Lars G.M. Pettersson
Department of Physics, AlbaNova University Center, Stockholm University, S-106 91, Stockholm, Sweden
I will discuss recent experimental and simulation data of liquid water and the picture of fluctuations between high-density (HDL) and low-density (LDL) liquid this has led to [1,2]. The HDL would be a more close-packed form, favored by entropy and dominates at high temperature. Below about 50°C correlated fluctuations into tetrahedral (LDL) structures, favored by hydrogen-bonding (enthalpy), begin to appear and become increasingly important upon further cooling [3,4]. These are evidenced through specific peaks in the O-O radial distribution function at 4.5, ~8.5 and ~9.7 Å, which become increasingly sharp upon supercooling. In addition, Maréchal [5] has shown that the temperature-dependent IR spectrum of liquid water (0-80 °C) can be decomposed as a linear combination of two temperature-independent, complete (0-4000 cm-1) spectra that can be associated with LDL and HDL environments. Here I will show computed IR spectra for two water models, chains and fused dodecahedra, which for the OH-stretch region remarkably well reproduces the extracted experimental HDL and LDL spectrum, respectively. I will show results from MD simulations of the dodecahedral model both in gas phase and as a seed in simulations of liquid water and discuss implications of such hypothetical structures for our understanding of water properties.
References:
[1] Anders Nilsson and Lars G.M. Pettersson, The Structural Origin of Anomalous Properties of Liquid Water, Nature Commun. 6, 8998 (2015).
[2] P. Gallo et al., Water: A Tale of Two Liquids, Chem. Rev. 116, 7463-7500 (2016).
[3] L. B. Skinner et al., The Structure of Water Around the Compressibility Minimum, J. Chem. Phys. 141, 214507 (2014).
[4] D. Schlesinger, K.T. Wikfeldt, L.B. Skinner, C.J. Benmore, A. Nilsson and L.G.M. Pettersson, The temperature dependence of intermediate range oxygen-oxygen correlations in liquid water, J. Chem. Phys. 145, 084503 (2016).
[5] Y. Maréchal, The molecular structure of liquid water delivered by absorption spectroscopy in the whole IR region completed with thermodynamics data. J. Mol. Struct. 1004, 146-155 (2011).