Effects of surface cations on the structure and dynamics of the hydrogen bonding network at the illite - water interface: a molecular dynamics simulation study

Document available in extended abstract form only. Safe and sustainable management of nuclear energy poses major scientific and engineering challenges, one of which is the necessity to make the environmental impacts of the long-term nuclear waste storage as small as possible. This requires detailed understanding and prediction of the behaviour of radionuclides and their migration and retention properties in the geological formations of nuclear waste repositories. The Callovo-Oxfordian rock formation of the French nuclear repository site is mainly composed of clay minerals, quartz, calcite, with some non-negligible amount of organic matter. The adsorption of water can change the properties of mineral surfaces, including protonation state, surface charge, structure and reactivity. Similarly, the properties of interfacial water are strongly affected by the mineral substrate structure and composition. Recent advances in experimental techniques such as FTIR, ellipsometry, synchrotron X-ray scattering, sum-frequency vibrational spectroscopy are capable of probing the properties of mineral-water interfaces at different levels of hydration. However, the surface-specific results of these experiments are often difficult to quantitatively interpret without having a reliable molecular scale picture of the underlying physical and chemical processes. Molecular computer simulations have become one of the most important tools in the study of such interfacial systems and phenomena by providing invaluable atomistic information on the underlying chemical and physical process. The present study is aimed at investigating the structural and dynamics effects of three different cations (K+, NH4+ and H3O+) exchanged at the hydrated surface of muscovite mica, which is taken here as a model illite. Molecular dynamics computer simulations were performed using the CLAYFF force field to investigate the important differences of the H- bonding configurations formed by the sorbed species, including H2O, NH4+ and H3O+, in contrast to the behaviour of spherical metal ions, such as K+. At the muscovite (001) surface, H2O can donate 2 H-bonds and accept 2 H-bonds, but it can also partially replace surface K+, because the hydrogens of H2O bear some positive charge. This behaviour was observed in previous MD simulations of the mica surface. Such surface - adsorbed H2O molecules have their negatively charged oxygen atoms exposed to the fluid phase and accessible for either H -bond acceptance from other H2Os or hydration of metal cations in outer - sphere coordination. For surface H3O+, the charges on the hydrogens are slightly higher than those of H2O, but the oxygen atom of hydronium is now almost hydrophobic, and cannot participate in a H-bond network. In contrast, NH4+ can equally well donate H-bonds to the surface O atoms and to the neighbouring H2O molecules, but it cannot participate in the hydration shell of a displaced metal cation. Thus, three similar species can provide for three greatly different structural, energetic and dynamical situations at the muscovite-water interface. Since the hydrogen-bonding network in any aqueous media provides a natural mechanism of forming low-barrier reaction paths for proton transfer in such systems, it is also an important phenomenon controlling the surface reactivity under various pH conditions. In addition, a detailed study of the structural characteristics of surface-adsorbed NH4+ provides a way for better understanding of the mechanisms of adsorption for organic molecules having amino-groups in their structure, which is quite common for natural organic matter. Each of the three systems was simulated at 7 different hydration states providing information on the structure and dynamics of the adsorbed water film in a wide range of relative humidity conditions. The atomic density profiles of water show significant layering at all hydration levels and the layering strongly depends upon the nature of the ionic species present on the surface. Our studies support the fact that the H3O+ ion is less strongly bound when compared to K+ on the muscovite surface as observed in earlier studies. At muscovite surfaces, both NH4+ and H3O+ cations establish strong hydrogen bonds with both the surface bridging oxygen atoms and also with the neighbouring H2O molecules at all hydration levels. However, we observed that the interactions are different for both species at low hydration levels. At low hydration levels, H3O+ prefers to strongly bind as 3-coordinated species to the surface than with the neighbouring H2O molecules. However, as the hydration levels increase, H3O+ binds as 2-coordinated species with the surface as is indicated by hydrogen bonding analysis. In contrast, irrespective of the hydration levels, NH4+ ion strongly interacts with the surface as 3- coordinated species because of its tetrahedral geometry. At the same time, we observe from hydrogen bond analysis that the hydrogen bonding network of water has been strongly influenced by the nature of the surface cations present at the mineral-water interface. The dynamics of water molecules were examined by self-diffusion coefficients from the mean square displacement of water oxygen. The diffusion mechanism is similar for K+ and NH4+ but was different for H3O+ in particular at the low hydration states. Furthermore, the spatial and orientations distributions of H2O and ions at the muscovite-water surface are analyzed in quantitative detail. All the simulation results are compared with available experimental data and the results of previous molecular simulations to provide reliable molecular view of the ions and water at the muscovite surface. (authors)