Banerjee D, Bhat S N, Bhat S V, Leporini D
Department of Physics, Indian Institute of Science, Bangalore 560 012, India.
Proc Natl Acad Sci U S A. 2009 Jul 14;106(28):11448-53. doi: 10.1073/pnas.0900734106. Epub 2009 Jun 25.
Using electron spin resonance spectroscopy (ESR), we measure the rotational mobility of probe molecules highly diluted in deeply supercooled bulk water and negligibly constrained by the possible ice fraction. The mobility increases above the putative glass transition temperature of water, T(g) = 136 K, and smoothly connects to the thermodynamically stable region by traversing the so called "no man's land" (the range 150-235 K), where it is believed that the homogeneous nucleation of ice suppresses the liquid water. Two coexisting fractions of the probe molecules are evidenced. The 2 fractions exhibit different mobility and fragility; the slower one is thermally activated (low fragility) and is larger at low temperatures below a fragile-to-strong dynamic cross-over at approximately 225 K. The reorientation of the probe molecules decouples from the viscosity below approximately 225 K. The translational diffusion of water exhibits a corresponding decoupling at the same temperature [Chen S-H, et al. (2006) The violation of the Stokes-Einstein relation in supercooled water. Proc Natl Acad Sci USA 103:12974-12978]. The present findings are consistent with key issues concerning both the statics and the dynamics of supercooled water, namely the large structural fluctuations [Poole PH, Sciortino F, Essmann U, Stanley HE (1992) Phase behavior of metastable water. Nature 360:324-328] and the fragile-to-strong dynamic cross-over at approximately 228 K [Ito K, Moynihan CT, Angell CA (1999) Thermodynamic determination of fragility in liquids and a fragile-to-strong liquid transition in water. Nature 398:492-494].
我们使用电子自旋共振光谱（ESR），测量了在深度过冷的大量水中高度稀释且受可能的冰相影响可忽略不计的探针分子的旋转迁移率。迁移率在假定的水的玻璃化转变温度T(g) = 136 K以上增加，并通过穿越所谓的“无人区”（150 - 235 K范围）平稳连接到热力学稳定区域，在该区域人们认为冰的均匀成核抑制了液态水。证明了探针分子存在两个共存部分。这两个部分表现出不同的迁移率和脆性；较慢的部分是热激活的（低脆性），并且在低于约225 K的脆弱到强动态转变的低温下更大。在约225 K以下，探针分子的重新取向与粘度解耦。水的平动扩散在相同温度下表现出相应的解耦[Chen S-H等人（2006年）。过冷水对斯托克斯 - 爱因斯坦关系的违反。美国国家科学院院刊103:12974 - 12978]。目前的发现与关于过冷水的静态和动态的关键问题一致，即大的结构波动[Poole PH，Sciortino F，Essmann U，Stanley HE（1992年）亚稳态水的相行为。自然360:324 - 328]和约228 K时的脆弱到强动态转变[Ito K，Moynihan CT，Angell CA（1999年）液体中脆性的热力学测定以及水中的脆弱到强液体转变。自然398:492 - 494]。
