Jorge Gulín-González

Dynamical Properties of Confined Water Nanoclusters: Simulation Study of Hydrated Zeolite NaA: Structural and Vibrational Properties

Water nanoclusters confined to zeolitic cavities have been extensively investigated by various experimental techniques. We report a series of molecular dynamics simulations at different temperatures and for water nanoclusters of different sizes in order to attempt an atomistic interpretation of the properties of these systems. The cavities of zeolite NaA are spherical in shape and about 1 nm in diameter and can host nanoclusters of water containing nearly up to 24 water molecules. A modified interaction potential, yielding a better reproduction of experimental hydration energy and water diffusivity across a number of different zeolites, is proposed. Molecular dynamics simulations reproduce the known experimental structural features obtained by X-ray diffraction. Variations of simulated vibrational IR and IINS spectra with temperature and size of nanoclusters are in good agreement with experiment. The simulated water nanoclusters in zeolite NaA are found to be too small to crystallize and, at low temperature, behave as amorphous ice, in agreement with recent experimental results for similar water nanoclusters in reverse micelles.

Structure and dynamics of hydrogen-bonded water helices in high pressure hydrated phase of natrolite studied by molecular dynamics simulations

Publisher Summary Classical molecular dynamics (MD) simulation technique is used to attempt a prediction of structural and dynamical properties of the recently discovered superhydrated phase of natrolite. In particular, also in consideration of the severe experimental conditions, as the new phase is stable under hydrostatic pressure higher than 1.5 GPa, it was not possible to derive from X-ray diffraction experiments the positions of the water hydrogens, which are important to ascertain the hydrogen bond pattern. Moreover, nuclear magnetic resonance (NMR) and vibrational spectroscopy data are still lacking, so that it is not known how much the dynamical properties of natrolite change due to the transition to the new phase. Once it was verified that an optimized potential model for the interatomic interaction was able to reproduce reasonably the properties of natrolite at ambient pressure and the known part of the structure of the superhydrated phase of natrolite at high pressures, one could extend the calculations to some still unknown properties of the new phase. The vibrational spectra should not be deeply influenced by the deformation of the zeolite framework caused by the phase transition, but more evident qualitative changes are predicted for the vibrational modes involving adsorbed water molecules and sodium cations. Finally, a preliminary study of the possible dehydration and super hydration mechanisms was attempted, suggesting that both are defect driven, and helped by jumps of the sodium cations.

Computer simulations of ethane sorbed in an aluminophosphate molecular sieve

Classical Molecular Dynamics (MD) simulations have been carried out to study the dynamic properties of ethane sorbed in an AlPO 4 -5 aluminophosphate. The main purpose is to gain new insight into the diffusive regime controlling the motion of ethane in such one-dimensional microporous structure. In recent experimental studies a standard MSD vs. t linear dependence (normal diffusion) was found, while earlier experiments seemed to reveal a single-file regime for this system. The present calculations show that ethane diffusion follows the standard regime on the time scale of MD simulations: particle passing (leading to normal unidirectional diffusion) is rather frequent over observation times of several nanoseconds. Consequently a single file regime for this system could only be achieved through the formation of stable clusters of ethane molecules that, once formed, move as single larger species in a single file regime. This possiblity has been tested by calculating the lifetimes of ethane clusters of different size: the large lifetimes observed are less than 1 ns, and are further reduced when the flexibility of the host lattice is incorporated in the model. Therefore these simulation support the experimental observation that normal diffusion dominates the motion of ethane in the AlPO 4 -5.