Abstract-Insights into the water status in hydrous minerals using terahertz time-domain spectroscopy

Yuanyuan Ma, Haochong Huang, Sibo Hao, Kunfeng Qiu, Hua Gao, Lu Gao, Weichong Tang, Zili Zhang, Zhiyuan Zheng


https://www.nature.com/articles/s41598-019-45739-2

The determinations of water status incorporated in hydrous minerals are of considerable significances in geoscience fields. Coincidentally, the aqueous sensitivity of terahertz radiation has motivated numerous explorations in several cross-domain applications. Terahertz time-domain spectroscopy is employed as a major probing technique coupling of traditional detecting methods to uncover the mask of water status in copper sulfate pentahydrate as well as mineral quartz in this article. Based on the quantitative identification of water status in copper sulfate pentahydrate, the water incorporated in mineral quartz is verified qualitatively. Notable differences of optical constants originating from the water content are obtained for copper sulfate pentahydrate and mineral quartz. These present works indicate that terahertz technology can be considered as a promising method to satisfy the ever-increasing requirements in hydrous mineral analyses.

Abstract-High Kerr nonlinearity of water in THz spectral range

A.N. Tcypkin , M.V. Melnik, M.O. Zhukova, I.O. Vorontsova , S.E. Putilin, S.A. Kozlov , X.-C. Zhang

https://arxiv.org/abs/1902.01129

The values of the nonlinear refractive index coefficient for various materials in the terahertz frequency range exceed the ones in both visible and NIR ranges by several orders of magnitude. We report the direct measurement of the nonlinear refractive index coefficient of liquid water by using the Z-scan method with broadband pulsed THz beam. Our experimental result shows that nonlinear refractive index coefficient in water is positive and can be as large as 3.5×10−10 cm2/W in the THz frequency range, which exceeds the ones for the visible and NIR ranges by 6 orders of magnitude

X-ray scientists create tiny, super-thin sheets of flowing water that shimmer like soap bubbles

A series of movies shows how increasing flows of gas that shape a stream of liquid affects the formation of liquid sheets and their soap-bubble-like sheen. Credit: SLAC National Accelerator Laboratory

by Glennda Chui,

 https://phys.org/news/2018-04-x-ray-scientists-tiny-super-thin-sheets.html#jCp

Water is an essential ingredient for life as we know it, making up more than half of the adult human body and up to 90 percent of some other living things. But scientists trying to examine tiny biological samples with certain wavelengths of light haven't been able to observe them in their natural, watery environments because the water absorbs too much of the light.
Now there's a way around that problem: A team led by scientists at the Department of Energy's SLAC National Accelerator Laboratory turned tiny liquid jets that carry samples into the path of an X-ray beam into thin, free-flowing sheets, 100 times thinner than any produced before. They're so thin that X-rays pass through them unhindered, so images of the samples they carry come out clear.

The new method opens new windows on critical processes in chemistry, physics and biology, including the nature of water itself, the researchers said in an April 10 report in Nature Communications.

The method was developed at SLAC's X-ray free-electron laser, the Linac Coherent Light Source (LCLS), but they said it can also work in experiments with synchrotron light sources, tabletop lasers and electron beams.

"This opens up possibilities in a lot of fields," said SLAC staff scientist Jake Koralek, who led the research with Daniel DePonte, leader of the LCLS Sample Environment Department.

"Until now, we haven't been able to examine samples suspended in water with two types of light – infrared and 'soft', lower-energy X-rays – that are important for making images and using spectroscopy to study basic processes in physics, chemistry and biology, including the physics of water," Koralek said.

"The new nozzle we developed, which can create flowing sheets of liquid just 100 water molecules thick that persist for days in a vacuum, solves that problem. The sheets can even be used to image samples with electron beams that resolve even smaller details."

