Physical Sciences and Neutron Science and Technology

Radioactive Ion Beam Facility Dedicated
ORNL Instrument Measures Fluid Densities under Extreme Conditions
Analyzing Biopolymer Mixtures Using Mass Spectrometry
New Insights into the Structure of Water
ORNL Designs Neutron Spallation Target System
ORNL Helps Develop Cleaner Ways To Make Plastics

Physical scientists at ORNL are engaged in many studies, ranging from the structure of heavy nuclei to the explosions in stars that cast out heavy elements to the formation of rocks and energy resources in the earth. Our nuclear physicists, both experimental and theoretical, are concerned with the effects of excitation on the shape, structure, and other properties of various nuclei. They carry out research at ORNL’s Holifield Radioactive Ion Beam Facility (HRIBF). which is the only U.S. facility that can produce and accelerate high-intensity, low-energy beams of radioactive nuclei, and the Oak Ridge Electron Linear Accelerator (ORELA), a unique, intense pulsed-neutron-source accelerator facility used for basic research in nuclear astrophysics and fundamental interactions.

This whimsical painting of the HRIBF by Edda Reviol (wife of a young research professor in the Physics Department at the University of Tennessee) indicates the production of radioactive ions when protons from the cyclotron smash a target (burst of color at bottom, left of center). After selection, a beam of these radioactive ions is accelerated to the top of the 100-foot, 25-megavolt tandem accelerator and then back down where the ions strike the target of the experimental apparatus, depicted in the lower right-hand portion of the figure. This sequence takes place in the HRIBF facility with ORNL’s landmark tower (center).

Atomic physicists at ORNL conduct experimental and theoretical investigations of a broad class of phenomena occurring when multiply charged heavy ions interact with gases, solids, free and bound electrons, photons, and other ions; the EN Tandem Van de Graaff Accelerator and the Electron Cyclotron Resonance Ion Source Facility are operated in support of the atomic physics research community.

ORNL also conducts broad programs in chemical energy, separations and analysis, heavy-element chemistry, and advanced battery technology. Basic research improves the fundamental understanding of methods for separating mixtures as well as systems related to chemical conversions that underpin new or existing concepts of energy production and storage. Geoscience research at ORNL focuses on fundamental geochemical processes that control the transport of matter and energy in the earth’s crust.

Stemming directly from the Laboratory’s original mission, ORNL’s competency in neutron-based science and technology includes the design and operation of neutron sources (reactors and accelerators) and the use of neutrons in science and technology (neutron scattering, isotope production, neutron activation analysis, materials irradiation, and molecular structure determination). This broad spectrum of research in the physical sciences at ORNL is supported by the Office of Basic Energy Sciences in DOE’s Office of Energy Research, which supports most research in the physical sciences in the United States.

Radioactive Ion Beam Facility Dedicated

Scientists from all over the world are coming to Oak Ridge to study atomic nuclei that cannot be produced naturally from elements that exist on the earth. On December 12, 1996, ORNL’s Holifield Radioactive Ion Beam Facility (HRIBF) was dedicated as an international user facility. The ceremony featured dignitaries from the White House’s Office of Science and Technology Policy, DOE headquarters, the state of Tennessee, Oak Ridge Associated Universities, Vanderbilt University, and the University of Tennessee. The dedication was followed the next day by a symposium on radioactive ion beam physics.

Dave Hendrie, director of DOE’s Division of Nuclear Physics, congratulated ORNL staff for designing and constructing the HRIBF. “You can count this effort as a success,” he said, “and today is one of those glorious days for science in general.”

Hendrie recounted the history of the project, which was conceived in 1991 as funding dwindled for ORNL’s Holifield Heavy Ion Research Facility (HHIRF). “When HHIRF ran into funding troubles a few years ago,” he said, “you had a good idea for a modest-sized, cost-effective facility. You sold the idea to DOE. You put it together over the years. It turned out to be timely.”

The Holifield Radioactive Ion Beam Facility
has produced its first beams for experiments
to study nuclear structure and astrophysics.

