mardi 19 mai 2015 à 16:00
Clay minerals consist of layered tetrahedral silicate sheets and octahedral hydroxide sheets. In many cases, various isomorphic substitutions are possible on both tetrahedral (e.g., Al3+ for Si4+) for and octahedral (e.g., Mg2+ for Al3+; Fe2+ for Al3+, etc.) sites. This leads to a very significant compositional and structural diversity of clays. The substitutions create a negative charge on the clay layer, which is usually compensated by various interlayer cations (alkali metals, alkaline earths, etc.)
which can be easily hydrated. Because of the potential large-scale use of clay minerals as host rocks for radioactive waste repository, good understanding of the molecular mechanisms controlling radionuclide sorption and mobility in clays is necessary.
Clay surfaces interacting with ions can be divided into two types: basal surfaces (parallel to the layering), and edge surfaces. When the cleavage of the clay crystal is imperfect, the sites can be heterogeneously protonated. Therefore, in addition to the ability of all surfaces to offer sites for complexation with various aqueous species, edge surfaces are also the place of active proton transfers. While molecular simulations involving the current classical force fields can be used to study the stable basal surfaces, quantum chemical calculations are necessary to fully describe the dynamics and reactivity of the edge surfaces. However, quantum methods are computationally much more demanding by many orders of magnitude than the classical modelling methods. Therefore, they are much more limited by the time scale and the sizes of the simulated systems and processes, and cannot fully describe the true compositional and structural complexity of clays. Consequently, it is still necessary to use classical molecular modelling methods for larger-scale simulations. This requires improvements in the description of edge surfaces at the classical level with the help of the higher level quantum modelling methods, such as DFT.
A new three-body term (3B) for the ClayFF force field has been recently introduced by Zeitler et al.  for dry brucite surfaces, using a vibrational analysis. As an extension of that work, we present here: (i) a detailed quantitative assessment of the capacity of ClayFF-3B to describe a wider range of brucite surfaces (fully coordinated edge surfaces, hydrated basal and edge surfaces); (ii) further development of the ClayFF-3B model to include the description of the surfaces of other similarly structured minerals (gibbsite, portlandite) and, later, to clays. To that end, DFT calculations were performed for these systems using the GGA approximation. The metal–O–H angular bending term was fitted for bulk, basal, and edge models of the hydroxyls using structural properties and vibrational modes. To measure the contribution of the 3B term to the description of surface hydroxyls, DFT, ClayFF and ClayFF-3B molecular dynamics were compared in terms of the metal–O–H angle distributions and the vibrational density of states.