Structural and topological changes across the liquid–liquid transition in water

Abstract

It has recently been shown that the TIP4P/Ice model of water can be studied numerically in metastable equilibrium at and below its liquid–liquid critical temperature. We report here simulations along a subcritical isotherm, for which two liquid states with the same pressure and temperature but different density can be equilibrated. This allows for a clear visualization of the structural changes taking place across the transition. We specifically focus on how the topological properties of the H-bond network change across the liquid–liquid transition. Our results demonstrate that the structure of the high-density liquid, characterized by the existence of interstitial molecules and commonly explained in terms of the collapse of the second neighbor shell, actually originates from the folding back of long rings, bringing pairs of molecules separated by several hydrogen-bonds close by in space.

Anomalies and Local Structure of Liquid Water from Boiling to the Supercooled Regime as Predicted by the Many-Body MB-pol Model

“…Figure c shows that the fully tetrahedral topology, where a water molecule simultaneously accepts and donates two hydrogen bonds (2 D 2 A ), is the most common hydrogen-bonding arrangement at all temperatures. The probability of finding 2 D 2 A molecules is ∼30% at 368 K and monotonically increases as the temperature decreases, displaying a steeper increase between 238 and 220 K before saturating to >90% below 220 K. An analogous rapid increase of tetrahedral structures at deeply supercooled temperatures was recently observed experimentally, and the predominance of the 2 D 2 A topology at low temperatures was also noted in the TIP4P/Ice empirical water model . In the present MB-pol results, water molecules that donate two hydrogen bonds but only accept one hydrogen bond constitute the second most probable topology over the entire temperature range, representing ∼20–25% of all possible hydrogen-bonding topologies between 368 and 278 K before quickly disappearing at lower temperatures.…”

Topological nature of the liquid–liquid phase transition in tetrahedral liquids

“…Here, r is the intermolecular oxygen-oxygen distance and θ is the smallest among the four angles between the intramolecular O-H and the intermolecular O-O lines. As shown previously 19 , no ambiguity in the identification of hydrogen bonds remains if the Chandler-Luzar geometric criterion is applied using inherent structure configurations. These are the local potential energy minimum (21) configurations reached via a steepest descent path that quenches the vibrational degrees of freedom.…”

“…The anomalous behaviour of liquid water, along with water polyamorphism, was then suggested to be a manifestation of the presence of this LLPT. Recent numerical studies 8,9 , as well as experiments [10][11][12] (despite the difficulties induced by the rapid formation of ice around the predicted critical temperature and pressure conditions), have provided strong support to this fascinating hypothesis.Several order parameters, based on geometric or energetic criteria, have been proposed to characterize the transition [13][14][15][16][17][18][19][20] . In particular, the local structure index 13 and the r 5 parameter 14 -two order parameters that effectively measure the distance between the first and second coordination shells-have been routinely applied.…”

“…21 ), V 4 (ref. 15 ) and the total node communicability 20 , which include some information on the connectivity properties of the hydrogen-bond network 19 . It has also become clear that network interpenetration 22-24 plays a key role in the LLPT in tetrahedral liquids 23,[25][26][27] , with the LDL being composed of a single random tetrahedral network and the HDL consisting of two (or more 23 ) disordered but locally interpenetrating networks.…”

“…Several order parameters, based on geometric or energetic criteria, have been proposed to characterize the transition [13][14][15][16][17][18][19][20] . In particular, the local structure index 13 and the r 5 parameter 14 -two order parameters that effectively measure the distance between the first and second coordination shells-have been routinely applied.…”