The HRIBF is the only facility in the world dedicated to the acceleration of radioactive ion beams with sufficient intensity and energy to be useful for nuclear physics and nuclear astrophysics. HRIBF’s radioactive ion beams provide a tool for creating new superheavy elements, extremely large deformed nuclei, and nuclei beyond their limits of stability, helping to answer important questions about the nature of the nucleus. Two-thirds of the experiments at the facility will be devoted to studying the structure of exotic nuclei that exist for just a fraction of a second.

About a third of the experiments using HRIBF radioactive ion beams will be performed to solve mysteries in nuclear astrophysics. These studies will focus on the formation and fate of stars. One goal is to better understand spectacular nova and supernova explosions in which temperatures at the center of doomed stars reach billions of degrees, fueling exotic fusion reactions that produce iron and all the naturally occurring elements in the universe heavier than iron. As the star blows itself up, it scatters heavy elements such as the iron, calcium, silver, tin, iodine, gold, mercury, lead, and uranium isotopes that are the basis of life and of our civilization.

On August 30, 1996, the HRIBF (whose chief components include the Oak Ridge Isochronous Cyclotron and the tandem accelerator housed in ORNL’s landmark tower) produced its first beams of radioactive ions and accelerated them using the tandem accelerator. Radioactive arsenic-70 ions were produced by bombarding a liquid germanium target with a proton beam from the Oak Ridge Isochronous Cyclotron. Although its intensity was only a few thousand ions per second, the beam was easily detected by stopping it on a moving tape system and observing its radioactive decay. A beam of arsenic-69 ions was produced in the same test. In March 1997, the HRIBF started its experimental program using beams of radioactive arsenic-69.

In 1997, scientists from universities and laboratories around the world will be conducting experiments they hope will answer questions about nuclear physics and nuclear astrophysics. The HRIBF facility will serve a national and international community of about 300 scientists from 33 states and 20 foreign countries, providing a unique new tool for understanding nuclear matter, the main constituent of the visible universe.

The HRIBF is the only facility in the world capable of supporting a wide range of nuclear astrophysics and nuclear structure physics studies—an exciting, fast-growing new field that has been identified by the U.S. nuclear science community as a top priority for future development.

Hendrie concluded the dedication by saying “I see the future of physics as quite bright.”

HRIBF is supported by DOE’s Office of Energy Research, Basic Energy Sciences, Division of Nuclear Physics.

ORNL Instrument Measures Fluid Densities under Extreme Conditions

Jim Blencoe (left) and Jeffery Seitz check the pumps on ORNL’s vibrating-tube densimeter, a facility that runs on the principle of a tuning fork. It enables geochemists to measure densities of fluid mixtures up to one part per million at temperatures up to 500°C and pressures up to 2000 bars. Photograph by Curtis Boles.

Hot fluids form and circulate deep in the earth’s crust. However, how they react with minerals and organic matter to form rock masses, magmas, ore deposits, oil, and natural gas is not well understood.

Using unique instrumentation, ORNL researchers are gathering data and developing “equations of state” that help geologists gain a more quantitative understanding of the origin and evolution of fluids in the earth’s crust. It is now evident that frequently used equations published by leading scientists are seriously in error. The new data and equations will lead to improved geologic models that will either support or refute current theories about how rock masses and energy resources form.

Natural fluids in the earth’s crust reach extreme, or “supercritical,” temperatures and pressures at great depths. When water reaches the supercritical state—at temperatures above 374°C and pressures above 222 bars (approximately 222 times atmospheric pressure)it acquires special properties. Compared with true liquids and gases, supercritical water-rich fluids are often more reactive with rocks, causing the formation of new minerals and hydrocarbons.

Using unique instruments, ORNL
researchers are helping geologists better
understand the origin and evolution
of fluids in the earth’s crust.

