Anomalies in Supercooled Water at ~230 K Arise from a 1D Polymer to 2D Network Topological Transformation

Abstract

Puzzling anomalous properties of water are drastically enhanced in the supercooled region. However, the nature of these anomalies is not known. We report here molecular dynamics simulations using the RexPoN force field from 298 to 200 K along the 1 atm density curve. At 298 K, there are 2.1 strong hydrogen bonds (SHBs), leading to a dynamic branched one-dimensional (1D) polymer. Water remains 1D down to 240 K, but at and below 230 K, the number of SHBs becomes 3.0, leading to a two-dimensional (2D) network that persists to 200 K. We propose that this 1D-to-2D topological transition accounts for the anomalous properties of supercooled water. Near 230 K, the power spectra show dramatic increases in the angular vibrational frequency modes, while the diffusivity decreases dramatically, both arising from the 1D-to-2D transformation. This transition is not first order because free energy changes uniformly but fluctuations in the entropy near 230 K suggest a possible second-order transition.