Unless you’re interested in isotopic labeling, neutrons don’t figure much into chemistry. Neutral in charge and a bit bigger than a proton, the neutron neither gives an atom its name nor determines much about its reactivity.
But neutrons have some unsung properties that make them useful for investigating matter. Because they are neutral, they can penetrate deeper into a sample than electrons can. Because they have mass and spin, they have a magnetic moment and can probe magnetism. Because they interact with nuclei rather than electron orbitals, they are sensitive to light elements and can even distinguish between hydrogen and deuterium. And they’re nondestructive. These features are inspiring researchers to use neutrons to analyze a variety of materials, from coal and complex fluids to cell membranes and membrane proteins and including magnetic materials.
“As far as I’m concerned, neutrons are the most powerful structural probe that inorganic and materials chemists have to characterize their materials,” says Robert J. Cava, a Princeton University chemistry professor who uses neutrons to characterize the structure and magnetic properties of transition-metal complexes. “Nothing is better than a neutron.”
Unlike most techniques that use electromagnetic radiation or electrons to probe samples, however, neutrons cannot easily be generated in individual labs. The two primary ways to obtain them involve nuclear reactions: One is through splitting uranium for a net yield of two neutrons. The other is through spallation, in which a beam of protons is aimed at a heavy target, such as tungsten or mercury, to induce a different nuclear reaction that also ejects neutrons.
The U.S. has three main neutron user facilities. The Center for Neutron Research at the National Institute of Standards & Technology (NIST) uses a uranium reactor. Oak Ridge National Laboratory (ORNL) has two neutron sources, one reactor and one spallation. And at Los Alamos National Laboratory (LANL), spallation rules.
All neutron experiments involve directing a beam of neutrons at a sample and then detecting the angle and the energy at which the neutrons are scattered after interacting with the atoms in the sample. The two kinds of neutron sources each have their own benefits. Spallation yields intense, pulsed neutron beams, allowing time-of-flight experiments, says Stephen E. Nagler, chief scientist of the Neutron Scattering Science Division at ORNL. The continuous stream of neutrons from reactors, however, can work better when researchers want to cool the neutrons so they have longer wavelengths, which can be important when looking at protein or polymer conformations in which the length scales of interest are hundreds or thousands of angstroms.
One use of neutron-scattering experiments is to study porous materials. In one example, a group led by Andrzej P. Radliski of Griffith University’s Nanoscale Science & Technology Centre, in Australia, and Yuri B. Melnichenko of ORNL investigated the ability of underground coal to sequester CO2 (Langmuir 2009, 25, 2385). When a neutron beam is directed at a sample of coal, how the neutrons scatter from the sample gives information about the sample’s size distribution, density, and chemical composition of pores.
The researchers found that coal can pack five times the CO2 present in the same volume of air at the same temperature and pressure. Condensation of CO2 is most pronounced in pores of about 20″“40 Ó¦ in diameter, compared with larger pores, and is also affected by local mineral composition. The coal microstructures also are not affected by exposure to CO2, at least for a period of several days. The results, Melnichenko says, show that neutron scattering can be used to pinpoint the sorption capacity of different coals. The technique may also be of use in studying membranes that capture CO2 from flue gases at coal-burning power plants.
But it’s not just solids that can be analyzed by using neutrons. Norman J. Wagner, a chemical engineering professor at the University of Delaware, is interested in the property of shear thickening, or the ability of some suspensions””think cornstarch mixed with water””to become solidlike under stress. He studies the neutron scattering of colloidal suspensions under flow conditions to try to understand the mechanism of shear thickening. “Small-angle neutron scattering is a unique way to probe something while it’s flowing so you can see how the nano- and microscale structure is changing,” Wagner says.
Shear thickening is a property to be exploited in applications such as “soft” body armor, but it can also be the bane of materials processing, leading to stopped-up equipment or ruined products, Wagner says. Using neutron scattering, he and colleagues studied silica particles in polyethylene glycol to try to validate the fundamental theory behind the behavior of shear-thickening fluids””to figure out what structures are being formed and compare the results against theoretical models (Rheol. Acta. 2009, 48, 897).
They found that, as predicted, fluids under stress self-assemble into individual close-packed, transient “hydroclusters”””colloidal particles that have been squeezed together””rather than larger aggregates. This flow-induced hydroclustering is responsible for the dramatic increase in solution viscosity and elasticity that occurs during shear thickening. The basic knowledge obtained from neutron-scattering measurements will lead to improved processes and products down the road, Wagner says. “If we can measure the nanostructure, we can rationally engineer better nanocomposites,” he says.
Solution properties such as diffusion dynamics can also be studied by neutron scattering. Eugene Mamontov, Huimin Luo, and Sheng Dai at ORNL have looked at protic ionic liquids, which are of interest as proton-conducting fuel-cell electrolytes and novel media for gas separation. Techniques such as X-ray scattering cannot illuminate hydrogen atoms, Dai notes, so neutron scattering is crucial to understanding how the liquids work.
In particular, the group found that the ionic liquid N,N,N´,N´-tetramethylguanidinium bis(perfluoroethylsulfonyl)imide has two different proton-diffusion processes at temperatures above its melting point (J. Phys. Chem. B 2009, 113, 159). The faster process is spatially restricted to an area with a radius of about 8 Ó¦ and is localized motion that is correlated to rotational dynamics, Dai says. The slower process involves long-range transfer of the protons on the NH2 group of the guanidinium cation and is likely the key to providing proton conduction””the quality necessary for an ionic liquid to be a good electrolyte.
