New insights into the supercritical state of water

Supercritical water has liquid-like and gas-like regimes which are separated by the so-called Widom line.© Christoph Schran

http://www.eurekalert.org/pub_releases/2016-01/rb-nii012116.php

Researchers predict terahertz spectra using computer simulations

RUHR-UNIVERSITY BOCHUM

Using molecular dynamics simulations, researchers have analysed the properties of supercritical water. The researchers showed which structure of the hydrogen bond network is formed in different supercritical states and also simulated the relevant terahertz spectra. This approach may help in future to interpret experimental results.

At temperatures of approx. 375 degrees Celsius and a pressure 220 times higher than normal, water reaches the supercritical state, where the liquid and the gaseous phases can no longer be clearly distinguished - according to traditional text-book opinion.

"Arguments that the supercritical state might be subdivided into a gas-like and a liquid-like regime, separated by the so-called Widom line, haven't been put forward until a few years ago," explains Christoph Schran from the Center for Theoretical Chemistry at the Ruhr-Universität Bochum, headed by Prof Dr Dominik Marx.

Three water states in comparison

Using molecular dynamics simulations, the team headed by Prof Marx analysed how to study the Widom line experimentally by means of terahertz spectroscopy. They published their results in collaboration with the Polish Gdansk University of Technology in Physical Review Letters. The simulations were partially conducted at the Leibniz Supercomputing Centre in Munich.

The theorists compared three states: the state of liquid water at room temperature; a supercritical state with high density; and a supercritical state with low density. The analyses revealed that the hydrogen bond networks between the hydrogen molecules are completely different in those three states.

States differ with regard to size and number of water clusters

In liquid water at room temperature, almost all hydrogen molecules are bound via hydrogen bonds. In supercritical water, however, isolated clusters are formed. They consist of water molecules that are bound inside the cluster via hydrogen bonds, but do not have any hydrogen bonds to other clusters.

The number of clusters of different sizes differs between supercritical states with high and low density. Properties of the gas phase are prevailing in supercritical water with low density, those of the liquid phase in supercritical water with high density.

The researchers simulated the vibrational spectra associated with the three states in the terahertz range, whose shape is largely determined by the structure of the hydrogen bond network. Experimentally, it is not possible to observe directly which factors affect the shape of the spectra on the molecular level. Theoretical chemistry can close this gap: the present study has shed light on the physical processes that determine the shape of the terahertz spectra of gas-like and liquid-like supercritical water.

"Our simulations have shown that terahertz spectroscopy should be an ideal method for analysing the properties of hydrogen bonds in the supercritical state of water - on both sides of the Widom line," concludes Schran. "Moreover, our findings will help to interpret the underlying molecular processes in the measured spectra."

Supercritical liquids as solvents for the industry

Supercritical water is relevant not only for academic research. The industry utilises it as an eco-friendly solvent. Minor variations of pressure or temperature affect its properties to a considerable extent. Consequently, supercritical water can be fine-tuned to adopt the properties required for the respective application.

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Funding

The German Research Foundation funded the study through the project MA 1547/11 and under the umbrella of the Cluster of Excellence RESOLV (Ruhr Explores Solvation, EXC 1069), which was approved in 2012. The simulations were conducted in the course of the federal project pr86fo at the Leibniz Supercomputing Centre in Munich.

Original publication

M. Smiechowsk, C. Schran, H. Forbert, D. Marx (2016): Correlated particle motion and THz spectral response of supercritical water, Physical Review Letters, DOI: 10.1103/PhysRevLett.116.027801 http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.027801

Further information

Prof Dr Dominik Marx, Center for Theoretical Chemistry, Faculty of Chemistry and Biochemistry at the Ruhr-Universitaet Bochum, 44780 Bochum, Germany, phone: +49/234/32-28083, email: dominik.marx@rub.de

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Terahertz Detects Dance of Water Molecules Turn Fire-Colored Beetles Into Antifreeze Artists

Changing of water dynamics: The antifreeze protein (blue) of fire-coloured beetle larva changes the dynamics of water on the ice-binding surface with threonine side chains (green). (Credit: Copyright Konrad Meister)
http://www.sciencedaily.com/releases/2013/01/130102083551.htm
Jan. 2, 2013 — Certain plants and animals protect themselves against temperatures below freezing with antifreeze proteins. How the larva of the beetleDendroides canadensis manages to withstand temperatures down to -30 degrees Celsius is reported by an international team of researchers led by Prof. Dr. Martina Havenith from the Department of Physical Chemistry II at the Ruhr-Universität in the journal PNAS. Together with American colleagues, the RUB-researchers showed that interactions between the antifreeze proteins and water molecules contribute significantly to protection against the cold.

Previously, it was assumed that the effect was only achieved through direct contact of the protein with ice crystals. The team obtained the results through a combination of terahertz spectroscopy and molecular dynamics simulations.

Protein-ice interaction: locally and over longer distances

The structure of the fire beetle antifreeze proteins resembles a triangular prism. The ice binding surface of the “prism” contains many exposed side chains, as fragments of the amino acid threonine protrude from the surface here. These side chains bind ice crystals. Up until now, it was assumed that the antifreeze proteins only interact locally with nano ice crystals and thus prevent the formation of larger ice crystals. The international group of researchers showed, however, that this interaction also takes place between proteins and ice crystals over longer distances via water molecules, which also contributes to the freeze-protection.

The dynamics of the water molecules is important for the freeze-protection

Close to the ice binding surface, the scientists observed a much slower movement of the water molecules, which differed significantly from the water movements on the non-ice-binding sides of the protein and of free water. The lower the temperature, the slower the water moved. “We suspect that the calmer water movement on the binding surface of the protein facilitates the docking of the nano ice crystals,” Martina Havenith speculated. In accordance with this, the researchers found little change in the movement of the water molecules in an inactive mutant of the antifreeze protein.

More efficient than in fish

The antifreeze proteins of the fire-coloured beetle are ten to one hundred times more active than those of Arctic and Antarctic fish that need to protect themselves against temperatures of -1.9 degrees Celsius. The insects achieve this high antifreeze activity through the combination of the two strategies: direct interaction between proteins and ice and interaction via water molecules.

Observing the role of the solvent

“The special role of water in natural antifreeze is an excellent example demonstrating that when looking at the function of a biomolecule, you not only have to consider its 3D structure, but also its entire environment - especially the solvent; in this case water,” Prof. Havenith said. This topic is the focus of the cluster of excellence RESOLV, which was launched on the 1st November 2012 at the RUB and whose spokesperson is Martina Havenith. The current studies were funded by the Volkswagen Foundation.