Molecular dynamics simulation of anomalous diffusion of one-dimensional water molecule chains in Li-ABW zeolite

Nanosecond scale classical molecular dynamics simulations of the behaviour of the one-dimensional water molecule chains adsorbed in zeolite Li-ABW, a lithium containing zeolite, were performed at different temperatures, both for the fully hydrated material, where diffusion of water is not present, and for the partially hydrated material, where diffusion occurs. The structure and the vibrational spectrum of fully hydrated Li-ABW were in agreement both with experimental data and Car-Parrinello molecular dynamics results. Classical molecular dynamics simulations were extended to the nanosecond time scale in order to study the flip motion of water molecules around their bisectors and the dehydration mechanism at high temperature. Based on the results of the simulations, it may be suggested that the dehydration proceeds by stepwise single-file diffusion. The diffusive mechanism appears as a single-file motion: the molecules never pass one another, even at temperatures as high as about 800 K, nor can switch between different channels. If the mean square displacements (MSDs) are evaluated with respect to the centre of mass of the molecules hosted in each channel, the expected dependence on the square root of time appears. The results are compared with those obtained for bikitaite, another Li containing zeolites showing a similar structure. Finally, the behaviour of Li cations at different temperature and hydration is discussed.