Thursday, April 12, 2012

How electrons outrun vibrating nuclei -- the X-ray movie

 Researchers at the Max-Born-Institute, Berlin, Germany, resolved spatial oscillations of electrons in a crystal by taking a real-time 'movie' with ultrashort x-ray flashes. Outer electrons move forth and back over the length of a chemical bond and modulate the electric properties while the tiny elongation of the inner electrons and the atomic nuclei is less than 1% of this distance.

A crystal represents a regular array of atoms in space, a so-called lattice, which is held together by interactions between the electron clouds of neighboring atoms. While most electrons are tightly bound to the positively charged nuclei, the outermost valence electrons form chemical bonds to the next neighbors. Such bonds determine the distance between atoms in the crystal as well as basic properties such as mechanical stability or the electrical behavior.

In the crystal lattice, atoms are not at rest but perform vibrational motions around their equilibrium positions. The spatial elongation of the vibrating nuclei together with their core electrons is a tiny fraction -- typically less than 1 percent -- of the distance between neighboring atoms. With respect to the outer valence electrons, the situation is much less clear and their elongations have remained unknown in many cases. Measuring the motions of valence electrons in space and time is important for understanding their fundamental role for the crystal's static and dynamic electric properties.

To address this issue, Flavio Zamponi, Philip Rothhardt, Johannes Stingl, Michael Woerner, and Thomas Elsaesser built an x-ray "reaction microscope" which allows for an in situ imaging of moving electrons and atoms in crystalline materials. As they report in PNAS (doi/10.1073/pnas.1108206109) vibrations in the ionic crystal potassium dihydrogen phosphate (KDP) are kicked off by excitation with an optical pulse of 50 femtosecond duration (1 fs = 10-15 seconds). The momentary position of atoms and electrons is measured with high spatial resolution by 100 fs hard x-ray pulses which are diffracted from the vibrating atoms. Measuring simultaneously many different x-ray diffraction peaks allows for reconstructing the momentary distances of atoms and in turn the three-dimensional distribution of electrons within the crystal. Taking x-ray snap shots at various delay times after initiating the vibrations creates a molecular movie according to the well known stroboscope effect.

It was a big surprise for the researchers that for a special kind of lattice vibrations (the so called soft mode of KDP) the involved valence electrons move a 30 times larger distance than the involved atoms (i.e. nuclei plus core electrons) when performing their oscillatory motion. Such a scenario is sketched in the electron density maps shown in Fig. 1. During the soft mode oscillation an electron initially residing on the phosphorus (P) atom moves to one of the neighboring oxygen (O) atoms (P-O bond length: 160 picometers (10-12 m)) and returns to the P-atom after half the oscillation period. However, when measuring the positions of the involved atoms one finds that the latter move just a few picometers. This is very surprising, because according to textbook knowledge one expects the same motion as that of the nucleus for all electrons of an atom. To understand this unexpected large-amplitude motion of valence electrons, one has to consider the electric forces the oscillating ionic lattice exerts on the electrons during the soft mode vibration. Theories developed in the 1960's predicted such a behaviour which is now experimentally proven for the first time and determines the ultrahigh-frequency electric behavior of the material. In the attached movie, we show the iso-electron density surface of the phosphate ion during the soft mode oscillation in a KDP crystal.

The femtosecond x-ray powder diffraction method demonstrated here can be applied to many other systems in order to map ultrafast structure changes in physical and chemical processes.


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The above story is reprinted from materials provided by Forschungsverbund Berlin e.V. (FVB), via AlphaGalileo.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

F. Zamponi, P. Rothhardt, J. Stingl, M. Woerner, T. Elsaesser. Ultrafast large-amplitude relocation of electronic charge in ionic crystals. Proceedings of the National Academy of Sciences, 2012; DOI: 10.1073/pnas.1108206109

New twist on 1930s technology may become a 21st century weapon against global warming

 Far from being a pipe dream years away from reality, practical technology for capturing carbon dioxide -- the main greenhouse gas -- from smokestacks is aiming for deployment at coal-fired electric power generating stations and other sources, scientists saidin San Diego March 27. Their presentation at the 243rd National Meeting of the American Chemical Society was on a potential advance toward dealing with the 30 billion tons of carbon dioxide released into the air each year through human activity.

"With little fanfare or publicity and a decade of hard work, we have made many improvements in this important new technology for carbon capture," said James H. Davis, Jr., Ph.D., who headed the research. "In 2002, we became the first research group to disclose discovery of the technology, and we have now positioned it as a viable means for carbon dioxide capture. Our research indicates that its capacity for carbon dioxide capture is greater than current technology, and the process is shaping up to be both more affordable and durable as well."

