By Neal Singer
It may seem obvious that dunking relatively spherical objects in a sauce — blueberries in melted chocolate, say — will result in an array of completely encapsulated berries.
MATT LANE examines a large projection of a computer model depicting polymer-coated silica nanoparticles. Here, the two 5-nanometer particles make contact in a water solution. Sandia researchers used molecular dynamics simulations to measure the forces between coated nanoparticles that were too small to measure experimentally. The observation of strongly asymmetric coatings led Matt (1435) and colleague Gary Grest (1114) to further study the coating properties on very small particles. These results were featured as the cover article in the June 11, 2010, issue of Physical Review Letters. (Photo by Randy Montoya)
Relying on that macroworld concept, fabricators of spherical nanoparticles have similarly dunked their wares in protective coatings in the belief such encapsulations would prevent clumping and unwanted chemical interactions with solvents.
Unfortunately, reactions in the nanoworld are not logical extensions of the macroworld, Sandia researchers Matthew Lane (1435) and Gary Grest (1114) have found.
In a cover article this past summer in Physics Review Letters (see cover image above), the researchers described using molecular dynamics simulations at the New Mexico Computational Center supercomputer to show that simple coatings are actually incapable of fully covering each spherical nanoparticle in a set. Instead, because the diameter of a particle may be smaller than the thickness of the coating protecting it, the geometry of the particle surface as it rapidly drops away from its attached chainlike coating molecules provokes the formation of a series of louvers rather than a solid protective wall (see illustration above).
“We’ve known for some time now that nanoparticles are special, and that ‘small is different,’” says Matt. “What we’ve shown is that this general rule for nanotechnology applies to how we coat particles, too.”
“It’s well-known that aggregation of nanoparticles in suspension is presently an obstacle to their commercial and industrial use,” says Carlos Gutierrez, manager of Surfaces and Interface Sciences Dept. 1114. “The simulations show that even coatings fully and uniformly applied to spherical nanoparticles are significantly distorted at the water-vapor interface.”
As the researchers put it in their paper, “Spontaneous Asymmetry of Coated Spherical Nanoparticles in Solution and at Liquid-Vapor Interfaces” . . . “The asymmetric response appears to be reinforced when particles are at a surface.”
“You don’t want aggregation because you want the particles to stay distributed throughout the product to achieve uniformity,” says Gary. “If you have particles of, say, micron-size, you have to coat or electrically charge them so the particles don’t stick together. But when particles get small and the coatings become comparable in size to the particles, the shapes they form are asymmetric rather than spherical. Spherical particles keep their distance; asymmetric particles may stick to each other.”
However, the simulation’s finding isn’t necessarily a bad thing, for this reason: Though each particle is coated asymmetrically, the asymmetry is consistent for any given set. Said another way, all coated nanoscopic sets are asymmetric in their own way.
A predictable, identical variation occurring in every member of a nanoset could open doors to new applications.
“What we’ve done here is to put up a large ‘dead end’ sign to prevent researchers from wasting time going down the wrong path,” says Matt. “Increasing surface density of the coating or its molecular chain length isn’t going to improve patchy coatings, as it would for larger particles. But there are numerous other possible paths to new outcomes when you can control the shape of the aggregation.” -- Neal Singer
An investigation by DOE and Sandia has concluded the Mixed Waste Landfill is not the source of very low concentrations of toluene detected in groundwater samples collected from monitoring wells at the site. Toluene is a common solvent found in paint thinners, gasoline, and other consumer products.
Don Schofield (4133, left) and Mike Mitchell (6765) check on the establishment of the new vegetation on the Mixed Waste Landfill cover in this photo from late 2009. Construction on the cover was completed a year ago. A recent investigation has found the landfill is not the source of trace amounts of toluene detected in nearby groundwater samples. (Photo by Randy Montoya)
The results of the investigation were published in the Mixed Waste Landfill Toluene Investigation Report and submitted in August to the New Mexico Environment Department (NMED), which requested the investigation in April.
The Mixed Waste Landfill was established in 1959 as a disposal area for low-level radioactive waste generated by Sandia’s research facilities.
The toluene concentrations detected in groundwater samples near the 2.6-acre site are significantly lower than the federal drinking water standard for toluene of 1,000 parts per billion. And, the concentrations are very close to the detection limit of 0.25 parts per billion set by the independent analytical laboratory subcontracted by Sandia.
This investigation also indicates that the analytical laboratory that tested the groundwater samples is the source of toluene reported in some samples.
