Bertrand Siboulet

Hydrophobic transition in porous amorphous silica.

Realistic models of amorphous silica surfaces with different silanol densities are built using Monte Carlo annealing. Water-silica interfaces are characterized by their energy interaction maps, adsorption isotherms, self-diffusion coefficients, and Poiseuille flows. A hydrophilic to hydrophobic transition appears as the surface becomes purely siliceous. These results imply significant consequences for the description of surfaces. First, realistic models are required for amorphous silica interfaces. Second, experimental amorphous silica hydrophilicity is attributed to charged or uncharged defects, and not to amorphousness. In addition, autoirradiation in nuclear waste glass releases hydrogen atoms from silanol groups and can induce such a transition.

Stability and instability of the isoelectronic UO2(2+) and PaO2+ actinyl oxo-cations in aqueous solution from density functional theory based molecular dynamics.

In this work, Pa(V) monocations have been studied in liquid water by means of density functional theory (DFT) based molecular dynamic simulations (CPMD) and compared with their U(VI) isoelectronic counterparts to understand the peculiar chemical behavior of Pa(V) in aqueous solution. Four different Pa(V) monocationic isomers appear to be stable in liquid water from our simulations: [PaO(2)(H(2)O)(5)](+)(aq), [Pa(OH)(4)(H(2)O)(2)](+)(aq), [PaO(OH)(2)(H(2)O)(4)](+)(aq), and [Pa(OH)(4)(H(2)O)(3)](+)(aq). On the other hand, in the case of U(VI) only the uranyl, [UO(2)(H(2)O)(5)](2)(+)(aq), is stable. The other species containing hydroxyl groups replacing one or two oxo bonds are readily converted to uranyl. The Pa-OH bond is stable, while it is suddenly broken in U-OH. This makes possible the formation of a broad variety of Pa(V) species in water and participates to its unique chemical behavior in aqueous solution. Further, the two actinyl oxocations in water are different in the ability of the oxygen atoms to form stable and extended H-bond networks for Pa(V) contrary to U(VI). In particular, protactinyl is found to have between 2 and 3 hydrogen bonds per oxygen atom while uranyl has between zero and one.

Water self-diffusion at the surface of silica glasses: effect of hydrophilic to hydrophobic transition

We study water dynamics at a silica aqueous interface. Both hydrophilic (hydroxylated) surfaces and hydrophobic surfaces (dehydroxylated upon irradiation) have been generated from atomistic simulations. A new method for the calculation of the normal self-diffusion coefficients based on the calculation of mean first passage times is proposed. It uses the Smoluchowski theory of Brownian motion and it takes proper account of the layering of the molecules. In the case of parallel self-diffusion coefficients, a decrease is found compared to the bulk values. It can be described in terms of hydrodynamic boundary conditions induced by the surface confinement. This hydrodynamic explanation is not enough to interpret the case of normal self-diffusion coefficients for which an important diminution is found. Normal self-diffusion coefficients appear to depend strongly on the hydrophilicity of the surface. They tend towards their bulk value only at long distances from the surfaces. The first layer of water molecules is found to be partially adsorbed.

Luminescence of trivalent lanthanide ions excited by single-bubble and multibubble cavitations.

This article focuses on the possibility of exciting some lanthanides (Ce(3+), Tb(3+), Gd(3+), and Eu(3+)) by ultrasound in aqueous solutions. Depending on the lanthanide ions and on the acoustic cavitation conditions (single-bubble or multibubble systems), the excitation mechanism is shown to be photoexcitation (e.g., for Ce(3+)) or collision-induced excitation (e.g., for Tb(3+)). The sonoluminescence of Tb(3+) is studied in detail at various ultrasonic frequencies, allowing quantification of the amount of quenching. The latter is much stronger in sonoluminescence than in photoluminescence due to the particular properties of acoustic cavitation. Complexation with citrate ions enhances manifold sonoluminescence of lanthanides due to reduction of intra- and inner-molecular quenching.

Modelling of mutual diffusion for associated electrolytes solution: ZnSO4 and MgSO4 aqueous solutions

We study mutual diffusion of associated electrolyte in water. A theory based on the Smoluchowski equation and on the mean spherical approximation is proposed. It is found to be valid up to molar concentrations. The values of the association constant obtained for ZnSO4 and MgSO4 aqueous solutions are consistent with further methods of measurements. Thanks to the elimination of fast variables method, the theory can be generalised to any association in solution. Comparison with molecular dynamics and Brownian dynamics simulations in the case of MgSO4 indicates that the pair is a global association phenomenon since it corresponds to several association phenomena (contact ion pair and solvent separated ion pair).