Shaping Liquid with Gas

The nozzle is a tiny glass chip with three microscopic channels. A stream of liquid flows through the middle channel, shaped by flows of gas coming in from the channels on either side. This particular nozzle was made with photolithography, a technique used

These images show the formation of tiny sheets of liquid shaped by jets of gas from a nozzle developed at SLAC. Top: As the gas flow increases, the liquid sheets become bigger. Bottom: The nozzle produces a series of liquid sheets; the one closest to the nozzle is the widest and thinnest. Each sheet is perpendicular to the previous one, so we are seeing the second and fourth sheets from the side. Credit: SLAC National Accelerator Laboratory


As the scientists turn up the speed of the gas flow, the liquid stream spreads into a series of sheets whose width and thickness can be precisely controlled. The sheet closest to the nozzle is the widest and thinnest; the farther they get from the nozzle, the narrower and thicker the sheets become until they finally merge into a cylindrical stream.

The sheets shimmer like soap bubbles in a variety of colors, the result of light reflecting off both the front and back surfaces of the sheet. And just as the contour lines on a topographic map mark differences in elevation, the hue and spacing of a sheet's ever-changing bands of color indicate how thick it is and how much the thickness changes from one point to another.

"It's a very flexible and reliable design for creating both ultrathin and slightly thicker liquid sheets, which can be desirable for some applications" said Linda Young, a distinguished fellow at DOE's Argonne National Laboratory and professor at the University of Chicago who was not involved in the study.

She said she will be using the nozzle to make slightly thicker sheets of water for an LCLS study of how water molecules behave after one of their electrons has been ripped away. These ionized water molecules persist for only a few hundred femtoseconds, or millions of a billionth of a second, and "the X-rays provide a completely new and clean wayto monitor their electronic response in their natural environment, so that's why we're excited about it," Young said.

A new way to study extreme forms of water

The liquid sheets have already been used in experiments that explore the properties of water in extreme environments like those on giant planets, said co-author Siegfried Glenzer, a SLAC professor and head of the lab's High Energy Density Science Division.

Those experiments were performed with the FLASH free-electron laser at Germany's Deutsches Elektronen-Synchrotron (DESY). Researchers used X-ray pulses to heat the liquid sheets to thousands of degrees to simulate the extremely warm, dense form of water present in giant planets like Jupiter. Then they measured the reflectivity and conductivity of the super-hot water with optical laser pulses in the instant before the water vaporized. These measurements could only be made on a flat sheet of water.

"There are many mysteries in those big planets and they're important for understanding the evolution of our planetary system as well as others," Glenzer said. "This is a beautiful tool for studying water itself, and in the future we will also study other materials that we can mix into it."

The team measured the thickness of the sheets with a beam of infrared light at the Advanced Light Source at the DOE's Lawrence Berkeley National Laboratory, and also demonstrated that the sheets could be used for infrared spectroscopy, where light absorbed by a material reveals its chemical makeup.

Abstract-Anharmonic Coupling between Intermolecular Motions of Water Revealed by Terahertz Kerr Effect

Tobias Kampfrath, Martin Wolf, Mohsen Sajadi

(Submitted on 24 Jul 2017)

https://arxiv.org/abs/1707.07622

Formation of local molecular structures in liquid water is believed to have marked effect on the bulk properties of water, however, resolving such structural motives in an experiment is challenging. This challenge might be handled if the relevant low-frequency structural motion of the liquid is directly driven with an intense electromagnetic pulse. Here, we resonantly excite diffusive reorientational motions in water with intense terahertz pulses and measure the resulting transient optical birefringence. The observed response is shown to arise from a particular configuration, namely the restricted trans-lational motion of water molecules whose motions are predominantly orthogonal to the dipole moment of the excited neighboring water molecules. Accordingly, we estimate the strength of the anharmonic coupling between the rotational and the restricted translational degrees of freedom of water.

