Molecular dynamics simulation of NH4-montmorillonite interlayer hydration: Structure, energetics, and dynamics

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The interaction of ammonium ions (NH4+) with clay is closely associated with many geochemistry, engineering, and environment issues. Molecular dynamics (MD) simulations were performed to investigate to the structure, energetics, and dynamics properties of ammonium-montmorillonite (NH4-Mt) at different hydration levels. The simulation results show that the layer spacing, immersion and hydration energy, atomic density, coordination number, self-diffusion coefficient and residence time of NH4-Mt vary with the water contents. The one-layer hydrate is the most energetically favorable at the water content of 5 H2O/uc but up to three-layer hydrate of NH4-Mt can potentially be formed at the high water content (13 H2O/uc). As the water content increases, some NH4+ ions which adsorbed in inner-sphere (IS) complexes at low hydration level can change to outer-sphere (OS) complexes, and further to random ions in a diffuse aqueous layer, with the interaction between NH4+ ions and basal surfaces becoming weaker. The H-bonding of N-Hn···Os, donated by hydrogen atoms of NH4+ to surface oxygen atoms, contributes to the adsorption of NH4+ ions besides the electrostatic attraction. NH4+ ions in inner-sphere complexes can reside above the hexagonal cavities longer and have more stable hydration shell at the low hydration level.