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International audienc

Within the wider context of water behavior in soils, and with a particular emphasis on clays surrounding underground radioactive waste packages, we present here the translational dynamics of water in clays in low hydrated states as studied by coupling molecular dynamics (MD) simulations and quasielastic neutron scattering experiments by neutron spin echo (NSE). A natural montmorillonite clay of interest is modeled by a synthetic clay which allows us to understand the determining parameters from MD simulations by comparison with the experimental values. We focus on temperatures between 300 and 350 K, i.e., the range relevant to the highlighted application. The activation energy Ea experimentally determined is 6.6 kJ/mol higher than that for bulk water. Simulations are in good agreement with experiments for the relevant set of conditions, and they give more insight into the origin of the observed dynamics.

No abstract

Diffusion of water in montmorillonite clays at low hydration has been studied on the microscopic scale by two quasi-elastic neutron scattering techniques, neutron spin-echo (NSE) and time-of-flight (TOF), and by classical microscopic simulation. Experiment and simulation are compared both directly on the level of intermediate scattering functions, I(Q, t), and indirectly on the level of relaxation times after a model of atomic motion is applied. Regarding the dynamics of water in Na- and Cs-monohydrated montmorillonite samples, the simulation and NSE results show a very good agreement, both indicating diffusion coefficients of the order of (1-3) x 10(-10) m(2) s(-1). The TOF technique significantly underestimates water relaxation times (therefore overestimates water dynamics), by a factor of up to 3 and 7 in the two systems, respectively, primarily due to insufficiently long correlation times being probed. In the case of the Na-bihydrated system, the TOF results are in closer agreement with the other two techniques (the techniques differ by a factor of 2-3 at most), giving diffusion coefficients of (5-10) x 10(-10) m(2) s(-1). Attention has been also paid to the elastic incoherent structure factor, EISF(Q). Simulation has played a key role in understanding the various contributions to EISF(Q) in clay systems and in clearly distinguishing the signatures of "apparent" and true confinement. Indirectly, simulation highlights the difficulty in interpreting the EISF(Q) signal from powder clay samples used in experiments.

In this paper, we compare the structure and the phase behavior of two kinds of magnetic fluids, also called ferrofluids. They are constituted of the same maghemite particles, the diameters of which lie around 8 nm, dispersed either in water or in cyclohexane. Both systems are constructed to get the same interparticle interactions and differ only through the nature of the repulsion. Repulsion is either electrostatic, due to the charges of citrate molecules adsorbed on the particles surface in water, or steric, due to the alkyl chains of adsorbed surfactants in cyclohexane. Small angle neutron scattering (SANS) experiments show that both systems are highly repulsive and that the structure factors are very similar. This is confirmed by stability measurements: the samples are stable if temperature is decreased and if a magnetic field is applied. If the repulsion is decreased by the addition of electrolyte in water or bad solvent in cyclohexane, a gas–liquid-like transition is observed in both systems. However, the standard electrostatic potential (Derjaguin–Landau–Verwey–Overbeek potential) fails to describe the electrostatic repulsion in the aqueous ferrofluid while the behavior of this system is very similar to the behavior of the sterically stabilized ferrofluid. This underestimate of the electrostatic repulsion is probably due to the finite size effects of the trivalent ions. The striking similarities in the structure and the behavior of both kinds of dispersions, despite their chemical differences, seems to be related to the presence, in both cases, of the adsorbed surface species which ensure the repulsion between particles. Moreover, this repulsion may be described by an effective Yukawa potential very similar in range and intensity in both systems.

A chemical core-shell strategy is developed here for the synthesis of ferrofluids based on nanoparticles of different ferrites with different mean sizes. A heterogeneity of chemical composition, associated with a superficial enrichment of iron, allows to obtain chemically stable ionic colloids. We propose here a coreshell model to describe the synthesized nanoparticles, which is tested by chemical and magnetic measurements performed at the various steps of the synthesis. The thickness of the superficial layer, rich in iron, is ranging between 0.4 and 1.3 nm, depending on the nanoparticle size and on the underlying ferrite. Its density is found close to that of maghemite, and its magnetization depends on the core ferrite. It is low with a cobalt ferrite core and larger for the three other ferrites investigated here (NiFe 2 O 4 , CuFe 2 O 4 , and ZnFe 2 O 4 ). Magnetic measurements prove that there is a strong redistribution of Zn 2+ ions inside the core of the synthesized nanoparticles based on ZnFe 2 O 4 .

We present a quasi-elastic neutron scattering study of water dynamics confined in a model clay system, a synthetic hectorite with Na+ compensating counterions. As shown by water adsorption gravimetry and neutron/X-ray diffraction, the clay system has, unlike its natural counterparts, very well-defined swelling characteristics, with a clear appearance of a monohydrated and a bihydrated state. This simplifies to a great extent neutron scattering analysis and interpretation. Initially, microscopic relaxation times as well as long-range self-diffusion coefficients for water in Na-hectorite at ambient temperature are determined using the time-of-flight (TOF) and neutron spin echo (NSE) neutron scattering techniques, applying a simple model of isotropic (three-dimensional) translational diffusion. Results from the two techniques are in excellent agreement, giving diffusion coefficient of approximately 1.5 × 10-10 m 2 s-1 and 4.5 × 10-10 m 2 s-1 for the monohydrated and bihydrated state, respectively. Concentrating on the monohydrated hectorite system, after an account is taken of short-time relaxation stemming from fast (vibration-like) motion, the data is analyzed using a geometrically more appropriate translational model:  powder averaged two-dimensional diffusion. This analysis yields a two-dimensional diffusion coefficient in the plane of the clay layers of 2.8 × 10-10 m 2 s-1. We demonstrate on model data that isotropic analysis applied to a system with powder averaged two-dimensional diffusion overall underestimates the diffusion coefficient by approximately 25%.