Abstract-Observation of Broadband Terahertz Wave Generation from Liquid Water

Qi Jin, Yiwen E, Kaia Williams, Jianming Dai, and Xi-Cheng Zhang

https://www.osapublishing.org/abstract.cfm?uri=NLO-2017-NW3A.1

Liquid water is a strong absorber in THz range, making the presence of water a nuisance when collecting measurements within this frequency regime. Here, we experimentally demonstrate the generation of broadband THz waves from liquid water with a femtosecond laser. The signal is obtained when a high-intensity laser beam is focused within a water film. This THz signal is distinguished from the signal generated with air plasma by translating the water film across the focal spot. Our observation demonstrates the possibility that liquid water can be a competitive THz source. Our research is also expected to enrich the study of laser-water interactions.

© 2017 OSA

Abstract-Water is an active matrix of life for cell and molecular biology

1. Philip Ball
2. 1. http://www.pnas.org/content/early/2017/06/06/1703781114.full
   2. 
      

Szent-Győrgi called water the “matrix of life” and claimed that there was no life without it. This statement is true, as far as we know, on our planet, but it is not clear whether it must hold throughout the cosmos. To evaluate that question requires a close consideration of the many varied and subtle roles that water plays in living cells—a consideration that must be free of both an assumed essentialism that gives water an almost mystical life-giving agency and a traditional tendency to see it as a merely passive solvent. Water is a participant in the “life of the cell,” and here I describe some of the features of that active agency. Water's value for molecular biology comes from both the structural and dynamic characteristics of its status as a complex, structured liquid as well as its nature as a polar, protic, and amphoteric reagent. Any discussion of water as life’s matrix must, however, begin with an acknowledgment that our understanding of it as both a liquid and a solvent is still incomplete.

My Note: From the paper-
Low-frequency, large-amplitude modes in the terahertz range are particularly important in controlling the conformational changes that dominate protein function, and are conveniently probed using terahertz spectroscopy….However, there is no simple qualitative account of how protein and solvent dynamics interact. Fluctuations of both take place over a wide range of timescales from milliseconds to picoseconds, influencing several aspects of protein function . Because no single technique can span so many temporal orders of magnitude, there has been considerable debate about how to reconcile the results of different experimental methods that explore dynamics, such as NMR relaxation, neutron scattering, ultrafast IR, and terahertz spectroscopies

Abstract-Spatially resolved dielectric constant of confined water and its connection to the non-local nature of bulk water

Christian Schaaf1,2 and Stephan Gekle2


1 Institute of Theoretical Physics, Technical University Berlin, Hardenbergstraße 36, 10623 Berlin, Germany
2 Biofluid Simulation and Modeling, Fachbereich Physik, Universität Bayreuth, Universitätsstraße 30, Bayreuth 95440, Germany
J. Chem. Phys. 145, 084901 (2016); http://dx.doi.org/10.1063/1.4960775
http://scitation.aip.org/content/aip/journal/jcp/145/8/10.1063/1.4960775?TRACK=RSS

We use molecular dynamics simulations to compute the spatially resolved static dielectric constant of water in cylindrical and spherical nanopores as occurring, e.g., in protein water pockets or carbon nanotubes. For this, we derive a linear-response formalism which correctly takes into account thedielectric boundary conditions in the considered geometries. We find that in cylindrical confinement, the axial component behaves similar as the local density akin to what is known near planar interfaces. The radial dielectric constant shows some oscillatory features when approaching the surface if their radius is larger than about 2 nm. Most importantly, however, the radial component exhibits pronounced oscillations at the center of the cavity. These surprising features are traced back quantitatively to the non-localdielectric nature of bulk water

Saykally continues quest for “universal first-principles” model of water

http://chemistry.berkeley.edu/news/saykally-continues-quest-for-water
by Michael Barnes

For a simple compound that is central to almost every aspect of our existence, water remains fiendishly difficult to understand. No one appreciates this better than chemistry professor Rich Saykally, who has devoted many years to studying water.

Even a minute sample contains far too many atoms and bonds to study, so Saykally has explored water from the bottom up, first by creating clusters of just two molecules and working up to larger and larger clusters.

