Inhaltszusammenfassung

Ice is ubiquitous in nature and all around us. Of particular interest is the surface, where interactions with the environment take place. The surface of the most stable polymorph, hexagonal ice I h , is known to pre-melt at temperatures close to the melting point. In this thesis, the low-index surfaces of ice I h —namely the basal (0001), the primary prismatic (101̄0), and the secondary prismatic (1̄21̄0) plane—are examined in a temperature range from 200 K to 270 K, using classical molec...Ice is ubiquitous in nature and all around us. Of particular interest is the surface, where interactions with the environment take place. The surface of the most stable polymorph, hexagonal ice I h , is known to pre-melt at temperatures close to the melting point. In this thesis, the low-index surfaces of ice I h —namely the basal (0001), the primary prismatic (101̄0), and the secondary prismatic (1̄21̄0) plane—are examined in a temperature range from 200 K to 270 K, using classical molecular dynamics simulations employing the TIP4P/Ice rigid water model. With structural analysis we probed the transition from ordered to disordered arrangements at the top surface layers. Our structural analysis, including radial distribution functions, hydrogen bond analysis, medium-range network topology, and order parameters, indicates that 2–3 layers (≈ 8 Å–12 Å) are disordered, with a structure similar to that of liquid water at 270 K for the basal plane, 1–2 layers (≈ 4.5 Å–8.5 Å) for the primary prismatic plane, and 2–4 layers (≈ 5 Å–9.5 Å) for the secondary prismatic plane. A sudden increase of disorder is detected for the second layer of the basal plane between 260 K–270 K explaining a peak shift observed in sum-frequency-generation spectroscopy measurements. [1] Even though local order is lost within the top layer at the highest temperatures, the surfaces retain an ordered structure averaging over several snapshots as revealed by two-dimensional density maps explained by a templating effect induced by the underlying layer. A different picture is obtained from dynamical analysis. According to the mean square displacement calculation only the top layer displays normal diffusion and can be considered liquid-like for all surfaces at high temperatures. Diffusion is isotropic. At lower temperatures, sub-diffusion is observed. The next few layers are only structurally similar to liquid water close to the melting point, but do not diffuse or display glass-like dynamics.» weiterlesen» einklappen

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Klassifikation

DDC Sachgruppe:
Chemie