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.
Relying on that 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.
The protective capabilities of a chain-link molecular coating are lessened when, instead of attaching to a flat surface to form an unbroken wall (fig. 1), the radius of the nanoparticle is so small that the extreme surface curvatuere opens spaces between the protective molecules (fig. 2). (Drawing courtesy Matt Lane, Sandia National Labs)
Unfortunately, reactions in the nanoworld are not logical extensions of the
macroworld, Sandia National
Laboratories researchers Matthew Lane and Gary Grest have found.
In a cover article this past summer in Physical Review Letters, the researchers
use molecular dynamics simulations to show that simple coatings are 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 curvature of the particle surface as it rapidly
drops away from its attached coating provokes the formation of a series of louvres
rather than a solid protective wall (see illustration).
“We’ve known for some time now that nanoparticles are special,
and that ‘small is different,’” Lane said. “What we’ve
shown is that this general rule for nanotechnology applies to how we coat particles,
too.”
Carlos Gutierrez, manager of Sandia’s Surfaces and Interface Sciences
Department, said, “It’s well-known that aggregation of nanoparticles
in suspension is presently an obstacle to their commercial and industrial use.
The simulations show that even coatings fully and uniformly applied to spherical
nanoparticles are significantly distorted at the water-vapor interface.”
Said Grest, “You don’t want aggregation because you want the particles
to stay distributed throughout the product to achieve uniformity. 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.”
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,”
Lane said. “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.”
More information: The paper, Spontaneous Asymmetry of Coated Spherical Nanoparticles
in Solution and at Liquid-Vapor Interfaces, was published on June 9 in Physical
Review Letters 104, 235501 (2010).