Distributions of single-molecule properties as tools for the study of dynamical heterogeneities in nanoconfined water.

The explicit trend of the distribution functions of single-molecule rotational relaxation constants and atomic mean-square displacement are used to study the dynamical heterogeneities in nanoconfined water. The trend of the single-molecule properties distributions is related to the dynamic heterogeneities, and to the dynamic crossovers found in water clusters of different shapes and sizes and confined in a variety of zeolites. This was true in all the cases that were considered, in spite of the various shapes and sizes of the clusters. It is confirmed that the high temperature dynamical crossover occurring in the temperature range 200-230 K can be interpreted at a molecular level as the formation of almost translationally rigid clusters, characterized by some rotational freedom, hydrogen bond exchange and translational jumps as cage-to-cage processes. We also suggest a mechanism for the low temperature dynamical crossover (LTDC), falling in the temperature range 150-185 K, through which the adsorbed water clusters are made of nearly rigid sub-clusters, slightly mismatched, and thus permitting a relatively free librational motion at their borders. It appears that the condition required for LTDC to occur is the presence of highly heterogeneous environments for the adsorbed molecules, with some dangling hydrogen bonds or weaker than water-water hydrogen bonds. Under these conditions some dynamics are permitted at very low temperature, although most rotational motion is frozen. Therefore, it is unlikely, though not entirely excluded, that LTDC will be found in supercooled bulk water where no heterogeneous interface is present.

Computer simulations of dynamic crossover phenomena in nanoconfined water.

In order to study dynamic crossover phenomena in nanoconfined water we performed a series of molecular dynamics (MD) computer simulations of water clusters adsorbed in zeolites, which are microporous crystalline aluminosilicates containing channels and cavities of nanometric dimensions. We used a sophisticated empirical potential for water, including the full flexibility of the molecule and the correct response to the electric field generated by the cations and by the charged atoms of the aluminosilicate framework. In addition, the full flexibility of the aluminosilicate framework was included in the calculations. Previously reported and new simulations of water confined in a number of different types of zeolites in the temperature range 100-300 K and at various coverage are discussed in connection with the experimental data. Dynamic crossover phenomena are found in all the considered cases, in spite of the different shape and size of the clusters, even when the confinement hinders the formation of tetrahedral hydrogen bonds for water molecules. Hypotheses about the possible dynamic crossover mechanisms are proposed.

The behaviour of water confined in zeolites: molecular dynamics simulations versus experiment.

In order to study the behaviour of water adsorbed in zeolites, which are microporous crystalline aluminosilicates, whose channels and cavities of nanometric dimensions can host many different molecules, we developed a sophisticated empirical potential for water, including the full flexibility of the molecule and the correct response to the electric field generated by the cations and by the charged atoms of the aluminosilicate framework. The reproduction of experimental data by our potential model is similar or even better than that obtained from the first principles methods. The results of molecular dynamics simulations of water confined in a variety of zeolites (worm-like clusters in silicalite, spherical nanoclusters in zeolite A and ice-like nanotubes in AlPO(4)-5 and SSZ-24) at different temperatures and coverage (loading) are discussed in connection with the experimental data, whose overall good reproduction encourages the attempt of an atomic-scale description of structural and dynamical phenomena occurring in confined water, in particular in the supercooled regime. The results are also compared with simulations and experimental data on bulk water.

Statics and dynamics of ethane molecules in AlPO(4)-5: a molecular dynamics simulation study.

From an experimental perspective, there has been disagreement among researchers on whether ethane would display single-file or normal diffusive behavior in the channels of AlPO(4)-5. Pulsed field gradient nuclear magnetic resonance measurements implied single-file diffusion, while quasielastic neutron scattering showed normal diffusion. In this paper we present the results of extensive classical molecular dynamics simulations of the diffusion of ethane molecules adsorbed in AlPO(4)-5. Our aim is to provide microscopic details of the static and dynamic properties of the adsorbed molecules in order to verify whether the conditions for the single-file regime can be achieved in a nondefective AlPO(4)-5 crystal structure.