On being heated, ice melts into liquid water. Although in practice this process tends to be heterogeneous, it can occur homogeneously inside bulk ice1. The thermally induced homogeneous melting of solids is fairly well understood, and involves the formation and growth of melting nuclei1,2,3,4,5. But in the case of water, resilient hydrogen bonds render ice melting more complex. We know that the first defects appearing during homogeneous ice melting are pairs of five- and seven-membered rings, which appear and disappear repeatedly and randomly in space and time in the crystalline ice structure6,7,8. However, the accumulation of these defects to form an aggregate is nearly additive in energy, and results in a steep free energy increase that suppresses further growth. Here we report molecular dynamics simulations of homogeneous ice melting that identify as a crucial first step not the formation but rather the spatial separation of a defect pair. We find that once it is separated, the defect pair—either an interstitial (I) and a vacancy (V) defect pair (a Frenkel pair), or an L and a D defect pair (a Bjerrum pair)9—is entropically stabilized, or ‘entangled’. In this state, defects with threefold hydrogen-bond coordination persist and grow, and thereby prepare the system for subsequent rapid melting.
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We thank D. J. Wales, N. Kosugi, H. Tanaka and S. Saito for their support and advice, and T. Yagasaki for performing the free-energy calculation to determine the melting temperature. Some of the calculations were carried out at the Research Center for Computational Science of NINS, Okazaki, Japan.
This video shows the successive transformation of Voronoi polyhedron along the trajectory of Figure 1.Voronoi tessellation is applied to show the volume occupied by each water molecule. The Voronoi polyhedra that have more than 5% larger (5% smaller) volume than the average volume are gradated in blue (red). Polyhedra with thickest colors have more than 10% higher or lower local volume than the average. At around 2150ps when many off-lattice molecules are accumulated in a locus (form a cluster), inside of which the density fluctuation is observed. Then, at 2210 ps, this cluster is separated into the high-density region (I-defect region, red) and the low-density region (V-defect region, blue).
This video shows the local HB structure change along the trajectory of Figure 1. Fragments are used to show the local HB structures; eight typical interfacial fragments of (a)~(h) in Supplementary Figure 5 are colored with from red to green, respectively. The other types of fragments in the interfacial region are colored in gray. White thin lines are for HBs belonging to the ice fragment, and blue lines are for other HBs. One can see that there are 4 stages in this trajectory; 0-2150 ps, 2150-2730 ps, 2730-3050 ps, and 3050 ps-. See the legend of Supplementary Figure 5 for the detail of each stage.
About this article
Scientific Reports (2015)