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Parruzot B.,CEA Marcoule Nuclear Site | Jollivet P.,CEA Marcoule Nuclear Site | Rebiscoul D.,Marcoule Institute for Separative Chemistry | Gin S.,CEA Marcoule Nuclear Site
Geochimica et Cosmochimica Acta

The long-term behavior study of archaeological artifacts and natural minerals and glasses revealed discrepancies between laboratory and field data. For a better understanding of the cause of these discrepancies and to reinforce the use of basaltic glass as an analog for nuclear waste glasses, this study focuses on the determination of alteration rates and processes of synthetic basaltic glass in residual rate regime. Laboratory batch experiments were performed at high surface-to-volume ratios at 90 and 30°C for more than 1000days. In all the experiments, the residual rate regime was reached after about 6months. The residual alteration rates at 30 and 90°C were 4.0±1.0×10-6 and 9.5±3.2×10-6g·m-2·d-1, respectively. At 90°C, this residual alteration rate is five orders of magnitude lower than the forward alteration rate (0.8g·m-2·d-1). Altered powders and monoliths were characterized by Transmission Electron Microscopy and Time-of-Flight Secondary Ion Mass Spectrometry. From glass core to solution, the altered materials are structured as follows: pristine glass, gel (corresponding to the palagonitic layer of natural glasses) and intergranular clays. To assess the passivating properties of this alteration film, we used solid characterization, an isotopically-tagged post-leaching experiment and the measurement of mobile species diffusion coefficients through the alteration film at different stages of reaction using various techniques (solution analysis and X-ray Reflectometry). These characterizations showed that the alteration film formed during residual rate alteration is passivating even without clogged porosity within the gel. Diffusion coefficients of water and alkali metals - respectively diffusing to and from the pristine glass - through the alteration film dropped from 10-20 to 10-19 m2·s-1 during the first alteration stages to 10-25 m2·s-1 in residual rate regime. © 2014 Elsevier Ltd. Source

Steins P.,CEA Marcoule Nuclear Site | Poulesquen A.,CEA Marcoule Nuclear Site | Diat O.,Marcoule Institute for Separative Chemistry | Frizon F.,CEA Marcoule Nuclear Site

Time-resolved rheology, small angle X-ray scattering (SAXS), and electron paramagnetic resonance (EPR) techniques were used to study the polymerization of geopolymers. These polymers are inorganically synthesized by the alkaline activation of an aluminosilicate source (metakaolin) in aqueous solution. The influence of the alkali activator (Na +, K +, and Cs +) was investigated at room temperature. As observed through the variation of the viscoelastic moduli (G′, G″), curing proceeds in steps that are well pronounced when NaOH is used. These steps correspond to a specific dissolution/polycondensation mechanism and are smoothed when the size of the alkali cations increases. This size effect also has an impact on the gelation time (maximum of tan δ). Structural analysis through SAXS experiments allows us to characterize these mechanisms on the nanoscale and to show that the growth of the geopolymer is due to the aggregation of oligomers with a size that is even smaller than the cation is chaotropic. Finally, water behavior during geopolymerization was assessed by using a spin probe. The results show that the spin-probe signal progressively disappears during the first moment of the reaction and reappears when the solid polymeric gel is formed, highlighting the role of water molecules in the different chemical reactions during the process. The EPR signal is in fact increasingly masked as the ion size decreases (because of the strength of the hydration shell). At the end of the reaction, some water molecules were released within the pores, restoring the visibility of the isotropic spin-probe signal. © 2012 American Chemical Society. Source

Guilbaud P.,CEA Marcoule Nuclear Site | Zemb T.,Marcoule Institute for Separative Chemistry
Current Opinion in Colloid and Interface Science

We assemble here all available descriptions of oil-soluble surfactant aggregates with or without solutes, assumed to be located in the polar cores of reverse micelles. The presence of solutes is crucial for the formation of a well-defined interface, thus inducing a transition from a loose reverse aggregate into a more structured micelle. This transition can be followed by the concomitant decrease of the "critical aggregation concentration" (c.a.c.). The less organized state as reverse aggregates is predominant when no "nucleating" species such as water, salts, or acids are present. One way to understand this weak aggregation is a depletion driving to aggregates as pseudo-phases introduced by Tanford. Analogues coexisting pseudo-phases seem to exist: weak oil-in-water (o/w) aggregation with the so-called surfactant-free microemulsions, containing loose aggregates, and re-entrant phase diagrams presenting a lowest aggregation concentration (l.a.c.), as described in the seventies. © 2014 Elsevier Ltd. Source

Guilbaud P.,CEA Marcoule Nuclear Site | Zemb T.,Marcoule Institute for Separative Chemistry

Reverse micelles? The transition from weak aggregation to water-poor reverse micelles triggered by the presence of extracted ion pairs is modeled using molecular dynamics simulations (see picture). The presence of the ion induces a polar/apolar segregation and the formation of a curved film microstructure consistent with the classical inverse micelle. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

Deme B.,Laue Langevin Institute | Zemb T.,Marcoule Institute for Separative Chemistry
Current Opinion in Colloid and Interface Science

We analyse the experimental evidence of the hydration force near phospholipid bilayers when the "solvent" is a solution of carbohydrates. Two cases must be clearly distinguished: when sugar is dissolved, depletion causes a supplementary attractive force, while in the case of sugar linked to the lipid the contact pressure increases by orders of magnitude. Attractive interaction inferred between bilayers is sometimes derived from indirect evidence, i.e. scattering, attraction between layers adsorbed, shape of phase boundary limits, and without the simultaneous determination of the osmotic compressibility. Generally, water molecules in the first hydration shell of sugar compete with water molecules bound (by more than one kT in free energy) to lipid head-groups. A general result is that the decay length of any repulsive effect remains close to 0.2. nm, even in concentrated sugar solutions. A tentative general explanation of this experimental fact is given together with consequences, such as the possibility of several types of critical points appearing in bilayer stacks. Decay length as well as effective contact pressure is considered with respect to carbohydrate activity. © 2011 Elsevier Ltd. Source

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