The new equations of state are based primarily on data obtained from a custom-designed, vibrating-tube densimeter constructed at ORNL. This unique instrument is used to determine the volumetric properties of fluids under extreme conditions. A commercial vibrating-tube densimeter that measures densities of fluids at ordinary temperatures and pressures was modified extensively to create an instrument that could be used at temperatures as high as 500°C and at pressures up to 2000 bars. Currently, measurements are being made on fluids in the carbon-oxygen-hydrogen-nitrogen (C-O-H-N) system. The fluid species of principal interest in this system are water (H2O), carbon dioxide (CO2), methane (CH4), and nitrogen (N2). Using the special ORNL densimeter, it is possible to mix these species in any proportion to create intermediate fluid compositions of interest. During experiments, fluids flow through a U-shaped tube, which vibrates in response to forces generated by magnets. By measuring the period of vibration of the U-tube (the denser the fluid, the slower it vibrates), the density of the fluid flowing through the tube can be calculated.

Additional research on C-O-H-N mixtures is being performed with another unique ORNL instrument: a hydrogen-service, internally heated pressure vessel. This apparatus is used to determine the effective concentrations (activities) of the species in supercritical aqueous fluids. By combining the data obtained from the vibrating-tube densimeter and the hydrogen-service, internally heated pressure vessel, ORNL researchers can determine how aqueous fluids of different compositions and densities behave at different temperatures and pressures.

The new equations of state derived from our research are useful not only to geologists but also to chemical engineers and waste management personnel. It has been shown in recent years that supercritical aqueous fluid plus oxygen converts hazardous wastes to harmless chemicals such as carbon dioxide and sodium chloride (ordinary table salt). The ORNL equations are also being applied to designing coal gasification facilities, fabricating specialty ceramics in which supercritical temperatures and pressures are needed to grow crystals of a particular size and shape, and predicting the availability of natural gas and geothermal energy. ORNL researchers are providing a “piece of the rock” of valuable information for understanding the earth and for improving human life on it.

The research is sponsored by DOE, Office of Basic Energy Sciences, Division of Engineering and Geosciences.

Analyzing Biopolymer Mixtures Using Mass Spectrometry

Scott McLuckey adjusts an electrospray ion trap mass spectrometer as researcher Jim Stephenson looks on. Photograph by Tom Cerniglio.

ORNL research using negative ions is having a positive effect on mass spectrometry applications. This work has led to sophisticated techniques for environmental analysis, explosives detection, engine exhaust analysis, and analysis of compounds of biomedical interest.

Several years ago, ORNL researchers connected different ion sources to a quadrupole ion trap mass spectrometer. One of these was the ORNL-developed and patented atmospheric sampling glow-discharge ionization source, which was originally developed to detect explosives. The technology was licensed to several ion trap mass spectrometer manufacturers. Although an explosives vapor detector has not been commercialized, the technology has been targeted for the environmental analysis market, which has been significantly larger and more reliable than the explosives detection market. These same ORNL researchers were also the first group to join the electrospray ion source (an effective method for generating ions of biopolymers such as proteins and DNA) with the quadrupole ion trap mass spectrometer. Two instrument manufacturers have recently started selling electrospray–ion trap instruments that were heavily influenced by the early ORNL work.

The glow-discharge source is particularly well suited to molecules present in air in trace concentrations, such as those that have vaporized from high explosives or organic pollutants. Many of these volatile and semi-volatile molecules form stable negative ions by readily capturing electrons in the glow discharge source. If a molecule in the source does not capture an electron efficiently, a proton usually can be added to it to form a positive ion. The positive or negative ions are injected into a quadrupole ion trap mass spectrometer where they can be analyzed according to their mass-to-charge ratio. By measuring the positions and sizes of the peaks in the mass spectrum, researchers can identify the constituents of a sample and their relative concentrations.

We are now studying the use of both negative and positive ions to improve methods for characterizing biological materials. Recently, the use of mass spectrometry to analyze biopolymers has been advanced by two techniques for forming gas-phase ions from involatile materials. One technique, called laser desorption, uses the laser. The other technique is electrospray, mentioned above, which is particularly well suited for liberating biopolymers in solution to yield gaseous ions. When the solution is passed through a hypodermic needle held at 3000 to 4000 volts, the liquid exiting the needle breaks up into a fine mist of droplets from which ions emerge. The ions are then drawn into the vacuum system of a mass spectrometer where they can be manipulated and measured.

