The structure and dynamics of the interface between the edges of clay nano-particles and aqueous solutions by molecular modelling
Maxime Pouvreau and Andrey Kalinichev
Clay minerals consist of layered tetrahedral silicate sheets and octahedral hydroxide sheets, where the octahedral sites are occupied by Al in the unsubstituted structure. In many cases, various so-called 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 potential host rocks for radioactive waste repository, good understanding of the molecular mechanisms controlling radionuclide sorption and mobility in clays is necessary.
Clay surfaces can be divided into basal (parallel to the layer), and edge surfaces. When the cleavage of the clay crystal is imperfect, the sites are heterogeneously protonated in the presence of water. The figure illustrates a typical case of the edge surfaces in pyrophillite, the unsubstituted clay structure. In this way, as an addition to the ability of all surfaces to complex with various aqueous species, edge surfaces are also the place of acid-base reactions.
While molecular simulations involving classical force fields can be used to study chemically inert surfaces, quantum chemistry calculations are necessary to fully describe their reactivity. In the literature, such calculations deal with the acidity of edge sites, and with the structure and dynamics of water and complexes at the edges.
Using ab initio molecular dynamics simulations and geometry optimizations, the goals of this PhD work are to improve the classical force field ClayFF parameters related to the surface hydroxyls and to further describe the molecular processes (acid-base reactivity, complexation) occurring at clay edges.