Jean-Pierre Simonin

Lanthanoids(III) and actinoids(III) in water: Diffusion coefficients and hydration enthalpies from polarizable molecular dynamics simulations

By using polarizable molecular dynamics (MD) simulations of lanthanoid(III) and actinoid(III) ions in water, we obtained ionic diffusion coefficients and hydration enthalpies for both series. These values are in good agreement with experiments. Simulations thus allow us to relate them to microscopic structure. In particular, across the series the diffusion coefficients decrease, reflecting the increase of ion–water interaction. Hydration enthalpies also show that interactions increase from light to heavy ions in agreement with experiment. The apparent contradictory result of the decrease of the diffusion coefficient with decreasing ionic radius is tentatively explained in terms of dielectric friction predominance on Stokes’ diffusive regime.

Varying the charge of small cations in liquid water: Structural, transport, and thermodynamical properties

In this work, we show how increasing the charge of small cations affects the structural, thermodynamical, and dynamical properties of these ions in liquid water. We have studied the case of lanthanoid and actinoid ions, for which we have recently developed accurate polarizable force fields, and the ionic radius is in the 0.995-1.250 Å range, and explored the valency range from 0 to 4+. We found that the ion charge strongly structures the neighboring water molecules and that, in this range of charges, the hydration enthalpies exhibit a quadratic dependence with respect to the charge, in line with the Born model. The diffusion process follows two main regimes: a hydrodynamical regime for neutral or low charges, and a dielectric friction regime for high charges in which the contraction of the ionic radius along the series of elements causes a decrease of the diffusion coefficient. This latter behavior can be qualitatively described by theoretical models, such as the Zwanzig and the solvated ion models. However, these models need be modified in order to obtain agreement with the observed behavior in the full charge range. We have thus modified the solvated ion model by introducing a dependence of the bare ion radius as a function of the ionic charge. Besides agreement between theory and simulation this modification allows one to obtain an empirical unified model. Thus, by analyzing the contributions to the drag coefficient from the viscous and the dielectric terms, we are able to explain the transition from a regime in which the effect of viscosity dominates to one in which dielectric friction governs the motion of ions with radii of ca. 1 Å.

ROTATING MEMBRANE CELL TECHNIQUE FOR THE STUDY OF LIQUID/LIQUID EXTRACTION KINETICS.

ABSTRACT The rotating membrane cell technique is described. Tim method has been tested on two types of systems: acetic acid extracted by isopropyl myristate, and Nickel and Zinc ions by di(2-ethylhexyl) pliosphoric acid (D2EHPA). The technique consists of a membrane, containing one oT the phases, that is glued on the base of a cylinder and that is rotated at. a definite speed. The main features of the technique are: simplicity of the set-up; good control of the transport in both phases; transfer occurring in a transient regime, limited by the kinetics of the chemical reactions at the beginning of the transfer process. Moreover, the technique offers the possibility of stirring only one of the two phases. An approximate mathematical solution is given for the case or an iuterfacial transfer reaction. The results for acetic acid are in accordance with such a mechanism. In contrast, the results for the metal cations cannot bo interpreted in the same way. The data may suggest the influence of a transfer of D2EH...

Kinetics of Liquid/Liquid Extraction of Europium(III) Cation by Two Malonic Diamides

In the framework of the DIAMEX separation process, the kinetics of extraction of Eu(III) ion by two diamides, DMDBTDMA and DMDOHEMA, have been studied using the rotating membrane cell technique. The kinetics with DMDBTDMA from a nitric acid solution were found to be very fast, in keeping with a previous work using the Nitsch cell, and in disagreement with another work using a Lewis-type cell. A simple model for turbulent transport in the latter cell is proposed to interpret this result. The extraction kinetics with DMDOHEMA was found to be slower than with DMDBTDMA at 22°C. The activation energy of the interfacial reaction was deduced from experiments at 11°C, 22°C, and 33°C. The activation energy of the distribution ratio is shown to have a particular effect on that of the overall kinetic rate constant in the general case.

Effect of polarization on the solubility of gases in molten salts

The solubility of noble gases and water in molten salts is predicted by developing an analytical parameter-free description in terms of polarizable hard sphere (the gas particle) in a medium composed of charged hard spheres of comparable size (the salt). The chemical potential of solute contains contributions from excluded volume, polarization, and dispersion forces. The polarization of the gas particle is calculated explicitly within the framework of the mean spherical approximation for the ion-dipole mixture. An additional contribution originating from the polarization of the salt is proposed. This effect has been overlooked in previous theoretical studies. Its magnitude is estimated in an approximate way. The description is found to give predictions that are in good agreement with experimental solubility data for noble gases and water in molten KCl and RbCl. The results suggest that volume exclusion and salt polarization may constitute the main two opposing factors affecting this phenomenon.