Although widely employed in the biomedical community and, in particular, the drug industry, the electrospray technique had limited ability to help scientists directly analyze mixtures, because it tends to form multiply charged ions with a distribution of charge states. Hence, each mixture component tends to give many peaks that can result in a complex mass spectrum, even with just a few components.

ORNL researchers developed the ion-ion
reaction technique for the electrospray
ion trap mass spectrometer to more
easily identify individual proteins
in complex mixtures.

This situation changed, however, when ORNL researchers developed the ion-ion reaction technique to enable identification of individual proteins in complex mixtures. In ORNL’s demonstration of this technique, singly charged negative ions formed by glow discharge were introduced into the quadrupole ion trap and reacted with a positively charged protein mixture formed by electrospray. The negative ions plucked the excess protons, or positive charges, off the proteins in the mixture, reducing dramatically the number of peaks per mixture component so that each protein in the mixture could be readily identified in the simplified spectrum.

ORNL researchers have also introduced positive rare-gas ions to react with negatively charged DNA molecules, causing their fragmentation. In this way, ion-ion reactions can enhance the ability to sequence pieces of DNA. This capability could be useful someday for detecting an infectious organism or identifying a genetic disease. Because the motion of each ion in the ion trap mass spectrometer’s oscillating electric field depends upon mass-to-charge ratio, ORNL researchers seek to develop methods for ion manipulation to enable ultrasensitive detection of targeted biopolymers in body fluids, such as proteins that signal the onset of disease. Because of its high speed, high sensitivity, and high specificity, ion trap mass spectrometry offers an attractive means for detecting disease in its early stages so it can be reversed in time to stop the negative effects.

Most of this research has been sponsored by DOE, Office of Basic Energy Sciences, Chemical Sciences Branch. Biomedical applications work has been sponsored by the National Institutes of Health, and bio-agent detection work has been initiated with DOE’s Office of Nonproliferation and National Security.

New Insights into the Structure of Water

As this true story illustrates, experimental
science and computational science can
make important contributions
to each other.

ORNL has long been a leading center for studying the molecular structure of substances using neutron scattering at research reactors. Using our powerful Intel Paragon supercomputers, we are now also a world leader in molecular simulation calculations. Here’s a true story that illustrates these complementary capabilities, demonstrating ORNL’s continuing leadership as a strong center for neutron science, computational science, chemical science, and materials science research.

This snapshot from a molecular dynamics computer simulation of supercritical water at a temperature of 573 K and a density of 0.72 g/cm3 shows a number of water molecules. The green spheres represent the oxygen atoms, and the silver bumps on them represent the hydrogen atoms. In the blown-up portion is an arrow connecting water molecules A and B. Note that the hydrogen atoms on molecule B point away from molecule A, and one hydrogen atom on molecule A points toward B. This geometric arrangement between pairs of water molecules is called a hydrogen bond and has a very important influence on the properties of water.

In 1991 Peter Cummings (who at the time was on the faculty of the University of Virginia and since 1994 has been an ORNL–University of Tennessee Distinguished Scientist) and ORNL researchers Hank Cochran, Mike Simonson, and Bob Mesmer performed molecular simulations of supercritical water on the Oak Ridge Cray computer. The simulations showed hydrogen bonding in supercritical water—a non-liquid, non-gaseous state of water produced at high temperature and pressure.

In December 1993 the prestigious scientific journal Nature published a paper that disputed the Oak Ridge results. It stated that neutron scattering experiments conducted at the Rutherford-Appleton Laboratory in the United Kingdom indicated that hydrogen bonding is essentially absent in supercritical water. The paper’s authors asserted that the simple water model used in the Oak Ridge simulations needed modification. Subsequent publications based on molecular simulation calculations by the ORNL group and by other research groups challenged the results of the U.K. laboratory scientists and their collaborators from the University of Rome, showing that the scattering results were unphysical and suggesting possible error in the correction for inelastic scattering.

