Magali Duvail

Revised Ionic Radii of Lanthanoid(III) Ions in Aqueous Solution

A new set of ionic radii in aqueous solution has been derived for lanthanoid(III) cations starting from a very accurate experimental determination of the ion-water distances obtained from extended X-ray absorption fine structure (EXAFS) data. At variance with previous results, a very regular trend has been obtained, as expected for this series of elements. A general procedure to compute ionic radii in solution by combining the EXAFS technique and molecular dynamics (MD) structural data has been developed. This method can be applied to other ions allowing one to determine ionic radii in solution with an accuracy comparable to that of the Shannon crystal ionic radii.

Building a polarizable pair interaction potential for lanthanoids(III) in liquid water: a molecular dynamics study of structure and dynamics of the whole series.

In this work we have extended our previously presented polarizable pair interaction potential for La(3+)-water [Duvail et al., J. Chem. Phys. 127, 034503 (2007)] to the whole lanthanoid(III) series (Ln(3+)) interacting with water. This was performed taking into account known modification of ionic radius and atomic polarizability across the series and thus changing potential parameters according to that. Our procedure avoids the hard task of doing expensive high level ab initio calculations for all the atoms in the series and provides results in good agreement with experimental data and with ab initio calculations performed on the last atom in the series (Lu(3+), the atom for which the extrapolation should be in principle much crude). Thus we have studied the hydration properties of the whole Ln(3+) series by performing classical molecular dynamics in liquid phase. This systematic study allows us to rationalize from a microscopic point of view the different experimental results on Ln(3+)-water distances, first shell coordination numbers and first shell water self-exchange reactivity. In particular, we found that across the series the coordination number decreases from 9 for light lanthanoids to 8 for heavy lanthanoids in a continuous shape. This is due to the continuous changing in relative stability of the two forms that can be both populated at finite temperature with different probabilities as a function of the Ln(3+) atomic number. The changeover of the Ln(3+) ionic radius across the series resulted to be the main driving physical properties governing not always the Ln(3+)-water distance changing across the series but also the observed coordination number and consequently ligand dynamics.

A dynamic model to explain hydration behaviour along the lanthanide series.

From nine to eight : Molecular dynamics simulations of all the lanthanide cations in water show that the change in first shell coordination number from nine to eight water molecules (see figure) is not a sudden change in behaviour. Instead, it results from a statistical predominance of one first hydration shell structure containing nine to eight water molecules.

Molecular Dynamics Studies of Concentrated Binary Aqueous Solutions of Lanthanide Salts: Structures and Exchange Dynamics

Concentrated binary aqueous solutions of lanthanide (Nd(3+) and Dy(3+)) salts (ClO(4)(-), Cl(-), and NO(3)(-)) have been studied by means of classical molecular dynamics (MD) simulations with explicit polarization and UV-visible spectroscopy. Pair interaction potentials, used for the MD simulations, have been developed in order to reproduce experimental hydration properties. Nd(3+) and Dy(3+) have been chosen because of their position in the lanthanide series: Nd(3+) being a light lanthanide and Dy(3+) a heavy one. They are respectively coordinated to nine and eight water molecules, in pure water, involving changes in their salt hydration structures. Both MD simulations and UV-visible experiments highlight the stronger affinity of nitrate anions toward Ln(3+) compared to perchlorates and chlorides. Dissociation/association processes of Nd(3+)-Cl(-) and Nd(3+)-NO(3)(-) ion pairs in aqueous solution have been analyzed using potential of mean force profile calculations. Furthermore, from MD simulations, it appears that the affinity of anions (perchlorate, chloride, and nitrate) is stronger for Nd(3+) than Dy(3+).

Complexation of lanthanides(III), americium(III), and uranium(VI) with bitopic N,O ligands: an experimental and theoretical study.

New functionalized terpyridine-diamide ligands were recently developed for the group actinide separation by solvent extraction. In order to acquire a better understanding of their coordination mode in solution, protonation and complexation of lanthanides(III), americium(III), and uranium(VI) with these bitopic N,O-bearing ligands were studied in homogeneous methanol/water conditions by experimental and theoretical approaches. UV-visible spectrophotometry was used to determine the protonation and stability constants of te-tpyda and dedp-tpyda. The conformations of free and protonated forms of te-tpyda were investigated using NMR and theoretical calculations. The introduction of amide functional groups on the terpyridine moiety improved the extracting properties of these new ligands by lowering their basicity and enhancing the stability of the corresponding 1:1 complexes with lanthanides(III). Coordination of these ligands was studied by density functional theory and molecular dynamics calculations, especially to evaluate potential participation of hard oxygen and soft nitrogen atoms in actinide coordination and to correlate with their affinity and selectivity. Two predominant inner-sphere coordination modes were found from the calculations: one mode where the cation is coordinated by the nitrogen atoms of the cavity and by the amide oxygen atoms and the other mode where the cation is only coordinated by the two amide oxygen atoms and by solvent molecules. Further simulations and analysis of UV-visible spectra using both coordination modes indicate that inner-sphere coordination with direct complexation of the three nitrogen and two oxygen atoms to the cation leads to the most likely species in a methanol/water solution.