In an article in the June 3 issue of Science magazine, Saykally and co-workers characterize the water octamer, a cluster of eight water molecules in a roughly cubic form. Says Saykally, “Understanding the octamer is important because it represents a transition to structures formed by stacking quasi-planar rings, a dominant pattern in larger systems. The water octamer has become an important benchmark.”

A huge effort has been devoted towards the development of computer models for water that can correctly describe its structure and physical properties, thermodynamics, phase behavior, solvation properties, and ultimately, its chemistry.

 “That we still do not have such a robust computational approach for water, let alone for aqueous solutions and aqueous interfaces, is a great impediment to science,” Saykally notes.

While isolated clusters do not exist as such in bulk water, highly detailed spectroscopic study provides accurate benchmarks for characterization of the complicated many-body forces that operate in bulk-water phases.

Graphic: The two cuboidal structures of the water octamer characterized by infrared and terahertz laser spectroscopy and theory. These cuboids are formed by stacking of two four-membered rings with the circular direction of the hydrogen bonds in the same (left) and opposite (right). Larger clusters form via a similar stacking of larger quasiplanar rings.

In the Science paper, the Saykally lab has presented the first high-resolution spectroscopic study of the water octamer. Terahertz vibration-rotation-tunneling (VRT) spectroscopy was used for the measurements. This work complements recent and elegant microwave spectroscopy characterization of the water heptamer and nonamer, also published in Science. The octamer could not be detected by that method because it lacks a dipole moment.

Nearly 100 individual measurements were made with parts-per-million accuracy and fitted to a standard model, which characterizes the structures and vibrational distortions of the cluster. Two distinct cuboidal structures were characterized via measurement of their torsional vibrations.  The results are in good agreement with recent theoretical predictions of the hydrogen-bond rearrangement tunneling rates and octamer cluster structures.

Considerable progress has been realized over the last several years by the Saykally group and their collaborators in the long-sought quest for a universal first-principles model of water. This goal has been aided by the development of several new potential energy surfaces, including new spectroscopically refined surfaces.

“Our approach is to help develop, test, and refine potential energy surfaces via our VRT spectroscopy results for water clusters, in combination with state-of-the-art theoretical calculations,” says Saykally.

It is important to continue these efforts in order to realize the ambitious but crucial goal of producing a water model that is relatively simple, yet capable of accurately reproducing and predicting observable properties of water in all of its forms (including the liquid-vapor interface) over large ranges of conditions. That is the goal that Saykally continues to pursue.

Abstract-Threshold for Terahertz Resonance of Nanoparticles in Water

Dao Xiang, Jian Wu, Joerg Rottler, and Reuven Gordon

Nano Lett., Just Accepted Manuscript

DOI: 10.1021/acs.nanolett.6b00770

Publication Date (Web): May 20, 2016

Copyright © 2016 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.6b00770?journalCode=nalefd

Nanoparticle vibrations are coupled to light through electrostriction, which gives nonlinear optical scattering. We investigated the acoustic response of 2 nm gold nanoparticles using nearly-degenerate four-wave mixing experimental configuration and show that the nonlinear response suddenly turns on at low powers (<100 mW) for continuous-wave (CW) lasers. The observed nonlinear response is a million times larger than typical electronic nonlinearities. The threshold implies a dramatic change in the quality factor of the vibrating nanoparticles, 4 orders of magnitude larger than usual hydrodynamic theory predicts. It is as if the water is removed altogether, which we speculate is the result of the vibrating particle pushing away the water molecules to form a stable cavity. Since these acoustic vibrations extend to terahertz frequencies, there is potential to harness this effect for high speed optical data processing; as well as to probe the dynamics of proteins all having acoustic modes in this range.

EU grants €2.5 million for terahertz probe into protein reactions

Team led by Bochum's Martina Havenith develops new method to measure real time heat and energy changes at protein-solvent interface.

