Neutron scattering provides window into surface interactions

To better understand the fundamental behavior of molecules at surfaces, researchers at the U.S. Department of Energy's Oak Ridge National Laboratory are combining the powers of neutron scattering with chemical analysis.

Scientists have a fundamental interest in how molecules behave at solid surfaces because surface interactions influence chemistry, such as in materials for catalysis, drug delivery and carbon sequestration. Understanding these interactions allows researchers to tailor materials for a specific desirable outcome.

Michelle Kidder and A.C. Buchanan, physical organic chemists, and Ken Herwig, neutron scattering scientist, used neutron scattering to study the physical motion of a chemically attached organic molecule inside a silica nanopore, MCM41.

"There is a connection between a molecule's dynamic behavior or motion to its surroundings." Herwig said. "In particular, restricting the ability of a molecule to freely move by confining it to a small volume dramatically affects both the range and character of its movement. We are trying to gain insight into the connection between the changes in molecular motion and the changes in chemistry that occur when molecules are attached to a solid surface."

Herwig used neutron scattering to gain a unique perspective into molecular motion because neutrons are sensitive to the hydrogen atoms, which are present in many molecules that researchers are interested in. Additionally, neutron scattering simultaneously tells researchers how rapid the motion is and what type of motion they are observing on the atomic and nanoscale.

If scientists understand how pore size affects surface interactions, they can modify pore size to change a chemical product outcome.

To study surface interactions, Kidder synthesized both the organic molecules and MCM41 of different pore sizes, then chemically attached the molecules to the silica pore surface, which forms an organic-inorganic hybrid material. This hybrid material is used in studies to understand chemical decomposition pathways, where surface interactions were presumed to play a role.

"We are interested in understanding the thermo decomposition of molecules similar to those found in biomass resources," Kidder said. "What we have seen is that there are many local environmental factors that influence chemical reactivity and products, and one of those large influences occurs when a molecule is confined to a pore wall, where even the pore size has a large impact on reactivity."

This research was funded by DOE's Office of Science.

Story Source:

The above story is reprinted from materials provided by DOE/Oak Ridge National Laboratory.

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

Journal Reference:

Edward J. Kintzel, Michelle K. Kidder, A. C. Buchanan, Phillip F. Britt, Eugene Mamontov, Michaela Zamponi, Kenneth W. Herwig. Dynamics of 1,3-Diphenylpropane Tethered to the Interior Pore Surfaces of MCM-41. The Journal of Physical Chemistry C, 2012; 116 (1): 923 DOI: 10.1021/jp209458a

Atoms dressed with light show new interactions, could reveal way to observe enigmatic particle

Physicists at the National Institute of Standards and Technology (NIST) have found a way to manipulate atoms' internal states with lasers that dramatically influences their interactions in specific ways. Such light-tweaked atoms can be used as proxies to study important phenomena that would be difficult or impossible to study in other contexts.

Their most recent work, appearing in Science, demonstrates a new class of interactions thought to be important to the physics of superconductors that could be used for quantum computation.

Particle interactions are fundamental to physics, determining, for example, how magnetic materials and high temperature superconductors work. Learning more about these interactions or creating new "effective" interactions will help scientists design materials with specific magnetic or superconducting properties.

Because most materials are complicated systems, it is difficult to study or engineer the interactions between the constituent electrons. Researchers at NIST build physically analogous systems using supercooled atoms to learn more about how materials with these properties work.

"Basically, we're able to simulate these complicated systems and observe how they work in slow motion," says Ian Spielman, a physicist at NIST and fellow of the Joint Quantum Institute (JQI), a collaborative enterprise of NIST and the University of Maryland.

According to Ross Williams, a postdoctoral researcher at NIST, cold atom experiments are good for studying many body systems because they offer a high degree of control over position and behavior of the atoms.

"First, we trap rubidium-87 atoms using magnetic fields and cool them down to 100 nanokelvins," says Williams. "At these temperatures, they become what's known as a Bose-Einstein condensate. Cooling the atoms this much makes them really sluggish, and once we see that they are moving slowly enough, we use lasers to 'dress' the atoms, or mix together different energy states within them. Once we have dressed the atoms, we split the condensate, collide the two parts, and then see how they interact."

According to Williams, without being laser-dressed, simple, low-energy interactions dominate how the atoms scatter as they come together. While in this state, the atoms bang into each other and scatter to form a uniform sphere that looks the same from every direction, which doesn't reveal much about how the atoms interacted.

When dressed, however, the atoms tended to scatter in certain directions and form interesting shapes indicative of the influence of new, more complicated interactions, which aren't normally seen in ultracold atom systems. The ability to induce them allows researchers to explore a whole new range of exciting quantum phenomena in these systems.

