New computer simulations could help settle a debate that’s raged for more than a century: Are H2O molecules “water-like” or “ice-like”?
The complex simulations offer evidence that the structure and dynamics of hydrogen bonding in liquid water are 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, and technologies such as desalinization and water-based energy production.
In condensed matter physics, phonons are considered to be a solid-state phenomenon and can be visualized as collective vibrations that propagate through a material.
More precisely, a phonon is the fundamental quantum mechanical unit of lattice vibration. Optical phonons are a type of phonon that interacts with electromagnetic radiation. These can be visualized as peaks in the infrared absorption spectrum in ice.
The new work shows that propagating vibrations or phonons can exist in water, just as in ice.
“No microscopes can allow us to directly see the behavior of water molecules and their pattern of hydrogen bonding. Therefore by simulating liquid water using the fundamental laws of physics, the structure and motion of molecules in water can be analyzed in great detail beyond what microscopes can reveal of liquid water,” Daniel C. Elton, a PhD candidate at Stony Brook University and lead author of the study.
“Our method involved both experimental data and extensive molecular dynamics simulations, and we found that the optical phonon coupling leads to similar absorption peaks also found in ice.”
Elton and colleagues used a powerful computer cluster to create the water dynamics simulations. By centering on water’s unique hydrogen bond network, they routinely 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.
“Our findings challenge older ideas about water dynamics, which characterized peaks in the absorption spectrum as being due to the vibrational motions of at most a few molecules,” says Professor Marivi Fernandez-Serra. “We found water peaks in spectra correspond to two different types of phonons, called longitudinal and transverse.
“The shifting of the position of the longitudinal and transverse peaks with temperature can be related to important structural changes in the hydrogen bond network, which provides a new window into how water’s structure changes with temperature.”
Grants from the US Department of Energy supported the work.
Source: Stony Brook University