Benedetta Mennucci

Department of Chemistry, University of Pisa,
via G. Moruzzi 13, 56124 Pisa (ITALY)

Web: molecolab.dcci.unipi.it

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Abstract

The environment plays a fundamental role in determining properties and processes of molecular systems. Its modeling however represents a real challenge and approximate methods have to be introduced. A very effective strategy is to partition the whole system in two subsystems (the one where the property and/or the process of interest is localized at and the rest) and describe the two parts with different but coupled approaches. In the most successful formulation of this strategy, the subsystem of interest is treated at an accurate quantum chemical level while the rest is modeled through a classical model. In this lecture, an overview of the resulting hybrid methods (both in their continuum and atomistic formulations) will be presented and discussed in terms of their applicability and limitations.

S. Perez-Conesa, J. M. Martínez and E. Sánchez Marcos

Department of Physical Chemistry
University of Seville
41012 Seville, Spain

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Abstract

Knowledge of actinide aqueous solution confined in smectite clay interlayers are crucial when designing permanent geological high level radioactive waste facilities. The actinide ions are sealed using montmorillonite containing clays to prevent radionuclide scape¹. A computational study of [UO₂]²⁺ (aq) hydration structure and diffusion inside montmorillonite clay was accomplished. The first actinide-clay ab initio interaction potential has been created based on the Hydrated Ion concept^2,3. Additionally it is the first application of the hydrated ion model to a confined medium chemical problem. The developed potential was used to carry out NPT classical molecular dynamics simulations for Montmorillonite clay containg [UO₂]²⁺ with two possible interlayer water contents. The z-density profiles reflect the formation of stable pentaaquo uranyl outer-shell complexes in agreement with EXAFS studies³ without any external imposed constraint. The complex interacts with the clay by the formation of 1.4 hydrogen bonds between its first hydration shell and the surface. Groups of three Mg substitutions were identified as strong interaction sites of the pentahydrate. The constrictivity factor ,δ_int, calculated from the self-diffusion coefficients agrees semiquantitatively with experiment. Diffusion is enhanced by an increase of concentration of uranyl in the clay interlayer.

References

[1] Birkholzer, J. et al. Ann. Rev. Environ. Resources 2012, 37, 79–106.
[2]Pappalardo, R. R. et al. J. Phys. Chem. 1993, 97, 4500–4504.
[3]Martínez J.M. et al. J. Am. Chem. Soc. 1999, 121, 3175–3184.
[4]Catalano, J. G. et al. Geochimica et Cosmochimica Acta 2005, 69, 2995 –3005