Structural characteristics of low-density environments in liquid water

Abstract

The existence of two structural forms in liquid water has been a point of discussion for a long time. A phase transition between these two forms of liquid water has been proposed based on evidence from molecular simulations, and experiments have also been very recently able to track the proposed transition of the low-density liquid (LDL) form to the high-density liquid form. We propose to use the average angle an oxygen atom makes with its neighbors to describe the structural environment of a water molecule. The distribution of this order parameter is observed to have two peaks, with one peak at ∼ 109.5°, corresponding to the internal angle of a regular tetrahedron, indicating tetrahedral arrangement. The other peak corresponds to an environment with a tighter arrangement of neighboring molecules. The distribution of O-O-O angles is decomposed into two skewed distributions to estimate the fractions of the two liquid forms in water. A good similarity is observed between the temperature and pressure trends of fractions of locally favoured tetrahedral structure (LFTS) form estimated using the new order parameter and the reports in the literature, over a range of temperatures and pressures. We also compare the structural environments indicated by different order parameters and find that the order parameter proposed in this work captures the structure of first solvation shell of LFTS accurately.

Multiple insights call for revision of modern thermodynamic models to account for structural fluctuations in water

“…LDL fraction isobars estimated using different methods. Solid lines are our estimations from PC‐SAFT with the parameter set P2 72 and dashed lines are from simulations with the iAMOEBA water model 60 …”

“…These results, in addition to results from Figures 2, 3, 5, and 6 indicate that modern thermodynamic models do not reflect the structure of water accurately since there are considerable differences to the degree of hydrogen‐bonding. This can be related to structural fluctuation which are believed to be caused by local structures that are characterized by different hydrogen bonded networks 11,51,60 …”

“…Solid lines are our estimations from PC-SAFT with the parameter set P2 72 and dashed lines are from simulations with the iAMOEBA water model. 60 F I G U R E 6 Variation of the average number of hydrogen bonds estimated from thermodynamic models (CPA, 70 PC-SAFT (P1), 46 ), experiments (NMR, 76 latent heat, 77 IR spectroscopy, 5 X-ray absorption, 9 pair correlation functions from X-Ray diffraction 78 ), and molecular simulations (of iAMOEBA water model). 66 The results from CPA and PC-SAFT (P1) are calculated using Equation ( 8).…”

“…Estimation of LDL fraction from O-O-O anglesWe recently showed60 that the structure of tetrahedral environments could be identified based on the average angle an oxygen atom makes with its neighbors (θ avg ). The distribution of θ avg is bimodal with one peak at around 109.5 , which is the internal angle of a regular tetrahedron.…”

Structural and Dynamical Properties of Liquid Water Increase Turbulent Flows

“…Contrary to the common approach that regards liquid water as an incompressible, isotropic and homogeneous fluid assessable through continuous models (Nakayama 2017;Gao et al, 2021), we regard liquid water as a compressible fluid. Indeed, water consists of a mixture of low-density water (LDW) and high-density water (HDW) transient assemblies (Muthachikavil et al, 2022) that generate patchy network inhomogeneities and micro-variations in local density which can be assessed just through discretized models. Focusing on a simple model of distilled liquid water flowing in a cylinder, we provide numerical simulations that illustrate how the everchanging, amorphous network configuration of liquid water could contribute to generate micro-vortices.…”