Neutrons can also be used to study biological systems, especially membranes and membrane proteins that have proven difficult to look at by other techniques. NIST research chemist Hirsh Nanda and colleagues have been using neutron scattering to investigate the formation of HIV type 1. One aspect of this research is to study the conformations of the Gag protein, which assembles on the inner surface of the infected cell’s membrane and is cleaved into several domains that eventually bud into a new virus.
Much of the work on Gag has focused on its “compact” solution conformations, but Gag is known to take on a more extended structure during viral assembly. Nanda and colleagues are using a tethered membrane system developed at NIST (Biointerphases 2007, 2, 21) to study the interaction of the protein with membranes. By analyzing the neutron-scattering patterns of Gag on the membrane, they’re hoping to elucidate the path of the conformational changes and eventual viral assembly.
So far, they’ve found that Gag by itself stays in its compact form on the membrane. “But another function of the protein is to drag RNA into the virus,” Nanda says. “If we also introduce nucleic acids, we can induce a change in conformation to the extended form. You need interactions of protein with both membrane and viral RNA to produce the extended conformation.” The RNA also appears to cross-link several Gag molecules together to form an extended conformation. The group is now studying other protein interactions that may also modulate Gag protein extension and lead to viral budding.
Other researchers are using neutron scattering to look in detail at cell membranes themselves. Ka Yee C. Lee, a chemistry professor at the University of Chicago, is using neutrons to study the interactions of lipids in cell membranes and other molecules, such as proteins, polymers, or additional lipids.
Lee does neutron experiments alongside X-ray measurements, in particular to find out how a protein associates with a membrane and how the interaction affects the ordering or packing of the lipids. “If we rely only on X-ray studies, we sometimes don’t get the contrast necessary to distinguish between the lipid group and a protein or peptide,” Lee says. If she selectively deuterates the lipids and uses deuterated water in her solutions, however, neutrons can then illuminate the details of the protein-membrane interactions.
One of Lee’s projects has focused on the role of cholesterol in membranes, to answer fundamental questions of whether and how cholesterol interacts with other membrane components. “There is a lot of interest in what people now refer to as a lipid raft, or domains that are formed at the membrane surface that can lead to local sequestration of proteins” for functions such as signaling, Lee says. One theory explaining how this happens involves cholesterol interacting with other membrane lipid components to give rise to the raft. In keeping with this theory, initial work from Lee’s lab indicates that lipids and cholesterol can associate to form a kind of lipidic alloy (Phys. Rev. Lett. 2009, 103, 028103), and her group is now examining the molecular interactions and their dynamics.
For his part, Princeton’s Cava uses neutron diffraction to study inorganic solids. For the ability to nail down the positions of light elements such as B, C, N, or O, or alkali metals such as Li and Na, neutron scattering “is the method of choice by which solid-state chemists determine the structure and formulas of the compounds they synthesize,” Cava says. The technique can also be used to elucidate the magnetic structure of materials.
In one recent study, Cava, his former graduate student D. Vincent West, and colleagues used neutron diffraction to study a new family of anhydrous sulfates, A2+Mn5(SO4)6, where A is Pb, Ba, or Sr (J. Solid State Chem. 2009, 182, 1343). They expected that the tetrahedral structure of SO4 2″“ would yield a triangular crystal lattice in the compounds. The triangular geometry in turn would lead to a phenomenon called geometric frustration, which neutralizes typical nearest-neighbor magnetic interactions and opens the system to ground states with novel properties.
The compounds turned out to be composed of a unique, Mn2+-containing Mn2O9 dimer along with chains of alternating MnO6-AO12 polyhedra. The structure is also layered, with the cations lining up akin to stacked honeycombs.
On the magnetism front, all three compounds transition from disordered to ordered magnetic moments below 10 K, which Cava, West, and colleagues believe stems from the magnetic interaction between the two Mn atoms in the Mn2O9 dimers. Other differences in how the compounds respond to magnetic fields likely involve the magnetic moments within the polyhedral chains, where the Pb, Ba, or Sr atom can more readily affect the compound’s magnetic properties. “The chemistry of these materials suggests a broader family of materials whereby the magnetic properties can be tuned through chemical substitution,” West says. “In addition, the unique triangular geometry in this structure represents a new perspective that may prove valuable for understanding magnetism in solids.”
All of the neutron user facilities in the U.S. are in various phases of expansion. NIST’s Center for Neutron Research is adding a second instrument hall, aiming to increase its measurement capacity and users by 25% when the project is completed in 2012. ORNL is also planning to add a second experimental hall to its spallation neutron source, a move that would double its capacity.
Doubling of capacity is also a goal at LANL, although the funding for that has not yet been secured, says Alan J. Hurd, director of LANL’s Manuel Lujan Jr. Neutron Scattering Center. Hurd notes that all of the neutron centers in the U.S. tend to have twice the requests for experiment time than they can accommodate, so he has no doubt that the extra capacity will be well used once it’s built.
“We’re finding more and more that neutrons can answer questions you really can’t get at in another way,” Delaware’s Wagner says. With greater knowledge of neutrons’ capabilities and increased availability, scientific progress undoubtedly awaits.
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