Abstract
Water diffusion within smectite clay interlayers is reduced by confinement and hence is highly determined by the interlayer spacings that are adopted during swelling. However, a molecular understanding of the short- and long-range forces governing interlayer water structure and dynamics is lacking. Using molecular dynamics simulations of water intercalated between pyrophyllite (smectite prototype) layers we provide a detailed picture of the variation of interlayered water mobility accompanying smectite expansion. Subtle changes in hydrogen bond network structure cause significant changes in water mobility that is greater for stable hydration states and reduced for intermediate separations. By studying pyrophyllite with and without external water we reveal that long-range electrostatic forces apply a restraining effect upon interlayer water mobility. Our findings are relevant for broad range of confining nanostructures with walls thin enough to permit long-range interactions that could affect the mobility of confined solvent molecules and solute species.
Highlights
Clays are very abundant layered minerals in the Earth’s crust
The dynamics of water molecules confined between smectite layers differ from bulk water
It has been shown that interlayer water mobility is determined by several short-range interactions including intermolecular hydrogen bonding[14,15] and water-mineral interactions that are significant for highly confined fluids[16,17]
Summary
Interlayers received: 02 February 2016 accepted: 14 April 2016 Published: 27 April 2016. As shown by the plots of interlayer water density profiles (Fig. 2c), well-defined 1, 2 and 3-layer hydration states develop at layer distances that agree with experimental observations of low-charge smectite[6,9,11,12,13]. These stable hydration states exhibit the most structured water perpendicular to the layers, the plot of. Both acceptors and donors predominantly occupy the center of interlayer (2W, 2–3W; Fig. 2d), but as expansion increases further the HB-donors start to dominate in the vicinity of the pyrophyllite layers (3W, 3–4W; Fig. 2d) These represent significant reorientations of the interlayer molecules, which strongly influenced water mobility. Long-range interactions across confining smectite layers may play an influential role in determining the mobility of intercalated ions and other species, and will be the topic of future work
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