Abstract
The structure of the hydrogen bond network is a key element for understanding water's thermodynamic and kinetic anomalies. While ambient water is strongly believed to be a uniform, continuous hydrogen-bonded liquid, there is growing consensus that supercooled water is better described in terms of distinct domains with either a low-density ice-like structure or a high-density disordered one. We evidenced two distinct rotational mobilities of probe molecules in interstitial supercooled water of polycrystalline ice [Banerjee D, et al. (2009) ESR evidence for 2 coexisting liquid phases in deeply supercooled bulk water. Proc Natl Acad Sci USA 106: 11448–11453]. Here we show that, by increasing the confinement of interstitial water, the mobility of probe molecules, surprisingly, increases. We argue that loose confinement allows the presence of ice-like regions in supercooled water, whereas a tighter confinement yields the suppression of this ordered fraction and leads to higher fluidity. Compelling evidence of the presence of ice-like regions is provided by the probe orientational entropy barrier which is set, through hydrogen bonding, by the configuration of the surrounding water molecules and yields a direct measure of the configurational entropy of the same. We find that, under loose confinement of supercooled water, the entropy barrier surmounted by the slower probe fraction exceeds that of equilibrium water by the melting entropy of ice, whereas no increase of the barrier is observed under stronger confinement. The lower limit of metastability of supercooled water is discussed.
Highlights
Several water anomalies with deep implications in biology, atmospheric phenomena, geology, and food technology are rooted in the supercooled liquid state [1,2,3,4,5,6]
The two amorphous ice phases were incorporated in the picture of the metastable and stable water by the liquid-liquid critical point (LLCP) scenario where [19]: i) liquid water displays polymorphism, i.e. it exists in two different phases, a highlydisordered high-density liquid (HDL), entropically favored, and a low density liquid (LDL) with local ice-like tetrahedral order, energetically favored, ii) the first-order LDL-HDL phase transition line terminates at a liquid-liquid critical point in the supercooled region
In conclusion, we investigated the rotational dynamics of a probe molecule localized in the interstitial supercooled water of polycrystalline ice
Summary
Several water anomalies with deep implications in biology, atmospheric phenomena, geology, and food technology are rooted in the supercooled liquid state [1,2,3,4,5,6]. The difficulty in specifying a few distinct states of liquid water motivated the growth of the continuum models. In this framework, first developed by Bernal and Fowler in 1933 [14] and Pople in 1951 [15], the picture of water structure is considered as a continuous distribution of approximately tetrahedral environments, corresponding to different degrees of distortion of the hydrogen bond (HB) ranging from strong HB’s such as those in ice to highly distorted or even broken HB’s [10]. It has been recently shown that LLCP scenario and the three other ones, including models that can reproduce more than one scenario [21,25,26], can be accounted for by one general scheme which predicts a LLCP at positive pressure [27]
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