnan effect, the different data can be interpreted using a single selectivity coefficient. The obtained Kc constant was successfully applied to interpret existing hydrogeochemical data on a natural aquitard. This most likely represents a more constrained and valid approach for the modeling of reactive element transport in natural media than does the poorly defined Kd parameter

Anisotropic Features of Water and Ion Dynamics in Synthetic Na- and Ca-Smectites with Tetrahedral Layer Charge. A Combined Quasi-elastic Neutron-Scattering and Molecular Dynamics Simulations Study

International audienceThe temperature- and orientation-dependent dynamics of water molecules and sodium or calcium cations confined in the interlayer space of synthetic saponite with contrasting layer charge were analyzed through the combination of three-axis neutron spectroscopy and molecular dynamics (MD) simulation. We first show that it is possible to generate MD simulated quasi-elastic spectra that are equivalent to the experimental ones. As a consequence, the analysis of spectra in terms of Lorentzian decomposition can be advantageously replaced by a direct exploitation of MD results. We show that such strategy provides 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. The same strategy applied to data obtained at higher temperature provided further confirmation of the validity of the atomic potentials used in simulations while allowing the extraction of activation energies for water and cations translational motions

Imaging Drosophila brain by combining cryo soft X ray microscopy of thick vitreous sections and cryo electron microscopy of ultrathin vitreous sections

Cryo soft X ray microscopy is an emerging imaging tool complementary to cryo electron microscopy, allowing to image frozen hydrated specimens ten to hundred times thicker, but at lower resolution. We describe how the method, so far restricted to isolated small cells or cell monolayers, can be extended to large cells and tissue. We image the synapses of the Kenyon cells in frozen hydrated Drosophila brains combining cryo soft X ray microscopy of thick vitreous sections, and cryo electron microscopy of ultrathin vitreous sections. We show how to obtain frozen hydrated sections of thicknesses ranging from 40 nm up to 2.5 m, by tuning the sectioning speed of the cryo microtome. A fluorescent stereo microscope mounted on the cryo microtome allowed us to target the regions of interest after GFP labeling of synapses. Thick cryo sections were imaged by cryo soft X ray microscopy at a resolution better than 25 nm, while ultrathin cryo sections of the same regions were explored in parallel at the nanometre level of resolution by cryo electron microscop

Bioweathering of nontronite colloids in hybrid silica gel: implications for iron mobilization

International audienceThis study aimed to study biotic iron dissolution using a new hybrid material constituted of well-dispersed mineral colloids in a silica gel matrix. This permitted to prevent adsorption of colloidal mineral particles on bacteria. Hybrid silica gel (HSG) permitted to study bioweathering mechanisms by diffusing molecules.Methods and ResultsHybrid silica gel was synthesized through a classical sol-gel procedure in which mineral colloidal particles (NAu-2) were embedded in a porous silica matrix. Rahnella aquatilis RA1, isolated from a wheat rhizosphere was chosen for its ability to dissolve minerals by producing various organic acids and siderophores. Pyruvic, acetic and lactic acids were the major organic acids produced by R.aquatilis RA1 followed by oxalic and citric acids at the end of incubation. Comparison of abiotic and biotic experiments revealed a high efficiency of R.aquatilis RA1 for iron dissolution suggesting an optimized action of different ligands that solubilized or mobilized iron.ConclusionsHybrid silica gel allowed focusing on the colloidal mineral weathering by metabolites diffusion without mineral adsorption on bacteria.Significance and Impact of the StudyHybrid silica gels are new and efficient tools to study colloidal mineral bioweathering. Adjusting HSG porosity and hydrophobicity should permit to precise the influence of limiting diffusion of siderophores or aliphatic organic acids on mineral weathering

Liquid–crystalline properties of aqueous suspensions of natural clay nanosheets

International audienceClay minerals, like beidellite or nontronite, spontaneously exfoliate in water and form colloidal suspensions of nanosheets. In a given range of concentration, these suspensions display a nematic liquid–crystalline phase whose structure and properties can be conveniently studied in detail by polarized-light microscopy and small-angle X-ray scattering (SAXS). Moreover, in situ SAXS investigations of sheared clay suspensions provide information about their flow properties, both in the isotropic and nematic phases. The colloidal nematic phase shows the classical properties of usual nematics, such as surface anchoring and electric-field and magnetic-field alignment. Thus, nematic single domains can be produced. The isotropic phase also displays strong electro-optic effects in moderate electric fields. Finally, we describe a few examples of applications of such systems and we show how these studies could be extended to suspensions of other types of nanosheets