The new approach has a back-to-the-future glint, leveraging technology that the petroleum industry has used since the 1930s to remove carbon dioxide and other impurities from natural gas. Davis, who is with the University of South Alabama (USA) in Mobile, explained that despite its reputation as a clean fuel, natural gas is usually contaminated with a variety of undesirable materials, especially carbon dioxide and hydrogen sulfide. Natural gas from certain underground formations, so-called "sweet" gas, has only small amounts of these other gases, while "sour" gas has larger amounts. Natural gas companies traditionally have used a thick, colorless liquid called aqueous monoethanolamine (MEA) to remove that carbon dioxide.

Several problems, however, would prevent use of MEA to capture carbon dioxide on the massive basis envisioned in some proposed campaigns to slow global warming. These involve, for instance, capturing or "scrubbing" the carbon dioxide from smokestacks before it enters the atmosphere and socking it away permanently in underground storage chambers. Vast amounts of MEA would be needed, and its loss into the atmosphere could create health and environmental problems, and it would be very costly.

Davis and his group believe that their new approach avoids those pitfalls. It makes use of a nitrogen-based substance termed an "ionic liquid" that binds to carbon dioxide very effectively. Unlike MEA, it is odorless, does not evaporate easily and can be easily recycled and reused.

Davis also described one important advantage the technology has over many other ionic liquid carbon-capture systems. He explained that the presence of water, like moisture in the atmosphere, reduces the effectiveness of many nitrogen-based ionic liquids, complicating their use. Water is always present in exhaust gases because it is a byproduct of combustion. Davis noted that the liquids prefer to interact with carbon dioxide over water, and thus are not hampered by the latter in real-world applications.

Although cautioning that the final application in power plants or factories may look different, Davis envisioned a possible set-up for power plants that would be similar to the one used in his laboratory. He described bubbling exhaust gas through a tank full of the nitrogen-based liquid, which the system could cycle out and replace with fresh liquid. Removing the carbon dioxide would create a new supply of ionic liquid. Once removed, companies could sequester the carbon dioxide by burying it or finding another way to keep it permanently out of the atmosphere. Others have suggested using captured carbon dioxide in place of petroleum products to make plastics and other products.

Davis suggested that in the future, people might also use the technology on a smaller scale in cars or homes, although he cautioned that these applications were likely a long way away. While his group has not fully explored the possible dangers of the chemicals his technology uses, Davis noted that his compounds are quite similar to certain compounds which are known to be safe for consumer use.

His presentation was part of a symposium on research advances involving "ionic liquids," strange liquids that consist only of atoms stripped of some of their electrons, with applications ranging from food processing to energy production.

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The above story is reprinted from materials provided by American Chemical Society (ACS), via Newswise.

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Analyzing food quality with an artificial intestine: the NutriChip

What happens in our bodies when we have eaten something? Are “healthy” food products actually good for us, once they have been digested and absorbed? Supported by Nano-Tera and Nestlé, the NutriChip project developed by Martin Gijs’s team at the Laboratory of Microsystems 2 (LMIS2) provides new insights to these questions. The NutriChip is a miniature artificial intestinal wall that can be used to identify foods that cause inflammation in the human body.

Preventing chronic inflammatory illness

“Generally, once a given food has been digested and absorbed by the intestine, it carries certain molecules into the body, such as Palmitic acid,” says Guy Vergeres, a member of the Agroscope Liebefeld-Posieux (ALP) Research Center, which is collaborating on the project. These molecules set off an immune response, in the form of slight, temporary inflammation. Biomarkers for inflammation, notably cytokines, can then be found in the blood. This is a normal phenomenon, but it must be monitored. “If this happens over and over for a long period of time, it can set the stage for inflammatory chronic illnesses,” warns Vergeres.

The NutriChip platform makes it possible to compare different foods in terms of their ability to lower the concentrations of those biomarkers – and thus possibly their ability to reduce inflammation itself. The research team began its tests with milk, a food that is widely consumed in Switzerland. “Some studies have shown that dairy products can reduce the concentration of inflammatory biomarkers in the blood, while others did not find any significant reduction in concentrations. With the NutriChip, we will be able to make a contribution to this debate,” says Martin Gijs.

The complexities of artificial digestion

The human body is complex, and designing a miniature artificial gastrointestinal system proved to be extremely exacting. The solution provided by researchers at EPFL ultimately took the shape of a two-level chip, whose levels are connected via a porous membrane.

