(Nanowerk News) Cage structures made with nanoparticles could be a pathway to the creation of organized nanostructures with mixed materials, and researchers at the University of Michigan have shown how to achieve this through computer simulations.
The findings could open new avenues for photonic materials that manipulate light in ways natural crystals cannot. It also exhibits an unusual effect the team calls entropy compartmentalization.
“We’re developing new ways to structure materials across scales, discovering what possibilities and strengths we can use them to,” said Sharon Glotzer, Chair of the Department of Chemical Engineering Anthony C. Lembke, who led the research published in Natural Chemistry (“Entropy compartmentalization stabilizes open host-guest colloid Clarate”). “Entropic forces can stabilize crystals that are more complex than we thought.”
Although entropy is often described as the disorder in a system, it more accurately reflects the tendency of a system to maximize the possibilities of its state. Most of the time, this ends up as a nuisance in the everyday sense. Oxygen molecules don’t cluster together in a corner—they spread out to fill the room. But if you put them in the right size box, they will naturally organize themselves into a recognizable structure.
Nanoparticles do the same. Previously, Glotzer’s team had shown that bipyramid particles—like two short, three-sided pyramids stuck together at the base—would form a structure that resembled fire ice if you put it in a small enough box. Fire ice is made of water molecules that form a cage around methane, and it can burn and melt at the same time. This substance is commonly found under the seabed and is an example of clathrate. Clathrate structures are being investigated for various applications, such as trapping and removing carbon dioxide from the atmosphere.
Unlike water clathrate, the former nanoparticle clathrate structure lacked gaps to be filled with other materials which might provide new and exciting possibilities for changing the structural properties. The team wants to change that.
“This time, we investigated what happens if we change the shape of the particles. We reasoned that if we cut the particles slightly, it would create space in the cage created by the bipyramid particles,” said Sangmin Lee, a recent doctoral graduate in chemical engineering and first author of the paper.
He took the three center corners of each bipyramid and found the sweet spot where the spaces appear in the structure but the sides of the pyramid are still intact enough that they don’t start to arrange differently. Space is filled with bipyramids which are more truncated when they are the only particles in the system. When a second shape is added, it becomes a trapped guest particle.
Glotzer had an idea for how to create a selective sticky side that would allow the different materials to act as both caged and guest particles, but in this case, there was no glue holding the bipyramid together. Instead, the structure is fully stabilized by entropy.
“What’s really interesting, looking at the simulations, is that the host network almost freezes. The host particles move, but they all move together like one rigid body, which is exactly what happens with water clathrates,” said Glotzer. “But the guest particle is spinning like crazy—like a system dumping all the entropy into the guest particle.”
This is the system with the highest degree of freedom that a truncated bipyramid can construct in a confined space, but almost all of the freedom belongs to the guest particles. The methane in the water clathrate is also swirling, the researchers said. What’s more, when they remove guest particles, the structures toss bipyramids that have become part of the networked cage structure into the cage interior—it is more important to have spinning particles available to maximize entropy than to have a complete cage.
“Entropy compartmentalization. Isn’t that cool? I bet it happens on other systems too—not just clathrate,” said Glotzer.