Two kinds of potential surfaces for supercooled water and percolation transition

本项目涵盖了水在体积和密闭状态下的相行为。(1)我们报道了过冷状态下体积水的分子动力学（MD）模拟以及相应的液-液相变。这已经通过检查单个水分子的局部结构、氢键数分布等进行了研究。还研究了温度为0 K时的压缩和减压循环。(2)我们报道的MD证据表明，在大气压下，在体积水的冰点以上出现了一种新型的一阶相变——从液体到双层非晶态的转变。当两层水被限制在宽度小于1纳米的疏水狭缝孔中时，这种多晶相变才会出现。在冷却后，具有不完美的随机氢键网络的封闭水转变为具有完美网络的双层非晶态，由于形成了各种氢键多边形，但没有长程有序。这种转变与在液态硅、液态碳和液态磷等四面体配位物质中观察到的转变有一些共同的特征。(3)本文报道了包封水相行为的MD模拟证据、准一维冰中新相的形成和固液临界点的存在。我们发现，在窄碳纳米管中，水可以冻结成七方、六边形、五边形或方形的冰纳米管，这取决于碳纳米管的直径（1-1.4 nm）和施加的轴向压力。基于树能计算，我们评估了将散装液态水封装成碳纳米管并转化为冰纳米管的实验条件。

The present project covers phase behaviors of water in bulk and confined states.(1) We have reported molecular dynamics (MD) simulations of bulk water in supercooled state and the associated liquid-liquid phase transition. This has been investigated by examining local structure of individual water molecules, the hydrogen bond number distribution etc. Also examined is compression and decompression cycle at temperature 0 K.(2) We have reported MD evidence suggesting a new type of first-order phase transition - a liquid-to-bilayer amorphous transition --- above the freezing temperature of bulk water under atmospheric pressure. This polyamorphic phase transition appears uniquely when a two-layer water is confined in a hydrophobic slit pore at a width of less than one nanometer. Upon cooling, the confined water which has an imperfect random hydrogen-bonded network undergoes the transition to a bilayer amorphous which has a perfect network due to the formation of various hydrogen-bonded polygons yet has no long-range order. This transition was visualized and the associated thermodynamic properties were examined The transition shares some characteristics with those observed in tetrahedrally coordinated substances such as liquid silicon, liquid carbon and liquid phosphorus.(3) We have reported MD simulation evidence of phase behavior of encapsulated water-formation of new phases of quasi-one-dimensional ice and existence of a solid-liquid critical point. We discover that in narrow carbon nanotubes water can freeze into heptagonal, hexagonal, pentagonal, or square ice nanotubes, depending on the diameter of carbon nanotubes (1-1.4 nm) and the applied axial pressure as well. Based on the tree energy calculation, we assessed experimental conditions under which bulk liquid water can be encapsulated into carbon nanotubes and can be transformed into ice nanotubes.