(63h) Anomalies in Supercooled Water at ~230 K Arise from Topological Transformation

Liquid water has many striking anomalous properties at ambient pressure and some of them become drastically enhanced in the supercooled region below the freezing point. Over the last 40 years, there has been confusion and debates about the nature of anomalies in supercooled water near 230 K. The thermodynamic response functions such as isothermal compressibility and heat capacity appear to diverge toward a singular temperature at 228 K (T_S) and 1 atm. The problem is, direct experiments on bulk water are challenging since liquid water transforms homogeneously to ice around 232 K at 1 atm. This has led to numerous experimental and theoretical studies (with liquid-liquid critical point being the most popular one) without any global agreement about the origin of the anomalies in supercooled water. We have now solved this problem.

In 2018, we published the RexPoN force field (FF) (Naserifar and Goddard, J. Chem. Phys. 149, 174502, 2018) fitted to highly accurate Quantum Mechanics (at the CCSDT ab-initio level), leading to the most accurate properties ever predicted by a FF (and more accurate than DFT), even though no empirical data was used. RexPoN reproduces the oxygen-oxygen radial distribution of the first water shell from neutron scattering, the first FF to do so.

In 2019, we reported (Naserifar and Goddard, PNAS, 116, 1998–2003, 2019) that at 298 K RexPoN finds an average of 2.1 Strong Hydrogen Bonds (SHB) with an average lifetime of 93 femtoseconds (fs). Connecting these SHBs leads to a one-dimensional polymer with occasional branches to sidechains. This established the revolutionary new paradigm for the structure of water as a dynamic polydisperse branched polymer (DynPol), leading to an average cluster size of 151 at 298 K that decreases to 36 at 400 K. The average number of SHB decreases from 2.1 down to 1.6 at 400 K while the average lifetime decreases from 93 to 68 fs.

In this talk, we report the changes in the structure and properties of liquid water as it is supercooled from 298 to 200 K along the 1 atm density curve. We find a topological transformation in which water changes from a dynamic branched one-dimensional polymer above 230 K to a different structural network at 230 K and below. The density of states calculations show a significant increase in the angular vibrational frequency modes at 230 K confirming the adjustment and transition of water molecules in to a strong network below 230 K. The free energy changes uniformly indicating that this topological transformation is not first order. However, the entropy shows fluctuations near 230 K, suggesting a possible second order transition.

We propose that this topological transition at ~230 K, accounts for the anomalous properties long associated with supercooled water near 230 K. This success in explaining the anomalous properties near 230 K shows the importance of the new DynPol paradigm for water as a dynamic polydisperse branched polymer (>230 K) or network (<230 K).

It is important to emphasize here that we did nothing special in explaining the phenomena near 230 K in water. We simply started with the new RexPoN FF for water (fitted to very accurate QM calculations and leading to unprecedented accuracy in the properties, despite eschewing experimental data) and followed the changes with temperature.

This revolutionary new insight on water, arguably the most important material to life, should be of great interest to many scientists in order to obtain a comprehensive understanding of water properties.