Water dynamics in chabazite

Abstract

Self-diffusion, libration and reorientation of water molecules in the zeolite chabazite are examined by molecular dynamics (MD) computer simulations at different temperatures and loadings. Comparison with experiment provides satisfactory agreement of the self-diffusivities and excellent agreement of the anisotropy factors and explains the failure of their prediction by the concept of structure-correlated diffusion anisotropy. Long-range electrostatic interactions are found to affect self-diffusion at high loadings and temperatures. Spectral densities of librational motion are similar to those in aqueous salt solutions, while reorientation is much slower and much more anisotropic. The vector in direction of the molecular dipole moment reorients only very slowly, as a consequence of the attraction by the almost immobile Ca + + -ions and by the walls of the zeolite. The other two vectors seem to reorient by jumps rather than by rotational diffusion.