Unveiling the Secrets of Nanoparticle Haloing

JUNE 9, 2008

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Representation of the way nanoparticles (red; not to scale) form “halos” around microspheres (blue-green) in solution.

A glass of milk, a gallon of paint and a bottle of salad dressing all look to the naked eye like liquids. But when viewed under a microscope these everyday liquids, called "colloids," actually contain small globules or particles that stay suspended in solution.

Colloids require a delicate balance of opposing forces for them to be stable: attractive forces must match repulsive ones. A new colloidal stabilization method characterized by scientists at the U.S. Department of Energy's (DOE) Argonne National Laboratory may give scientists a new way to control the stability of some colloidal suspensions.

In this approach, known as nanoparticle haloing, highly charged nanoparticles and negligibly charged colloidal microspheres are mixed together in solution. The nanoparticles self-organize around the microspheres to form a halo-like structure that stabilizes the solution. This new pathway to produce materials would not be possible through traditional routes.

The structure of the halo–the key to understanding this kind of stable colloid–has remained a mystery because the nanoparticles that form it are more than 100 times smaller than the microspheres they surround.

By using x-rays produced by Argonne's Advanced Photon Source (APS), Argonne scientists, in collaboration with researchers from the University of Illinois at Urbana-Champaign, were able to finally discover the structure of the nanoparticle halo.

The researchers used the ultra-small-angle x-ray scattering instrument at X-ray Operations and Research beamline 33-ID the APS to discover that nanoparticles form a loosely organized layer a small distance from the microspheres' surfaces. This discovery suggests a weak attraction between nanoparticle and microsphere, corroborating earlier theoretical predictions that the halo can form only in such an environment.

“Because we have established a methodology to determine the structure of nanoparticle halo, it opens a window to the systematic study of the entire nanoparticle-microsphere phase diagram for this type of novel colloidal stabilization mechanism,” said Argonne's Fan Zhang, a coauthor on the Langmuir paper.

Contact: G. Long, gglong@aps.anl.gov

See: F. Zhang, G.G. Long, P.R. Jemian, J. Ilavsky, V.T. Milam, J.A. Lewis, “Quantitative Measurement of Nanoparticle Halo Formation around Colloidal Microspheres in Binary Mixtures,” Langmuir 2008; ASAP Article.  DOI: 10.1021/la702968n

The original press release can be found here.

Research at the Advanced Photon Source, Argonne National Laboratory is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-06CH11357. The material is based in part on work supported by the U.S. Department of Energy, Division of Materials Sciences under Award Nos. DEFG-02-91ER45439 (V.T.M. and J.A.L.) and DE-FG02-07ER46471 (J.A.L.).

Argonne National Laboratory brings the world's brightest scientists and engineers together to find exciting and creative new solutions to pressing national problems in science and technology. The nation's first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America 's scientific leadership and prepare the nation for a better future. With employees from more than 60 nations, Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy's Office of Science.

This research was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences as part of its mission to foster and support fundamental research to expand the scientific foundations for new and improved energy technologies and for understanding and mitigating the environmental impacts of energy use.