A molecular modeling exploration of smectite interlayers as adsorption sites for inorganic and organic molecules.

摘要

The abundance and reactivity of smectite minerals have prompted considerable scientific and engineering interest in these negatively charged, shrinking and swelling, layer type aluminosilicates. Molecular modeling techniques provide a uniquely molecular scale view of the complex interactions that occur within a smectite interlayer region between interlayer species and surrounding mineral surfaces. Atomistic simulations describing two hydrated smectite interlayers illustrate the effects of mineral structure and solution properties on adsorption mechanisms available to inorganic and organic species.; Detailed understanding of Cs-smectites is essential to accurate prediction of clay liner permeability to radiocesium cations at nuclear waste containment facilities. Monte Carlo (MC) and molecular dynamics (MD) calculations performed on three Cs-smectites suggested that Cs-saturated interlayers contained less than a monolayer of water molecules, organized into partial hydration shells around the cations. Cs+ formed strong inner sphere complexes with the mineral surface and displayed jump diffusional motion. Water molecules exhibited typical diffusional movements that accelerated with increased water content. Animation of the MD trajectories of interlayer species revealed the phenomenon of water sharing, when a single water molecule hydrates two cations simultaneously for hundreds of picoseconds. Differences in smectite mineral structure, including charge site distribution and hydroxyl group orientation, affected the prevalence of water sharing interactions, as well as the location and mobility of Cs+.; To manage the landscape to sequester carbon and reduce the severity of climate change, we must expand our knowledge of the organo-mineral interactions responsible for long-term recalcitrance of humic substances. A model humic molecule, subjected to energy minimization (EM) and MD calculations for different hydration and protonation states, mimicked experimental properties of humic substances, including bulk density, infrared spectrum, pseudomicellar reorientation with hydrated, and relative complexation of Na+ and Ca 2+. When placed within a hydrated Ca-montmorillonite interlayer, the deprotonated, Ca-saturated version of this model humic molecule adsorbed to the mineral surface via numerous cation bridges, a few water bridges, and indirect H-bonding interactions mediated by water molecules. The protonated version of this organic molecule formed direct hydrophobic and H-bonding interactions with the mineral. Both studies highlight the complex molecular scale surface chemistry associated with smectite surfaces.