In response to the challenges, in 1996 the British-Italian group reexamined its neutron scattering data, using an improved correction for the results of inelastic scattering. From their reanalysis, group members found that hydrogen bonding is indeed present in supercritical water as predicted by the Oak Ridge simulation group. They also reanalyzed the data they had obtained a decade earlier for ambient water—data that had been the basis for understanding and modeling water structure by researchers worldwide.

As a result of the new analysis, the British-Italian group, led by Alan Soper, has revised the description of water at ambient conditions (i.e., at room temperature and atmospheric pressure). The structure of water at ambient conditions is extremely important because of water’s role in many biological and chemical processes at ambient conditions.

Interestingly, the widely accepted data disagreed with an earlier understanding of water structure, as determined more than 20 years ago by ORNL chemist Al Narten through his neutron scattering studies at our High Flux Isotope Reactor (HFIR). Soper’s revised results now agree much better with Narten’s. The British-Italian group’s paper correcting its previous neutron scattering results was published in the January 1997 issue of the Journal of Chemical Physics.

Meanwhile, using the Intel Paragon supercomputers in the Center for Computational Sciences, the ORNL simulation group is refining models used in simulations of ambient water to get quantitative agreement with the results of the reanalyzed neutron scattering data. An improved model shows the ability to predict accurately the properties of water all the way from ambient conditions to supercritical conditions. And another ORNL group is currently studying supercritical water using neutron scattering at the HFIR.

This story illustrates an unusual interplay between neutron scattering experiments and molecular simulation calculations in which both the interpretation of scattering results and the models used in molecular simulations have been improved. In the past, it has been believed that calculations alone must be revised to agree with the results of experiments. Now, it appears that experimental science and computational science can make important contributions to each other.

With its combination of world-class scattering and simulation capabilities, ORNL is well positioned to lead a new era of molecular-based engineering and science. Clearly, such research has improved our understanding of water, the most important but least understood substance on the earth.

The work has been funded by DOE’s Office of Energy Research, Office of Basic Energy Sciences, Division of Chemical Sciences.

ORNL Designs Neutron Spallation Target System

Like the fist of the legendary Superman, neutrons can punch through centimeters of steel. X-rays and positrons can penetrate only fractions of a centimeter inside many materials, giving us only a glimpse of a material’s surface layers. Neutrons, on the other hand, can reveal bulk structure and properties of materials. Although Superman achieves astonishing feats with the help of his X-ray vision, scientists rely on a more powerful tool—neutron vision—to “see” through matter. That’s why researchers are excited about the proposed National Spallation Neutron Source (NSNS), which will offer pulsed neutrons of the highest intensity ever. No wonder the NSNS, which DOE seeks to locate in Oak Ridge, has been called “the microscope of the 21st century.”

To get neutrons from the NSNS, you need protons. But first you have to start with negative hydrogen ions fed from an ion source into a high-energy linear accelerator. This “linac” accelerates the ion beam to energies of 1 billion electron volts (GeV) and injects it into an accumulator ring that produces short bursts of protons. Some 60 proton pulses per second (less than 1 microsecond per pulse, one pulse every 17 milliseconds) are directed onto a heavy metal target, where a nuclear reaction called spallation occurs, resulting in the production of neutrons for research into materials.

Schematic of the mercury target system for the National Spallation Neutron Source.

The NSNS is a collaborative project involving five DOE national laboratories. In addition to coordinating the NSNS project, ORNL’s responsibilities include designing and constructing the liquid mercury target for neutron production. Because of the enormous amount of energy that the short, powerful pulses of the incoming 1-GeV proton beam will deposit in the spallation target, it was decided not to use a solid target such as tantalum or tungsten. The sledgehammer effect of the proton pulses will cause a large, rapid rise in temperature resulting in thermal shock, which could break the solid target material. In addition, the protons and neutrons will produce severe radiation damage in the solid target material.

View of mercury target (black area, lower center) struck by a proton beam. Intense neutron beams will be produced from this target of the National Spallation Neutron Source.

