Molecular dynamics study on water and ions transport mechanism in nanometer channel of 13X zeolite

Abstract

Understanding the atomistic transport within nanoporous zeolite is essential in a broad field. Herein, molecular dynamics was systematically utilized to elucidate the migration mechanism of pure water and NaCl solution in the 13X zeolite. At nano-scale level, molecular simulation clearly distinguishes the wetting and saturating stages of water transport in porous zeolite. The interior skeleton of 13X zeolite provides plenty of oxygen sites with strong polarity, which attracts frontier water molecules to infiltrate the hydrophilic zeolite surface rapidly in nearly 2 ns. The ladder-like ingress curve for water molecules highlight the discontinuous remain-jump wetting process that is induced by the competition adsorption with oxygen sites between the subsequent water molecules and the front water molecules. Initial wetting on interior surface reduces significantly the permeability of subsequent water transport. Furthermore, the penetrated water molecules, competing oxygen sites with extra-framework cations, can cause cations to exhibit different hydration behavior. Additionally, the permeability of water flux throughout zeolite channel is reduced by 10% as the Na+ and Cl- are incorporated in solution. Na+ with large hydration shells are sieved by bottlenecks formed by the windows of 12-ring structure with a diameter of 7.38 Å. The accumulated Na+ at the entrance region associate with neighboring water molecules and hydroxyl oxygen atoms to form hydrated clusters and ionic pairs, which plays as a physical barrier in blocking water transport. Decoding of the transport mechanism of water and salt solutions in zeolites at the molecular level facilitates breaking the existing upper limit of energy storage density and water treatment efficiency.