Ice-like Phonons in Liquid Water Discovered

In January 2016 Nature Communications published an article called “The hydrogen-bond network of water supports propagating optical phonon-like modes“. We asked one of the authors, Daniel Elton from Stony Brook University, to comment on this study.

The molecular structure of water

For more than 100 years, scientists have debated what the underlying molecular structure of water is. Now through computer simulation conducted at the Institute for Advanced Computational Science (IACS) at Stony Brook University, researchers can illustrate that the structure and dynamics of hydrogen bonding in liquid water is more similar to ice than previously thought. The finding, published in Nature Communications, changes the common understanding of the molecular nature of water and has relevance to many fields, such as climate science and molecular biophysics.

Countless experiments have documented the many anomalous properties of water, and it is understood that these anomalies arise because of water’s complex network of hydrogen bonds. The average structure of these bonds has been debated as far back as 1892, when W.K. Roentgen proposed that water contains a mixture of two structural motifs – “ice like” and “liquid like”. Today, two state mixture models are still discussed, but in a more nuanced form. There is a debate about whether water’s hydrogen bond network contains clearly defined domains of slightly different densities, or whether the network is homogeneous. Experimental measurements, while offering many clues, are not able to completely settle this debate, so computer simulation using the underlying laws of physics is a valuable tool.

Water’s dielectric properties

In the debate about water’s structure and dynamics, relatively little attention has been paid to water’s dielectric properties, which describe how a water interacts with electromagnetic waves. By comparing the longitudinal and transverse dielectric functions for the first time, and comparing them with infrared and Raman spectra, the researchers realized that previous Raman peak assignments were problematic. The solution to the problem was to assign peaks to longitudinal and transverse phonon modes, just as in ice. In condensed matter physics, phonons can be visualized as collective vibrations that propagate through a material. Propagating phonons are usually only possible in solids, but the water’s hydrogen bond network allows for them to exist in the liquid.

Computer simulation of liquid water

By simulating liquid water, the structure and motion of molecules in water can be analyzed in great detail beyond what experiments can reveal of liquid water. The method used by the researchers involved both experimental data and extensive molecular dynamics simulations.

The authors used a new high-powered computer cluster at Stony Brook’s IACS to perform the simulations. By calculating the wavelength and frequency dependent dielectric properties, they demonstrated that optical phonon-like modes can propagate the hydrogen bond network, just as in ice. Unlike in ice, however, hydrogen bonds in water are constantly being broken and reformed, so the phonons only last for about one trillionth of a second yet can travel over long distances up to two nanometers.

The authors believe that the findings challenge older ideas about water dynamics, which characterized peaks in the absorption spectrum as being due to the vibrational motions of at most small clusters of a few molecules.

Additionally, by comparing several different simulation techniques, the authors also found that the current non-polarizable water models used in biophysics fail to capture the higher frequency optical phonons. This work builds on their previous work, which showed that polarizable models are more accurate than the more often used non-polarizable models.

The researchers hope that this work will help resolve the debate about water’s structure. The splitting of frequencies between longitudinal and transverse phonons can in principle be related to structural changes in the hydrogen bond network, providing a new window into how water’s structure changes with temperature.