Atmospheric ice grows by uptake of water from the vapor, and the crystal structure of ice determines the equilibrium water vapor pressure in ice clouds. Experiments and molecular simulations indicate that ice grown from deeply supercooled liquid water is the metastable stacking disordered polymorph with a comparable fraction of cubic ice and hexagonal ice layers. The structure of the ice grown from vapor, however, has not yet been reported, and measurements of vapor pressure in ice clouds give conflicting accounts. Hence, it is not known whether the ice formed by vapor growth is the stable hexagonal ice polymorph or the mestastable stacking disordered ice. These results pose the questions of whether the same ice structures are grown from the vapor and liquid water, and what kinetic or thermodynamic factors control the polymorphism. In this talk we present a study using molecular simulations with the mW water model to elucidate the structure of the ice grown by uptake of water from the vapor, compare it with the structure grown from supercooled liquid water, and investigate the factors that control ice polymorphism. We find that cubic ice is strongly destabilized at the vapor interface, but only mildly so at the liquid-ice interface. Comparison of free energies and and energetic considerations indicate that entropy is the main factor in the stabilization of hexagonal ice at the vapor interface. We conclude that ice grown from the vapor is the purely hexagonal ice polymorph, and that the selection of the polymorph is effected by the interface. Our results suggest that it may be possible to synthesize the metastable cubic ice polymorph by control of interfacial properties of the growing ice.

Stacking disorder in ice grown from liquid but not from vapor: Polymorph selection by interfaces

251st National Meeting (2016)

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