A systematic investigation of monatomic spherical sorbates in the supercages of zeolites Y and A by molecular dynamics technique is presented. Rates of intercage diffusion, rates of cage visits, and the diffusion coefficients have been calculated as a function of the sorbatezeolite interaction strength. These properties exhibit markedly different dependences on interaction strength for the two zeolites. The observed behavior is shown to be a consequence of the two principal mechanisms of intercage diffusion and the energetic barrier associated with them. The diffusion coefficient and other properties associated with intercage diffusion are found to be directly proportional to the reciprocal of the square of the sorbate diameter when the sorbate diameter is significantly smaller than the window diameter. As the sorbate diameter increases, a peak is observed in all the transport properties investigated including the diffusion coefficient. We call this surprising effect as the ring or levitation effect and it explains several anomalous results reported in the literature and suggests a breakdown of the geometrical criterion for diffusion of sorbates. It shows that under certain conditions nongeometrical factors play a major role and geometrical factors become secondary in the determination of the molecular sieve property. A generalized parameter has been proposed which suggests conditions under which one can expect the ring or levitation effect in any porous medium. Inverse size selectivity becomes operative under these conditions.

The sorption properties of xenon sorbed in sodium Y zeolite as a function of adsorbate concentration and temperature have been obtained from molecular dynamics simulations. The properties reported include the various site-site a center of self-diffusion coeffcients, the power spectra, and the distribution of site residence times. The location of the physical adsorption site for xenon is the same as for methane. The guest-host energy distribution function for xenon differs significantly from the bimodal function observed for methane. It is shown that the mean square displacement shows a crossover from ballistic to diffusive behaviour and the activation energy for diffusion is 4.1 kJ/mol. We suggest that the 15-cm-l frequency mode seen in the power spectra of the autocorrelation function corresponds to the XeXe dimers. The nonavailability of sorption sites at higher adsorbate concentrations is reflected in the guest-host distribution function, center of cage-center of mass radial distribution function, power spectra, and other properties.cage-center of mass radial distribution functions, distribution of guest-host energy, guest-guest pair and bon d ' g energy, IntroductionThe interest in the study of adsorption properties in restricted regions is growing. Several materials in nature permit the adsorption of small to medium sized guest molecules in voids of the order of several molecular diameters. These are known by the general name micropores. Some of these are simple adducts such as the urea-alkanes with purely physical interaction. The others

Molecular dynamics calculations on sorbate atoms of varying diameters in zeolites A and Y are reported. Rates of cage-to-cage diffusions, rate of cage visits, and diffusion coefficients show unexpected behavior. A generalized parameter has been defined that exhibit a peak in diffusion coefficient around 0.84 for both the zeolites investigated.

Various geometrical and energetic distribution functions and other properties connected with the cage-to-cage diffusion of xenon in sodium Y zeolite have been obtained from long molecular dynamics calculations. Analysis of diffusion pathways reveals two interesting mechanisms-surface-mediated and centralized modes for cageto-cage diffusion. The surface-mediated mode of diffusion exhibits a small positive barrier, while the centralized diffusion exhibits a negative barrier for the sorbate to diffuse across the 12-ring window. In both modes, however, the sorbate has to be activated from the adsorption site to enable it to gain mobility. The centralized diffusion additionally requires the sorbate to be free of the influence of the surface of the cage as well. The overall rate for cage-to-cage diffusion shows an Arrhenius temperature dependence with Ea = 3 kJ/mol. It is found that the decay in the dynamical correction factor occurs on a time scale comparable to the cage residence time. The distributions of barrier heights have been calculated. Functions reflecting the distribution of the sorbate-zeolite interaction at the window and the variations of the distance between the sorbate and the centers of the parent and daughter cages are presented.

Simulations were carried out on 138-molecule clusters freezing isothermally at 130, 120, and 80 K. At 120 K, the nucleation rate was the same as in our prior simulations performed adiabatically but the final product was different. During the nanosecond period of the runs, clusters transforming adiabatically had frozen to bcc crystals while warming from 120 K to about 130 K. On the other hand, isothermal clusters at 120 and 130 K changed to monoclinic clusters after passing through the bcc phase. Clusters cooled to 80 K froze to a variety of structures. The number of molecules whose Voronoi polyhedra qualified them as being in bcc embryos grew in size erratically, and in most runs it was difficult to use the Voronoi information by itself to identify a well-characterized nucleation time. Therefore, a more discriminating criterion for the onset of nucleation was devised. The 138-molecule clusters proved to be too small to yield definitive profiles of the several order parameters characterizing the change from the liquid phase to the critical nucleus. Even though the sizes of the nuclei were not established accurately, it was clear that critical nuclei were considerably larger than the five-molecule size forecast by the classical theory of homogeneous nucleation. At the deep supercooling of the simulations, precritical and critical nuclei were extremely ramified and haphazard in molecular orientation, but the chaotically organized nuclei at 120 and 130 K quickly annealed and grew to single crystals in most clusters. Clues were found suggesting that surface molecules may participate in the formation of critical nuclei, contrary to our long-standing belief. From nucleation rates were derived the kinetic parameters σsl, the solid−liquid interfacial free energy of the classical nucleation theory, and δ, the interface thickness of Gránásy's diffuse interface theory (DIT). In addition, the effect of pressure on the DIT, a new treatment of errors, and an improved weighted least-squares analysis of nucleation data were developed.