Mercury was chosen for the target for three reasons: it is not damaged by radiation, as are solids; it has a high atomic number, making it a source of numerous neutrons (the average mercury nucleus has 120 neutrons and 80 protons); and, because it is liquid at room temperature, it is better able than a solid target to move heat away by flowing. It is calculated that only one cubic meter of mercury is needed as the target material and that it will last for the life of the NSNS facility. The NSNS will be the first scientific facility to use pure mercury as a target for a proton beam.

The production of neutrons through spallation reactions can be visualized by the following analogy. The nucleus is like a bucket of baseballs representing the neutrons and protons; throw another baseball into the bucket and watch a few balls spray out of the bucket. In a similar way, part of the neutrons are produced in what is called the fast, or high-energy, part of the nuclear reaction, or the “microscopic intranuclear cascade.” After these fast particles are ejected, the remaining particles are still in a heated state, like the balls in the bucket bouncing around. Some of these lower-energy baseballs can still escape, and so can neutrons in the so-called evaporation phase. These evaporated neutrons make up the bulk of neutron production. In addition to the incoming proton beam, the higher-energy neutrons and protons that are produced can also collide with mercury nuclei in a “macroscopic cascade.” The bottom line is that, for every incident 1-GeV proton, approximately 20 neutrons will be produced in the evaporation phase for potential use in neutron scattering experiments.

To maximize the safety of the facility, the NSNS will be designed with many levels of containment to keep potentially hazardous material from getting into the environment. Containment levels include primary and secondary vessels, concrete barriers, and the main building. For example, between the mercury target container and its shroud, helium gas will be circulated in a flow loop. If the container for the liquid mercury should rupture, the mercury would flow into the loop containing the helium, which would then be collected in a dump tank. The system will be monitored, and if escaping mercury is detected, the accelerator and mercury pump system will be shut down. The remaining mercury in the system would then be passed to the dump tank, and the container would be replaced.

The neutrons coming out of the mercury must be turned into low-energy neutrons suitable for research—that is, they must be at room temperature or colder. The emerging neutrons will be channeled by a beryllium or lead reflector into vessels to reduce their energy. Neutrons will be slowed down by passing them through cans of water (to produce room-temperature neutrons) or through cans of liquid hydrogen at a temperature of 20 K (to produce cold neutrons).

In 1997, containers of mercury will be irradiated with protons at DOE’s Brookhaven and Los Alamos national laboratories. Results of these tests will help ORNL researchers refine their computer models and improve their capabilities for predicting neutron production, thermal shock, heating, and other phenomena in a mercury target system. Model results will be used to improve the design of the mercury target system, which is described in the NSNS Conceptual Design Report (published in mid-1997)—an important document about a super source of neutrons.

The NSNS is funded by DOE’s Office of Energy Research, Office of Basic Energy Sciences, Division of Materials Sciences.

ORNL Helps Develop Cleaner Ways To Make Plastics

Plastics are popular, but pollution is not. There’s the rub. Our lifestyles are being shaped by products increasingly made of plastics, such as polystyrene in coffee cups, Teflon® in pans, and fluoropolymers (e.g., Scotchgard® ) that protect carpets against staining and soiling. These plastics are formed by joining many (poly) small molecules (mers) into giant, chainlike macromolecules called polymers. But the process of synthesizing polymers from monomers generates undesirable discharges that may include contaminated water, toxic wastes, or ozone-destroying chlorofluorocarbons. A cleaner way to make plastics is needed to improve our environment and meet environmental regulations.

At the University of North Carolina (UNC), Professor Joseph DeSimone is developing environmentally friendly methods for synthesizing polystyrene, fluoropolymers, and other plastics. The process produces no large volumes of toxic waste or contaminated water. The reason: it uses supercritical carbon dioxide (CO2) instead of water or other solvents as the medium in which reactions or separations can be performed. Supercritical CO2 is a solvent with almost no undesirable characteristics with regard to safety, environmental, health, or cost considerations. Supercritical CO2 is made by pressurizing the gas from our atmosphere; when it’s discharged by the process, it replaces the same amount of CO2 that was extracted from the air, so there’s no net addition to the greenhouse gas.

