Dynamical behavior of one-dimensional water molecule chains in zeolites: Nanosecond time-scale molecular dynamics simulations of bikitaite

Abstract

Nanosecond scale molecular dynamics simulations of the behavior of the one-dimensional water molecule chains adsorbed in the parallel nanochannels of bikitaite, a rare lithium containing zeolite, were performed at different temperatures and for the fully and partially hydrated material. New empirical potential functions have been developed for representing lithium-water interactions. The structure and the vibrational spectrum of bikitaite 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 the hydrogen bonds connecting adjacent molecules in the chains, which has been observed by NMR experiments, and the dehydration mechanism at high temperature. Computed relaxation times of the flip motion follow the Arrhenius behavior found experimentally, but the activation energy of the simulated system is slightly underestimated. Based on the results of the simulations, it may be suggested that the dehydration proceeds by a defect-driven stepwise diffusion. The diffusive mechanism appears as a single-file motion: the molecules never pass one another, even at temperatures as high as about 1000 K, nor can they switch between different channels. However, the mean square displacement ͑MSD͒ of the molecules, computed with respect to the center of mass of the simulated system, shows an irregular trend from which the single-file diffusion cannot be clearly evidenced. If the MSDs are evaluated with respect to the center of mass of the molecules hosted in each channel, the expected dependence on the square root of time finally appears.

A Synthetic Zwitterionic Water Channel: Characterization in the Solid State by X‐ray Crystallography and NMR Spectroscopy

“…This very narrow resonance is characteristic of water that tumbles very rapidly and was therefore assigned to adsorbed or surface water. It is not clear if these water molecules are released from the channel as a direct consequence of water transport through the channel, as found in other one-dimensional water-channel systems, [26][27][28] or due to a rather unlikely diffusion process through the "walls" of the nanotubes. In samples with less than 100 % water content, water molecules do not appear to leave the tubes when the temperature increases, but move along the vacant sites.…”

“…The existence of vacant sites was shown to be critical for diffusion processes observed in zeolites containing one-dimensional water channels. [27] Studies on helical zinc complexes of carboxylic acids have been previously reported, [29,30] including conducting helical polymers, [31] of which zwitterionic polymers are of special interest. [32] In this work, however, the water molecules in the helical channel have been studied by 2 H NMR spectroscopy, and it was possible to differentiate the two different types of water molecules.…”

Nanofluidics: what is it and what can we expect from it?

“…This reversal in orientation is thought to prevent proton conduction by the Grotthus mechanism (Tajkhorshid et al 2002). Single-file motion of water is predicted by MD simulations of carbon nanotubes, aquaporins and also for nanochannels naturally occurring in zeolites (Demontis et al 2004).…”

Spreading in narrow channels

“…Such systems show the interesting feature of non-diffusive behavior of tracer particles, which stimulated experimental (see, e.g., Refs. [33,34]) and numerical [27,29,30,31] interest. Here we present a lattice model for ultrathin films in which multiple occupancy of a site is allowed (generalizing the single-occupancy model of Refs.…”

“…[25,26,27,28,29,30,31,32,33,34]). Such systems show the interesting feature of non-diffusive behavior of tracer particles, which stimulated experimental (see, e.g., Refs.…”