Prof Dr Martina Havenith from the Ruhr-Universität Bochum (RUB) http://optics.org/news/7/4/25 The European Research Council(ERC) is to finance a research team led by Prof Dr Martina Havenith from the Ruhr-Universität Bochum (RUB) with €2.5 million over five years to develop an optical procedure to investigate the behavior of proteins in biochemical reactions. In a process called time-resolved terahertz calorimetry, the RUB scientists say they will “tinker with terahertz laser spectroscopy to deduce with nanosecond precision how proteins and solvent exchange energy and rearrange themselves in biological processes”. Professor Havenith, who is Chair of the Department of Physical Chemistry II at RUB and speaker for the cluster of known as European Research Council (“Resolv”), will lead an international team with Prof Dr Irit Sagi from the Weizmann-Institute in Israel and Dr Matthias Heyden from the Max-Planck-Institute for Coal Research in Mülheim at the Ruhr, Germany. ’Underestimated role of water’ Fundamental biological processes such as protein folding, protein-protein interactions and enzymatic reactions take place in watery solutions and depend upon the ability of the reacting molecules to recognize themselves in a solvent. Molecular recognition is mediated through various chemical interactions that can be studied by calorimetry, measuring enthalpy, entropy d other associated energetic parameters. Unfortunately, current calorimetric approaches are all based on heat transfer, hence they deliver results on time scales of 1 to 100 seconds and can analyze the system only at equilibrium, that is when molecules are already bound. However, as Havenith points out, a successful reaction is the result of a dynamic interplay between the partner molecules and the solvent. "Terahertz-Calorimetry promises to investigate these processes, for the first time in real time with a million time better time resolution," the scientists claim. The new technique will use terahertz light sources to excite solvent molecules and biomolecules at the time scales of hydrogen bond dynamics in water as well as large-amplitude motions of proteins and nucleic acids. “If we are able to collect full information on these time-scale dynamics, we can deduce useful values of entropy, enthalpy, and so on associated with molecular recognition processes”, said Havenith. Time-resolved terehaertz calorimetry will also offer, the opportunity to directly access the distinct contributions of the solute entropy changes versus those of the solvent. To this end, the Bochum-based project will combine the expertise gathered in terahertz laser development with microwave technology, biophysics, calorimetry and theoretical modeling. “Interdisciplinary discussions within Resolv were crucial to inspire us the THz calorimetry idea”, added Havenith. Two grants In the latest application round for the ERC's Advanced Grants, two further researchers from the Ruhr-Universität Bochum have been successful: IT security expert Prof Dr Christof Paar also raised such a grant. Via these two projects, the RUB will receive up €5 million in total. The European Union launched the European Research Council in 2007 to fund excellent research in Europe. The ERC Advanced Grants program targets senior researchers with ground-breaking ideas that may be risky to pursue, awarding up to €2.5 million for five years. The 2015 call received almost 2000 project applications from 29 countries, but only about 14% of them were selected after a peer review process. “I received the confirmation message at Zürich airport after an 11-hour flight. I had to read it three times before I believed it. It is such a great chance to test some exciting new ideas”, said Havenith.

Abstract-The Crucial Role of Water in Shaping Low-Barrier Hydrogen Bonds

Michael T Ruggiero and   Timothy Michael Korter  

Phys. Chem. Chem. Phys., 2016, Accepted Manuscript

DOI: 10.1039/C5CP07760E
Received 15 Dec 2015, Accepted 25 Jan 2016
First published online 25 Jan 2016

http://pubs.rsc.org/en/content/articlelanding/2016/cp/c5cp07760e#!divAbstract

Low-barrier hydrogen bonds (LBHBs) are key components in a range of chemical processes, often appearing in metal-mediated catalytic applications. The formation of LBHBs has generally been attributed to the particular geometric arrangement of the surrounding atoms and molecules, yet few specific atomic-level details have emerged. In this study, several metal maleate tetrahydrates have been studied using a combination of solid-state density functional theory, terahertz spectroscopy, and X-ray diffraction to evaluate the significance of both water molecules and metal cations in guiding LBHB formation and function. The findings reveal the assumption that metal identity is of paramount importance to be incomplete, and that the metal cation does not directly influence the LBHB in the maleate ligand. Rather, the characteristics are regulated by water molecule positioning, asserting the critical role of water in governing LBHBs and providing new insight into their formation mechanisms.

Probing how alcohol affects the structure of water

http://atlasofscience.org/probing-how-alcohol-affects-the-structure-of-water/

Water, despite its ubiquity, is an extraordinarily complex substance. The structure of water on a molecular level is defined by the interactions, “hydrogen bonds”, that individual molecules have with each other. These are the reason that upon turning to solid ice, water takes up more space that it does as a liquid. The addition of other substances to water can dramatically alter these interactions and hence the properties of water. The impact of adding alcohols to water is particularly interesting. Not only are alcohol/water mixtures widely used in chemical processes, for example as solvents, but they provide an excellent model system for considering how more complex species such as proteins influence the structure of water. The same interactions also take place in alcoholic beverages where ethanol is mixed with water and hence play a key role in the properties of those, perhaps influencing their flavor.

Fig. 1. Schematic showing a “molecular diffusive jump” of 2-propanol in water.

In order to probe the structure of alcohol/water mixtures it is necessary to apply a range of advanced techniques. In this work we have utilized NMR relaxation time analysis, terahertz time-domain spectroscopy and neutron diffraction. These techniques are ideally suited for probing the structure of alcohol/water structures on the most relevant lengthscales: NMR relaxation time analysis can provide the energy barrier for a molecule to break its interactions with its neighbors and form new ones with other molecules; terahertz time-domain spectroscopy can probe the hydrogen-bonding interactions and give a measure of the average number of water molecules surrounding an alcohol molecule in solution’ while neutron diffraction probes the structure of the solutions, yielding data on the number, type and strength of hydrogen bonds present. The results of this study show that even the addition of a small quantity of alcohol, in this case 2-propanol, has a significant impact on the structure of water.

Fig. 2. Spatial probability densities of 2-propanol (methyl group – green; oxygen – red) and water (blue) at 90 mol % H2O.

Specifically, at a composition ratio of nine water molecules to one 2-propanol molecule, the mixture shows the greatest deviation from ideality; i.e. deviation from what would be expected from a simple additive mixture of the two components. By employing the range of advanced techniques described above we have shown that there are four-to-five water molecules in the immediate surroundings of an alcohol molecule. The alcohol molecules are therefore intimately mixed throughout the water, altering its hydrogen bonding structure and hence its properties. For example, this directly impacts on the mobility of hydrogen through the liquid media – a key step in many chemical processes including hydrogenation reactions; the viscosity of the mixture; and other related properties such as the velocity of sound through the mixture. Figure 1 shows a schematic representation of a 2-propanol molecule moving through water, the energetics of which are directly probed in this study, while Figure 2 shows Spatial probability densities of 2-propanol and water in a 90% water / 10% 2-propanol mixture as derived from neutron diffraction studies.

Understanding the impact of additives, e.g. 2-propanol, has the potential to have a significant impact in any area where water finds application: from catalysis, to fuel cells, to biological processing to alcoholic beverages. The knowledge of the meso-scale structure of such mixtures directly informs the design of such processes and products. A similar approach could be applied to, for example, understand the impact of salts on the structure and dynamics of water. Water surrounds us, and plays a crucial role in supporting life, however we are only now beginning to understand the properties of this highly complex substance.

James McGregor
University of Sheffield

Publication

Structure and dynamics of aqueous 2-propanol: a THz-TDS, NMR and neutron diffraction study.
McGregor J, Li R, Zeitler JA, D’Agostino C, Collins JH, Mantle MD, Manyar H, Holbrey JD, Falkowska M, Youngs TG, Hardacre C, Stitt EH, Gladden LF.
Phys Chem Chem Phys. 2015 Nov 11

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.

###

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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