While the researchers used rubidium atoms, which are bosons, for this experiment, they are modifying the scheme to study ultracold fermions, a different species of particle. The group hopes to find evidence of the Majorana fermion, an enigmatic, still theoretical kind of particle that is involved in superconducting systems important to quantum computation.

"A lot of people are looking for the Majorana fermion," says Williams. "It would be great if our approach helped us to be the first."

Story Source:

The above story is reprinted from materials provided by National Institute of Standards and Technology (NIST).

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

Journal Reference:

R. A. Williams, L. J. Leblanc, K. Jiménez-García, M. C. Beeler, A. R. Perry, W. D. Phillips, I. B. Spielman. Synthetic Partial Waves in Ultracold Atomic Collisions. Science, 2011; DOI: 10.1126/science.1212652

New equation predicts molecular forces in hydrophobic interactions

The physical model to describe the hydrophobic interactions of molecules has been a mystery that has challenged scientists and engineers since the 19th century. Hydrophobic interactions are central to explaining why oil and water don't mix, how proteins are structured, and what holds biological membranes together. Chemical engineering researchers at UC Santa Barbara have developed a novel method to study these forces at the atomic level, and have for the first time defined a mathematical equation to measure a substance's hydrophobic character.

"This discovery represents a breakthrough that is a culmination of decades of research," says Professor Jacob Israelachvili. "The equation is intended to be a tool for scientists to begin quantifying and predicting molecular and surface forces between organic substances in water."

Using a light-responsive surfactant -- a soap-like molecule related to fats and lipids -- the researchers developed an innovative technique to measure or change the forces between layers of the molecule in water by using beams of UV or visible light. The result is a general equation that applies to even more complicated systems, such as cellular membranes or proteins.

"We were fortunate to find the right combination of experimental methods and theory," said Brad Chmelka, UCSB Chemical Engineering professor and co-author of the study. "The keys to our research were using a light-responsive surfactant molecule, a means of measuring these delicate surface forces, and applying knowledge of what to look for."

The highly-sensitive instrument they used to sense these molecular-level hydrophobic forces, called a surface forces apparatus, is a now-standard technique that was originally pioneered by Israelachili and colleagues in the 1970s.

"In basic chemistry, students learn about van der Waal forces -- the weak forces that act between all molecules. That theory was developed more than 100 years ago," explains Professor Israelachvili.

"According to the van der Waals theory, however, oil and water shouldn't separate and surfactants shouldn't form membranes, but they do. There has been no proven theory to account for these special hydrophobic interactions. Such behaviors are crucial for life as we know it to exist."

Hydrophobic and hydrophilic interactions are central to the disciplines of chemistry, physics, and biology that have fueled modern developments in industries from detergents to pharmaceuticals and new biotechnologies. The new equation is expected to impact applications in water filtration, membrane separations, biomedical research, gene therapy methods, biofuel production, and food chemistry.

Virus and disease propagation in the human body are directly linked to hydrophobic properties on a cellular level. One of the problems related to chemotherapy treatments for cancer is being able to direct a drug specifically to cancer cells, instead of the entire body. Israelachvili and his colleagues foresee their discovery having an impact in biomedical research that attempts to understand and treat diseases.

"Cell membranes are complex and discriminating structures, allowing the transmission of various signals into cells and mediating specific interactions with bacteria and viruses," said Jean Chin, Ph.D., who oversees membrane structure grants at the National Institute of General Medical Sciences of the National Institutes of Health. "This study, by enhancing our understanding of the role played by hydrophobic forces in membrane dynamics, will expand what we know about membrane structure and function, as well as microbial infection pathways."

"Understanding how water and oil-like substances interact is enormously important for explaining the properties and functions of many biological and engineering materials," says Dr. Robert Wellek, Program Director in the Directorate for Engineering at the National Science Foundation. "The UCSB and USC teams have elegantly combined concepts from synthetic chemistry, photophysics, and chemical engineering to unravel and quantify the elusive hydrophobic interaction. NSF is very pleased that its grantees have been able to contribute important fundamental knowledge in this important area."

Details of the research were published this month in the Proceedings of the National Academy of Sciences. Their research was made possible by support from the National Science Foundation, the National Institutes of Health, and the Procter & Gamble Company.

"We've known for a long time what we were aiming for. It's a bit like climbing a mountain," said Professor Israelachvili. "The whole thing started at the very bottom. I've been searching for the keys to this interaction for thirty years. We are thrilled with the findings, but it took a lot of steps over carefully chosen paths to get there."