The upper level, which represents the intestinal wall, is made of a homogeneous layer of cultured epithelial cells. The lower level represents the circulatory system and is made up of immune system cells, and in particular macrophages. The macrophages’ job within the human body is to keep it clean: when they encounter any potentially dangerous agents they release molecules such as cytokines that activate other immune-system cells. The NutriChip platform uses CMOS high-resolution optical sensors developed by Sandro Carrara’s team in the EPFL’s Integrated Sytems Lab in order to precisely detect and measure cytokine production by the immune cells that are on the other side of the layer of intestinal wall cells. These measurements, which are performed using fluorescence, show exactly how much inflammation is caused by a given food.

“We have to reproduce every stage in the digestive process before food hits the intestine,” says Professor Gijs. Milk, for instance, is successively digested by the enzymes and chemical components from the saliva, gastric juices, pancreatic juices and bile. The mixture that emerges from this process is then applied to the upper level of the NutriChip.

Is milk an anti-inflammatory?

Some studies have found that milk can reduce the concentration of inflammatory biomarkers in humans. However, these results need to be confirmed. “On another front, studies are being done on volunteers at the Bern University Hospital to find links between body mass, diet, and pro-inflammatory cytokine production,” says Martin Gijs. “The study participants eat various types of meals, and then afterwards their cytokine levels are measured via a blood test. Blood tests could tell us whether we obtain the same results with the NutriChip artificial intestine.” If the project team does obtain the same results, this will pave the way for in vitro screening of various types of foods to determine their pro- or anti-inflammatory potentials. The most promising foods could then be tested more intensely, via nutritional studies.

Provided by Ecole Polytechnique Federale de Lausanne

Forces among molecules: Tiny but important

 Forces are not only associated with machines or muscles. You can also find them elsewhere, for instance between molecules. Theoretical chemists like Dr. Łukasz Tomasz Rajchel (University of Warsaw) are familiar with that. However, they -- or rather their computers -- are not capable of calculating them with high accuracy and efficiency at the same time.

The scholarship holder of the Alexander von Humboldt Foundation wants to get to the bottom of the computational problem while working in Prof. Dr. Georg Jansen's Theoretical Organic Chemistry team at the University Duisburg-Essen (UDE).

Since intermolecular forces are very small, the computational technique must be very precise. Furthermore, getting significant results by experiment is difficult. For solving the task Łukasz Rajchel refers to various approximations of quantum chemistry. "They form my theoretical basis and shall help me develop new approaches for calculating intermolecular energies." The 30-year-old chemist solves the underlying equations with the help of self-developed computer codes.

The more Łukasz Rajchel and his colleagues get to know about the interactions between chemical compounds, the better they can understand matter and predict its characteristics. The significance of those tiny forces cannot be stressed enough. "They are substantial in nature," says Dr. Rajchel. For example: they are responsible for DNA and RNA's stability in genetic information or for the existence of molecular crystals and the proteins' structure. Interestingly, they also let the gecko walk on vertical glass surfaces.

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The above story is reprinted from materials provided by Universität Duisburg-Essen, via AlphaGalileo.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

New process converts polyethylene into carbon fiber

( -- In a paper published in , a team led by Amit Naskar of the Materials Science and Technology Division outlined a method that allows not only for production of carbon fiber but also the ability to tailor the final product to specific applications.

"Our results represent what we believe will one day provide industry with a flexible technique for producing technologically innovative fibers in myriad configurations such as fiber bundle or non-woven mat assemblies," Naskar said.

Using a combination of multi-component fiber spinning and their sulfonation technique, Naskar and colleagues demonstrated that they can make polyethylene-base fibers with a customized surface contour and manipulate diameter down to the submicron scale. The patent-pending process also allows them to tune the , making the material potentially useful for filtration, and harvesting.

Naskar noted that the sulfonation process allows for great flexibility as the exhibit properties that are dictated by processing conditions. For this project, the researchers produced carbon fibers with unique cross-sectional geometry, from hollow circular to gear-shaped by using a multi-component melt extrusion-based fiber spinning method.

The possibilities are virtually endless, according to Naskar, who described the process.

"We dip the fiber bundle into an acid containing a chemical bath where it reacts and forms a black fiber that no longer will melt," Naskar said. "It is this sulfonation reaction that transforms the plastic fiber into an infusible form.

"At this stage, the plastic molecules bond, and with further heating cannot melt or flow. At very , this fiber retains mostly carbon and all other elements volatize off in different gas or compound forms."

The researchers also noted that their discovery represents a success for DOE, which seeks advances in lightweight materials that can, among other things, help the U.S. auto industry design cars able to achieve more miles per gallon with no compromise in safety or comfort. And the raw material, which could come from grocery store plastic bags, carpet backing scraps and salvage, is abundant and inexpensive.

More information: "Patterned functional carbon fibers from polyethylene," http://onlinelibra … 01104551/pdf

Provided by Oak Ridge National Laboratory (news : web)