Researchers Create 2D Liquid Made of Nanoparticles at Oil-Water Interface

Researchers at the University of Pennsylvania have used 'soft' nanoparticles to create a system that behaves as a 2D liquid. A 2D world exists at the place where oil and water meet. This interface has properties that could be useful for engineers and chemists.

The researchers created a 2-D liquid consisting of nanoparticles at the interface between a drop of oil and surrounding water.

Researchers have been able to make a soap molecule stay at the interface and make it behave in a predictable manner. However, they have not been able to make more complex molecules behave in the same manner.

The University of Pennsylvania research team has demonstrated the way to make nanoparticles not get attracted to each other, but only to this interface. This creates a 2-D liquid system, and the team has measured the pressure and density of the liquid in this system. This measurement shows the way for using the system in different types of applications, including photonic devices, catalysis and nanomanufacturing.

In this system the particles do not clump into skins or clusters. This allows study of the physical fundamentals of the manner in which nanoscale objects interact with other nanoscale objects in two dimensions.

The team of researchers included, Valeria Garbin, a postdoctoral researcher; Ian Jenkins, a graduate student; and Talid Sinno, Kathleen Stebe, and John Crocker, who are professors at the Department of Chemical and Biomolecular Engineering in Penn’s School of Engineering and Applied Science. Garbin is presently at the Imperial College London as an assistant professor in the Department of Chemical Engineering.

“Things get stuck at the interface between oil and water,” Stebe said. “That’s of tremendous fundamental and technological interest, because we can think of that interface as a two-dimensional world. If we can start to understand the interactions of the things that accumulate there and learn how they are arranged, we can exploit them in a number of different applications.”

However, it is difficult to make nanoparticles to go the interface and make them stay there. Adapting the surface chemistry of the nanoparticles to either oil or water is easy, however, balancing the adaptation so that the particles stay within the 2-D regime is quite difficult.

“We understand how particles work in 3-D,” Crocker said. “If you put polymer chains on the surface that are attracted to the solvent, the particles will bounce off each other and make a nice suspension, meaning you can do work with them. However, people haven't really done that in 2-D before.”

When the particles manage to stay at the interface, they form a skin as they demonstrate a tendency to clump together. This skin cannot be pulled apart into the particles that it is made of.

“All particles love themselves,” Stebe said. “Just due to Van der Waals interactions, if they can get close enough, they aggregate. But because our nanoparticles have protective ligand arms, they don’t clump together and form a liquid state. They’re in two-dimensional equilibrium.”

In order to address this problem, the team decorated gold nanoparticles with ligands, which are surfactants. These ligands are made up of an oil-loving tail and a water-loving head. The manner in which they are joined to the central particle enable them to contort in a manner such that when the particle is at an interface, it satisfies both the sides.

This leads to a “flying saucer” shape, and the ligands stretch comparatively lesser above and below than at the interface. These particles are prevented from clumping together by the ligand bumpers.

“This is a very beautiful system,” Stebe said. “The ability to tune their packing means that we can now take everything we know about the equilibrium thermodynamics in two dimensions and start to pose questions about particle layers. Do these particles behave like we think they should? How can we manipulate them in the future?”

The research team had to understand the basics of the system, and for that they had to infer the relationships of specific properties. This included the way in which the 2-D liquid’s pressure changed as a function of the particle’s packing. They used a modified version of the pendant drop method. In this method, among particles suspended in water, an oil droplet was formed. Over a period of time, more particles attached themselves to the oil-water interface, which led to the formation of a 2-D liquid in a manner where its properties could be measured.

“We can infer the pressure of this 2-D fluid by the shape of the drop,” Stebe said. “Once we compress the drop by pulling some of the oil back into the syringe, we can determine how the shape changes and relate it to the pressure in the layer.”

The density of the particle packing also had to be determined. When the drop was compressed, the particle density increased which made the drop become more opaque. The amount of light that shines through the drop cannot be easily measured. This is due to the plasmonic behavior that leads to change in the properties of the gold nanoparticles when they come closer.

“Fortunately, we discovered another interesting feature of this nanoparticle system,” