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Livermore Scientists Solve Molecular Transport Mystery
Determining how single-valence negatively charged ions or “monovalent anions,” can squeeze through a carbon nanotube 20,000 times thinner than a human hair despite their similarity in size and lack of chemical reactivity was no easy feat, but a team of Lawrence Livermore scientists solved this molecular mystery. Their findings were published in the May 14, 2020, issue of ACS Nano.
With funding from the Department of Energy’s Office of Science and the LLNL Grand Challenge Program, the team used fluorescence assays and stopped-flow spectrometry to uncover how four monovalent anions—chloride, bromide, iodide, and thiocynate—manage to travel through carbon nanotube porins (CNTPs) so narrow that just one water molecule or one ion can traverse the nanotube at a time. The measurements revealed unexpectedly strong differential ion selectivity with permeabilities of different ions varying by up to two orders of magnitude. “Seeing differential selectivity for diverse anions is important because of the need to design very selective membranes that could separate these ions,” says Livermore scientist Alex Noy, lead author of the article. The team then applied first principles molecular dynamics simulations and determined that “In general, an ion with lower hydration energy permeates more readily than an ion with higher hydration,” according to Tuan Anh Pham, co-author on the study and modeling director for the project.
Understanding which anions permeate the nanotube pore could be critical to many separation processes, including desalination. “The observation of this strong differential selectivity is based on a mechanism unique to nanometer-scale pores and could pave the way for a new generation of custom-designed separation membranes,” says Zhongwu Li, the first author of the paper.
Contact: Alex Noy (925) 423-3396 (noy1@llnl.gov).
Quantum Simulations Reveal Missing Physics
Engineering models for shock initiation safety and explosive detonation performance rely on physics models that focus on hot spots of elevated temperatures which accelerate the chemical reactions governing explosions. Models for 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) explosions based on the hot spot paradigm, however, have been unable to describe both the initiation and detonation regimes, indicating missing physics in the fundamental understanding of what processes drive insensitive high-explosives detonations. Two Lawrence Livermore scientists discovered a new ignition mechanism that explains the unusual detonation properties of TATB. Their research appears in the May 22, 2020, online edition of Physical Review Letters.
To uncover the missing physics, the team used supercomputer simulations involving millions of atoms to examine the material response behind a detonation shock wave. They discovered the formation of networked shear bands, regions of highly disordered material produced under extreme stress. Illuminating the chemical reactivity of shear bands required deploying quantum-based molecular dynamics (QMD) simulations and high-performance computing. “The main challenge with QMD is that it can only be applied to small systems, so we developed a multiscale modeling technique to look at the chemistry of shear band and crystal regions,” explains Matthew Kroonblawd, lead author of the study.
Through scale bridging with QMD, the team also discovered that disordered material in shear bands becomes chemically activated, forming in strongly shocked TATB, and reacting 200 times faster than the crystal. The scientists describe this newly discovered phenomena as “chemical activation through shear banding,” which leads to enhanced reaction rates without the local heating typically evoked by hot spots.
Contact: Matthew Kroonblawd (925) 422-2221 (kroonblawd1@llnl.gov).
Iron Performance Under Pressure
The heaviest element produced by stellar nucleosynthesis, iron is also the most abundant heavy element in Earth’s interior, and most studied for its socio-technological and planetary importance. Stable at ambient conditions in a body-centered cubic form, iron transforms to a nonmagnetic hexagonal close-packed structure as pressures rise above 13 gigapascals, 130,000 times the atmospheric pressure on Earth. To better understand this transition, Lawrence Livermore physicist Hyunchae Cynn and a team of international collaborators identified the sub-nanosecond phase transitions in laser-shocked iron, revealing all of iron’s known structural types. Their research appears in the June 5, 2020, edition of Science Advances.
The team used a combination of a short-pulse optical laser and an ultrashort x-ray Free Electron Laser (XFEL) probe to observe the atomic structural evolution of shock-compressed iron at an unprecedented time resolution at 50 picosecond (ps) intervals. Team members also discovered the appearance of new phases after 650 ps with densities similar to or even lower than the ambient phase. This transition happens to be one of the fastest, occurring in less than 50 ps, with one of the highest strain rates recorded.
The research could further reveal the physics, chemistry, and magnetic properties of the Earth and other rocky planets by measuring time-resolved high-resolution x-ray diffractions for the entire duration of shock compression. “This is the first direct, complete observation of shock wave propagation associated with the crystal structural changes recorded by high-quality time series data,” says Cynn, co-author of the paper.
Contact: Hyunchae Cynn (925) 422-3432 (cynn1@llnl.gov).
