Translational dynamics of water in a nanoporous layered silicate

Abstract

Neutron time-of-flight and backscattering spectroscopy have been used to study the translational diffusion of water molecules in the unusual layered material AMH-3, which consists of (zeolitelike) three-dimensionally nanoporous silicate layers spaced by (claylike) interlayer regions. The synthesis of AMH-3 and its characterization by $^{29}\mathrm{Si}$ NMR, Raman, and infrared spectroscopy, are described. An analysis of quasielastic neutron scattering (QENS) spectra using the random jump diffusion model reveals two translational diffusive motions clearly separated in time scales: a fast process ($D\ensuremath{\sim}{10}^{\ensuremath{-}9}\phantom{\rule{0.3em}{0ex}}{\mathrm{m}}^{2}∕\mathrm{s}$ at 300 K), and a much slower process ($D\ensuremath{\sim}{10}^{\ensuremath{-}11}\phantom{\rule{0.3em}{0ex}}{\mathrm{m}}^{2}∕\mathrm{s}$ at 300 K). Considering the structural model of AMH-3 and the transport properties extracted from the QENS data, it is suggested that the slower motion corresponds to diffusion by water molecules in the interlayer spaces whereas the fast process involves diffusion in the silicate layer. This first investigation of transport phenomena in nanoporous layered silicates like AMH-3 indicates that they have the potential to offer mass transport properties different from zeolite materials and layered clays.