Solid-liquid phase transitions
 
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Phase transitions in neutral and charged water clusters

To understand forming of clusters nucleation phenomena in general is of great importance to build a link between single molecules and their liquids and solids temperature [Laaksonen, Talanquer & Oxtoby, Annu. Rev. Phys. Chem. 46, 489-524 (1995)].  We have also performed MD simulations of solid-liquid phase-transitions in small water clusters,  pure and containing charged or neutral solute particles with (N = 8 – 64) in order to understand of the mechanisms behind nucleation, growth, melting, and freezing in small water aggregates. We are focusing on water cluster phase behaviour in pure clusters or in the presence of charged and neutral particles,  related to their size, charge, shape. Several different solid-liquid phase transition criteria: Lindemann index, caloric curve, analysis of the number of hydrogen bonds, visual observation of cluster structure, analysis of heat capacity behavior, radial distribution functions and diffusion motion, monitoring of average value of modulus of the cluster total dipole moment, have been used in recent studies of water clusters melting. All these criteria indicate a finite temperature interval in which the cluster undergoes a transition between the solid and liquid-like forms. The phase transition in small water aggregates is a continuous process, and its temperature interval may be as much as 20 K. The structure of water clusters undergoes numerous solid-solid transitions during a thermal evolution masking the precise melting point as the curves fluctuate. Two types of processes were observed:  Reorientation of water hydrogen atoms without oxygen displacements and cluster structure reorganization including oxygen movements. In all considered systems, numerous solid-solid polymorphic transitions were observed during the heating.  In freezing studies the structure analysis shows that the temperature decrease results in the forming of a fused ice-like crystal structure (in other words, the crystallization) only for small clusters with N up to 20. The difference between the melting and freezing temperatures is inside uncertainty limits. In the case of larger systems water becomes a glass under cooling. This glass-like structure mostly consists of 4- and 5-site ring fragments. The glass transition temperature is increased with cluster size and their values are in the range from 130 to 160 K close to results from bulk water simulations, where solidification occurred at about 177 K. Possible non-ergodicity may lead to differences in the solid-liquid phase transition temperatures about 10-15 K. One of the objectives was also to systematically study the dependence of the melting temperature Tm on the number of molecules. The melting temperature changes to a monotonic increase for clusters with N ≥ 20 and the asymptotic trend of Tm with N approaches the classical liquid drop model:  Tm = T0 (1-C·N -1/3), where T0 is the bulk ice melting point while C is a constant ; .  Enclosing one methane molecule inside the cage cavity strongly affects the cluster structure evolution. The presence of the guest molecule significantly stabilizes the dodecahedral configuration.  It appears that the methane-water interactions create an excluded volume for the water molecules that hampers some of the rearrangements of the aggregate configuration in spite of the higher energy compared to the pure water cluster. This excluded volume mechanism for stabilization of a cluster structure has been observed in previous MD and ab initio studies

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REFERENCES:

[Egorov, Brodskaya & Laaksonen, Comp. Mater. Sci., 36, 166 (2006)
Egorov, Brodskaya  & Laaksonen, J.Chem.Phys., 118, 6380 (2003)
Brodskaya, Egorov, Lyubartsev  & Laaksonen, J. Chem. Phys., 119, 10237 (2003)
Egorov, Brodskaya & Laaksonen, Mol. Phys., 100, 941 (2002).