No abstract

No abstract

The paper presents experimental investigation of the crystallization kinetics of thin (from several and up to 100 monolayers, ML) amorphous ice films grown in a vacuum on clean and adsorbate modified Pt(111) and graphite (0001) surfaces. The crystallization kinetics was followed by the associated decrease of the desorption rate by isothermal desorption mass spectroscopy. The process is strongly substrate dependent for thin films (below 20-30 ML), gradually becoming substrate independent in 30-100 ML range, and approaching the bulk ice characteristics for thicker films. For thin films, the enthalpy of vaporization is 52 kJ/mol for the amorphous and 54 kJ/mol for the crystalline films. The activation energy for crystallization was estimated to be 75 kJ/mol. The crystallization is accompanied by an enhanced mobility in the film as demonstrated experimentally by following O2 release during crystallization of ice films on O2 precovered Pt(111).

No abstract

No abstract

Abstract:Monte Carlo and molecular dynamics simulations were performed to calculate solubility, S, and diffusion, D, coefficients of oxygen and water in polyethylene, and to obtain a molecular-level understanding of the diffusion mechanism. The permeation coefficient, P, was calculated from the product of S and D. The AMBER force field, which yields the correct polymer densities under the conditions studied, was used for the simulations, and it was observed that the results were not sensitive to the inclusion of atomic charges in the force field. The simulated S for oxygen and water are higher and lower than experimental data, respectively. The calculated diffusion coefficients are in good agreement with experimental data. Possible reasons for the discrepancy in the simulated and experimental solubilities, which results in discrepancies in the permeation coefficients, are discussed. The diffusion of both penetrants occurs mainly by large amplitude, infrequent jumps of the molecules through the polymer matrix.

The Gibbs-ensemble Monte Carlo methods based on the extended single point charge [H. J. C. Berendsen, J. R. Grigera, and T. P. Straatsma, J. Phys. Chem. 91, 6269 (1987)] potential-energy surface have been used to study the clustering of vapor phase water under vapor-liquid equilibrium conditions between 300 and 600 K. It is seen that the number of clusters, as well as the cluster size, increase with temperature. This is primarily due to the increase in vapor density that accompanies the temperature increase at equilibrium. In addition, due to entropic effects, the percentage of clusters that have linear (or open) topologies increases with temperature and dominates over the minimum-energy cyclic topologies at the temperatures studied here. These results are insensitive to the number of molecules used in the simulations and the criterion used to define a water cluster.

No abstract