Recycling metals by controlled transfer of ionic species between complex fluids: en route to “ienaics”

Recycling chemistry of metals and oxides relies on three steps: dissolution, separation and material reformation. We review in this work the colloidal approach of the transfer of ions between two complex fluids, i.e. the mechanism at the basis of the liquid-liquid extraction technology. This approach allows for rationalizing in a unified model transformation such as accidently splitting from two to three phases, or uncontrolled viscosity variations, as linked to the transformation in the phase diagram due to ion transfer. Moreover, differences in free energies associated to ion transfer between phases that are the origin of the selectivity need to be considered at the meso-scale beyond parameterization of an arbitrary number of competing “complexes”. Entropy and electrostatics are taken into account in relation to solvent formulation. By analogy with electronics dealing about electrons transported in conductors and semi-conductors, this “ienaic” approach deals with ions transported between nanostructures present in colloidal fluids under the influence of chemical potential gradients between nanostructures coexisting in colloidal fluids. We show in this review how this colloidal approach generalizes the multiple chemical equilibrium models used in supra-molecular chemistry. Statistical thermodynamics applied to self-assembled fluids requires only a few measurable parameters to predict liquid-liquid extraction isotherms and selectivity in multi-phase chemical systems containing at least one concentrated emulsified water in oil (w/o) or oil in water (o/w) microemulsion.

Polarizable interaction potential for molecular dynamics simulations of actinoids(III) in liquid water.

In this work, we have developed a polarizable classical interaction potential to study actinoids(III) in liquid water. This potential has the same analytical form as was recently used for lanthanoid(III) hydration [M. Duvail, P. Vitorge, and R. Spezia, J. Chem. Phys. 130, 104501 (2009)]. The hydration structure obtained with this potential is in good agreement with the experimentally measured ion-water distances and coordination numbers for the first half of the actinoid series. In particular, the almost linearly decreasing water-ion distance found experimentally is replicated within the calculations, in agreement with the actinoid contraction behavior. We also studied the hydration of the last part of the series, for which no structural experimental data are available, which allows us to provide some predictive insights on these ions. In particular we found that the ion-water distance decreases almost linearly across the series with a smooth decrease of coordination number from nine to eight at the end.

Accounting for adsorption and desorption in Lattice Boltzmann simulations

We report a Lattice-Boltzmann scheme that accounts for adsorption and desorption in the calculation of mesoscale dynamical properties of tracers in media of arbitrary complexity. Lattice Boltzmann simulations made it possible to solve numerically the coupled Navier-Stokes equations of fluid dynamics and Nernst-Planck equations of electrokinetics in complex, heterogeneous media. With the moment propagation scheme, it became possible to extract the effective diffusion and dispersion coefficients of tracers, or solutes, of any charge, e.g., in porous media. Nevertheless, the dynamical properties of tracers depend on the tracer-surface affinity, which is not purely electrostatic and also includes a species-specific contribution. In order to capture this important feature, we introduce specific adsorption and desorption processes in a lattice Boltzmann scheme through a modified moment propagation algorithm, in which tracers may adsorb and desorb from surfaces through kinetic reaction rates. The method is validated on exact results for pure diffusion and diffusion-advection in Poiseuille flows in a simple geometry. We finally illustrate the importance of taking such processes into account in the time-dependent diffusion coefficient in a more complex porous medium.

Atomistic Description of Binary Lanthanoid Salt Solutions: A Coarse-Graining Approach

The experimental difficulties inherent to the solution chemistry of actinoids and lanthanoids have led to the use of a wide variety of models, from the microscopic to the macroscopic scale, in an attempt to represent their solution properties. Molecular dynamics (MD) simulations, with explicit solvents, have been successfully used to describe the structural characteristics, but the limits on the accessible length and time scales do not allow for an equivalent description of the macroscopic properties. In this study, we propose a multiscale approach, based on MD simulation results, to study the thermodynamic and structural properties of a series of lanthanoid-chloride aqueous solutions. An inversion procedure, based on the approximate hypernetted chain (HNC) closure and the Stillinger-Lovett sum rules for ionic liquids, is used to obtain the effective ion-ion potentials from MD-generated radial distribution functions (RDF). Implicit solvent Monte Carlo (MC) simulations are then performed to compute the osmotic coefficients of the salt solutions. This coarse-grained strategy provides accurate effective pair potentials for the lanthanoid salts, derived from an atomic model. The method presented here is an attempt to bridge the gap between MD and the thermodynamic properties of solutions that are experimentally measured.

Predicting for thermodynamic instabilities in water/oil/surfactant microemulsions: A mesoscopic modelling approach

The thermodynamics and structural properties of flexible and rigid nonionic water/oil/surfactant microemulsions have been investigated using a two level-cut Gaussian random field method based on the Helfrich formalism. Ternary stability diagrams and scattering spectra have been calculated for different surfactant rigidities and spontaneous curvatures. A more important contribution of the Gaussian elastic constants compared to the bending one is observed on the ternary stability diagrams. Furthermore, influence of the spontaneous curvature of the surfactant points out a displacement of the instability domains which corresponds to the difference between the spontaneous and effective curvatures. We enlighten that a continuous transition from a connected water in oil droplets to a frustrated locally lamellar (oil in water in oil droplets) microstructure is found to occur when increasing the temperature for an oil-rich microemulsion. This continuous transition translated in a shift in the scattering functions, points out that the phase inversion phenomenon occurs by a coalescence of the water droplets.