Professor Jacob Israelachvili, Professor Bradley Chmelka, and Stephen Donaldson, Ph.D. student, are with the Department of Chemical Engineering at UCSB. Dr. Israelachvili is a Fellow of the Royal Society of London and member of the U.S. National Academy of Science and the U.S. National Academy of Engineering. Dr. Israelachvili received his Ph.D. in Physics from University of Cambridge and joined UCSB in 1986. He was recently named by the American Institute of Chemical Engineers as one of the "100 Chemical Engineers of the Modern Era" for his achievement and leadership in the field.

 

The above story is reprinted (with editorial adaptations ) from materials provided by University of California - Santa Barbara, via EurekAlert!, a service of AAAS.

Journal Reference:

S. H. Donaldson, C. T. Lee, B. F. Chmelka, J. N. Israelachvili. General hydrophobic interaction potential for surfactant/lipid bilayers from direct force measurements between light-modulated bilayers. Proceedings of the National Academy of Sciences, 2011; 108 (38): 15699 DOI: 10.1073/pnas.1112411108

New equation predicts molecular forces in hydrophobic interactions

The physical model to describe the hydrophobic interactions of molecules has been a mystery that has challenged scientists and engineers since the 19th century. Hydrophobic interactions are central to explaining why oil and water don't mix, how proteins are structured, and what holds biological membranes together. Chemical engineering researchers at UC Santa Barbara have developed a novel method to study these forces at the atomic level, and have for the first time defined a mathematical equation to measure a substance's hydrophobic character.

"This discovery represents a breakthrough that is a culmination of decades of research," says Professor Jacob Israelachvili. "The equation is intended to be a tool for scientists to begin quantifying and predicting molecular and surface forces between organic substances in water."

Using a light-responsive surfactant – a soap-like molecule related to fats and lipids – the researchers developed an innovative technique to measure or change the forces between layers of the molecule in water by using beams of UV or visible light. The result is a general equation that applies to even more complicated systems, such as cellular membranes or proteins.

"We were fortunate to find the right combination of experimental methods and theory," said Brad Chmelka, UCSB Chemical Engineering professor and co-author of the study. "The keys to our research were using a light-responsive surfactant molecule, a means of measuring these delicate surface forces, and applying knowledge of what to look for."

The highly-sensitive instrument they used to sense these molecular-level hydrophobic forces, called a surface forces apparatus, is a now-standard technique that was originally pioneered by Israelachili and colleagues in the 1970s.

Enlarge

This is Israelachvili?s equation. Credit: UCSB

"In basic chemistry, students learn about van der Waal forces – the weak forces that act between all molecules. That theory was developed more than 100 years ago," explains Professor Israelachvili.

"According to the van der Waals theory, however, oil and water shouldn't separate and surfactants shouldn't form membranes, but they do. There has been no proven theory to account for these special hydrophobic interactions. Such behaviors are crucial for life as we know it to exist."

Hydrophobic and hydrophilic interactions are central to the disciplines of chemistry, physics, and biology that have fueled modern developments in industries from detergents to pharmaceuticals and new biotechnologies. The new equation is expected to impact applications in water filtration, membrane separations, biomedical research, gene therapy methods, biofuel production, and food chemistry.

Virus and disease propagation in the human body are directly linked to hydrophobic properties on a cellular level. One of the problems related to chemotherapy treatments for cancer is being able to direct a drug specifically to cancer cells, instead of the entire body. Israelachvili and his colleagues foresee their discovery having an impact in biomedical research that attempts to understand and treat diseases.

"Cell membranes are complex and discriminating structures, allowing the transmission of various signals into cells and mediating specific interactions with bacteria and viruses," said Jean Chin, Ph.D., who oversees membrane structure grants at the National Institute of General Medical Sciences of the National Institutes of Health. "This study, by enhancing our understanding of the role played by hydrophobic forces in membrane dynamics, will expand what we know about membrane structure and function, as well as microbial infection pathways."

"Understanding how water and oil-like substances interact is enormously important for explaining the properties and functions of many biological and engineering materials," says Dr. Robert Wellek, Program Director in the Directorate for Engineering at the National Science Foundation. "The UCSB and USC teams have elegantly combined concepts from synthetic chemistry, photophysics, and chemical engineering to unravel and quantify the elusive hydrophobic interaction. NSF is very pleased that its grantees have been able to contribute important fundamental knowledge in this important area."

Details of the research were published this month in the Proceedings of the National Academy of Sciences. Their research was made possible by support from the National Science Foundation, the National Institutes of Health, and the Procter & Gamble Company.

"We've known for a long time what we were aiming for. It's a bit like climbing a mountain," said Professor Israelachvili. "The whole thing started at the very bottom. I've been searching for the keys to this interaction for thirty years. We are thrilled with the findings, but it took a lot of steps over carefully chosen paths to get there."

