Nanoscopic particles resist full encapsulation, Sandia simulations show

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.”