“The toluene groundwater results reflect the ubiquitous nature of toluene and the very low analytical detection limit (of 0.25 parts per billion),” the report’s executive summary states. “The detections do not represent a release to the environment or widespread low-concentration toluene contamination in the regional aquifer.”
Toluene is a common volatile organic liquid, which means it easily turns to vapor at ambient temperatures. It is present in many workplaces and consumer products and has been identified by the US Environmental Protection Agency as a common laboratory contaminant. It is also released into the atmosphere by manufacturing plants and automobile emissions.
Toluene is present in urban air at very low concentrations of 0.01 to 0.05 parts per million — in other words, at concentrations higher than was detected in the groundwater samples collected near the Mixed Waste Landfill.
The extensive investigation on the source of the toluene included a review of the results from all previous Mixed Waste Landfill field investigations; a review of soil-vapor surveys conducted in 1994 and 2008; a review and analysis of all Sandia groundwater monitoring results from 2001 through April 2010; and evaluation of the possible introduction of toluene from the sampling equipment and process, the drilling and well construction materials and processes, and during testing at the analytical laboratory. DOE and Sandia also completed additional groundwater studies as requested by NMED.
Results of the current and previous investigations show that toluene has never been found in concentrations that would affect regional groundwater, which occurs at a depth of 500 feet, so the chemical is not considered a significant contaminant at the site.
Toluene has been sporadically detected in groundwater samples collected prior to 2008 and continues to be detected in groundwater samples from all four groundwater monitoring wells installed in 2008, including the background well, at very low concentrations, typically less than one part per billion.
The Mixed Waste Landfill is located in Technical Area 3 in the west-central part of Kirtland Air Force Base about five miles southeast of the Albuquerque airport. For the past 20 years, groundwater monitoring beneath the site has demonstrated that the materials disposed in the landfill have not contaminated the groundwater. The landfill stopped accepting waste in 1988.
DOE and Sandia, in coordination with the NMED, will continue to closely monitor the groundwater beneath the landfill and will continue testing for toluene. -- Heather Clark
By Mike Janes
Transportation experts are proposing that the research and development of next-generation biofuels must be done in conjunction with the development of advanced combustion engines, if those biofuels are to become a reality and experience long-term success in the US transportation sector, according to a new report issued by Sandia.
The recommendations came out of a Sandia-hosted workshop held in the Bay Area late last year, “Next Generation Biofuels and Advanced Engines for Tomorrow’s Transportation Needs.” Participants included researchers at Sandia’s Combustion Research Facility (CRF) and Joint BioEnergy Institute (JBEI), as well as representatives from oil companies, biofuel developers, engine manufacturers, suppliers, and experts from the university, regulatory, finance, and national laboratory communities.
The workshop, says Ron Stoltz (8302), was designed to identify opportunities for codevelopment of biofuels and engines, an often overlooked issue.
“The oil companies and the automobile and truck engine companies have engaged in a dialogue and collaboration on fuel and engine issues for almost 100 years,” Ron says. “But the same cannot be said for the majority of biofuel start-up companies, especially those that are thinking ‘beyond ethanol.’ The report highlights how fragmented the biofuels industry is today and how, by putting serious thought behind some key issues like fuel chemistry linked to engine performance, great strides can be made.”
The primary goal of the workshop, Ron says, was to foster dialog among researchers and experts from industry, academia, and government, with the ultimate hope of finding ways to accelerate the transition to biofuels.
Workshop participants agreed that a series of key attributes are necessary to make the introduction of next-generation biofuels a reality in the transportation sector. Biofuels, they concluded, must be:
Among other observations, participants also concluded that a consolidated, federally funded research program on biofuels and advanced engine concepts is necessary to accelerate the transition to biofuels.
Four key recommended actions emerged from the workshop:
Bob Carling, director of Transportation Energy Center 8300, says Sandia’s role as a national laboratory is to look to the future and inform policymakers and others about the potential of advanced technologies and the technical challenges that stand in the way of commercialization.
“In the biofuel arena, or more generally advanced liquid fuels, the need is to understand how chemistry has an impact on the performance of any advanced fuel, either through efficiency gains or losses while meeting current emission regulations,” says Bob. “Sandia will continue to work with the scientific and engineering community as new, advanced liquid fuels are being developed. We want to integrate the community’s research into our work as we develop increased knowledge and understanding of chemistry and its role in new internal combustion engine architectures.” - Mike Janes