Provided by University of California - Santa Barbara (news : web)

Supramolecules get time to shine: Technique reveals interactions between nanotubes, photoluminescent materials

What looks like a spongy ball wrapped in strands of yarn -- but a lot smaller -- could be key to unlocking better methods for catalysis, artificial photosynthesis or splitting water into hydrogen, according to Rice University chemists who have created a platform to analyze interactions between carbon nanotubes and a wide range of photoluminescent materials.

The microscopic particles assembled in the lab of Angel Martí, an assistant professor of chemistry and bioengineering, combine single-walled carbon nanotubes with porous silicate materials that can absorb various molecules -- in this case, a ruthenium complex.

Martí, graduate student and lead author Avishek Saha and their colleagues reported their results July 12 in the Royal Society of Chemistry journal Chemical Science.

The ability to immobilize individual carbon nanotubes on a solid surface is interesting enough, but combining supramolecular systems with nanomaterials to produce hybrids is unique, they said.

"This can be used as a general platform to study the interaction of not only ruthenium complexes, but most photoactive molecules can be encapsulated within these porous silicates in a very simple way without chemical modification, without anything," Marti said.

Saha endured trial and error at every step in bringing the new particles to fruition, first figuring out the best way to keep long, single-walled carbon nanotubes produced by the Rice-born HiPco process from aggregating into bundles while allowing them to adhere to the particles.

The solution suggested by co-author Matteo Pasquali, a Rice professor in chemical and biomolecular engineering and in chemistry, involved dissolving the bundles in chlorosulfonic acid, which added protons -- and thus a positive charge -- to each nanotube.

That was the key to making nanotubes attractive to the three types of silicate particles tested: a commercial version of MCM-41, a mesoporous material used as a molecular sieve; another version of MCM-41 synthesized at Rice by Saha, and microporous Zeolyte-Y.

"We don't fully understand the mechanism, but the truth is they have a very strong affinity to silicon oxide networks," said Marti, describing the nanotube-wrapped particles. "Once they're protonated, they just bind."

But that wasn't enough to create a proper platform because protonated nanoparticles are no longer photoluminescent, a quality the researchers required to "see" such tiny structures under a spectroscope. "Protonated nanotubes are cool, but we want to have pristine nanotubes," Martí said.

"We were stuck there for a while. We tried a lot of things," he said. Acetone, ammonia, chloroform and other substances would deprotonate the nanotubes, but would also release them from the silicate sponges and allow them to clump. But vinylpyrrolidone (VP) did the trick by giving the nanotubes a polymer-like coating while returning them to their pristine states.

"This becomes interesting not only from the standpoint of getting individualized nanotubes on top of a surface, but also because we got fluorescence of nanotubes not from a solution, but from a solid material," Martí said.

The experiment went one critical step further when the researchers introduced ruthenium molecules to the mix. The silicates absorbed the ruthenium molecules, putting them into close proximity with an array of nanotubes. Under a spectroscope, the ruthenium complexes would photoluminesce, but they saw something unexpected in the interaction.

"Basically, we found out that if you put a photoactive species (ruthenium) there and excite it with light, two different processes happen. If it has carbon nanotubes close by, it will transfer an electron to the nanotubes. There's a charge transfer, and we knew that would happen," Martí said. "What we didn't expect when we analyzed the spectrum was seeing two different species of ruthenium complexes, one with a very short photoluminescence lifetime and one very long."

The researchers theorized that ruthenium in the center of the sponge was too far from the nanotubes to transfer electrons, so it retained its standard luminescence.

The research leads to some interesting possibilities for materials science, Saha said. "MCM itself has many applications (as a mesoporous sieve in fuel refineries, for instance), and carbon nanotubes are wonderful materials that many people are interested in. We're just combining these two into a hybrid material that might have the virtues of both."

While pore sizes in zeolites are locked by their crystalline structure at 0.7 nanometers, pores in MCM can be customized, as Saha has done, to absorb specific materials. "There are many things we can do to tune the system that we haven't explored," he said; combining metal molecules or even quantum dots with MCM and nanotubes might lead to interesting results.

Martí said putting charged nanotubes on the surface of a solid also opens the door to use them as catalysts in solar-energy conversion. "You need that driving force, that charge separation, for artificial photosynthesis," he said.

Co-authors of the paper are Rice graduate students Saunab Ghosh and Natnael Behabtu.

The Welch Foundation supported the research.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by Rice University.

Journal Reference:

Avishek Saha, Saunab Ghosh, Natnael Behabtu, Matteo Pasquali, Angel A. Mart. Single-walled carbon nanotubes shell decorating porous silicate materials: A general platform for studying the interaction of carbon nanotubes with photoactive molecules. Chemical Science, 2011; DOI: 10.1039/C1SC00323B