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Volume 141, 2009
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On thin ice: surface order and disorder during pre-melting

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Abstract

The effect of temperature on the structure of the ice Ih (0001) surface is considered through a series of molecular dynamics simulations on an ice slab. At relatively low temperatures (200 K) a small fraction of surface self-interstitials (i.e. admolecules) appear that are formed exclusively from molecules leaving the outermost bilayer. At higher temperatures (ca. 250 K), vacancies start to appear in the inner part of the outermost bilayer exposing the underlying bilayer and providing sites with a high concentration of dangling hydrogen bonds. Around 250–260 K aggregates of molecules formed on top of the outermost bilayer from self-interstitials become more mobile and have diffusivities approaching that of liquid water. At ∼270–280 K the inner bilayer of one surface noticeably de-structures and it appears that at above 285 K both surfaces are melting. The observed disparity in the onset of melting between the two sides of the slab is rationalised by considering the relationship between surface energy and the spatial distribution of protons at the surface; thermodynamic stability is conferred on the surface by maximising separations between dangling protons at the crystal exterior. Local hotspots associated with a high dangling proton density are suggested to be susceptible to pre-melting and may be more efficient at trapping species at the external surface than regions with low concentrations of protons thus potentially helping ice particles to catalyse reactions. A preliminary conclusion of this work is that only about 10–20 K below the melting temperature of the particular water potential employed is major disruption of the crystalline lattice noted which could be interpreted as being “liquid”, the thickness of this film being about a nanometre.

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Publication details

The article was received on 30 Apr 2008, accepted on 24 Jun 2008 and first published on 09 Oct 2008


Article type: Paper
DOI: 10.1039/B807377P
Citation: Faraday Discuss., 2009,141, 277-292
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    On thin ice: surface order and disorder during pre-melting

    C. L. Bishop, D. Pan, L. M. Liu, G. A. Tribello, A. Michaelides, E. G. Wang and B. Slater, Faraday Discuss., 2009, 141, 277
    DOI: 10.1039/B807377P

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