A supercritical CO2 fluid is formed by raising the temperature and pressure of CO2 so that all differences between CO2’s liquid and gaseous states disappear. By making small adjustments in the pressure or temperature of the CO2, the solubility of monomers and other components needed for polymer formation can be very widely varied, ensuring that the CO2 that is eventually released does not contain undesirable contaminants. But supercritical CO2 is not a perfect solvent. Not all monomers are sufficiently soluble in CO2 to mix well and form polymers. In such cases, a surfactant, or chemical “soap,” must be added to enable the supercritical CO2 to suspend enough of these small molecules that otherwise would sit at the vessel’s bottom.

To remove grease from dirty clothes, a detergent must be added to water, which by itself cannot dissolve and remove oil and grease. Detergent molecules surround oil and grease molecules, holding them in the water so they can be washed away (see drawing above). A detergent is a surfactant (short for surface-active agent). Surfactant molecules form a cluster called a micelle (shown in all drawings), which consists of spaghetti-like chains whose tails prefer oil (on the inside) and whose heads are attracted to water (on the outside). For CO2 surfactants, block copolymers (two different polymers covalently bonded together) are used. The different ends, which are either attracted to, or repelled, by CO2, form micelles that suspend the growing polymer chains inside and shield them from contact with the supercritical solvent. Drawing by Reneé Balogh.

A surfactant is a surface-active agent such as a detergent. Water normally cannot dissolve oil and grease to remove them from dirty clothes, but detergent molecules surround oil and grease molecules, holding them in the water so they can be washed away. A cluster of surfactant molecules, called a micelle, may look like twisted spaghetti lying around a meatball in the middle of a plate. The outside ends on the detergent molecule chains are attracted to water, and the inside ends prefer oil (and hold it there just as the spaghetti keeps the meatball from rolling on a slightly tilted plate); for polymer synthesis, the outside ends of the surfactant chains in a micelle are attracted to supercritical CO2, and the inside ends have an affinity for monomers, which are strung together (synthesized) in the center of the micelle, where they are shielded from contact with CO2.

Using small-angle neutron scattering, we
helped determine the best ways to form
polymers from chemicals in supercritical
CO2 , an environmentally friendly
solvent for making plastics.

Polymers can be formed the pollution-free way by suspending small molecules in supercritical carbon dioxide with the help of detergent-like molecules called surfactants. When pressure is adjusted at constant temperature to increase the density of the supercritical fluid, surfactant molecules aggregate to form micelles with the CO2 -loving (green) segments, suspending the CO2-phobic (red) segments in solution and shielding them from the carbon dioxide. Drawing by Jamie Payne.

Why do some monomers and polymers dissolve in CO2 while others don’t? Which surfactants are the most effective at suspending different monomers and polymers to keep them from dropping out of solution? Can we control the process and make the surfactants aggregate into micelles or break apart into individual molecules simply by “tuning” the CO2 pressure? DeSimone, who devised more than a dozen surfactants that are effective with CO2 and specific polymers, asked ORNL researchers if small-angle neutron scattering (SANS) could determine the answers. We said that it could, based on our experience in employing SANS to characterize micelles in water. Using SANS, in collaboration with visiting researchers from the University of Palermo in Italy, we characterized a number of different micelles and determined their relative effectiveness in suspending different monomers in supercritical CO2 to enable the formation of polymers. We don’t make plastic, but we take our best tools and talent to help others find better, cleaner ways to make it.

As a result of this work, ORNL’s George Wignall, Hank Cochran, and David Londono were formally recognized for their contributions to research on the “Design and Application of Surfactants for Carbon Dioxide,” which was selected to receive a Presidential Green Chemistry Challenge Award. The research team was led by Joseph DeSimone. The neutron studies at ORNL were initiated under the Laboratory Directed Research and Development Program. Additional funding was provided by DOE’s Office of Energy Research (Division of Materials Sciences and Division of Chemical Sciences within the Office of Basic Energy Sciences) and Office of Computational and Technology Research.

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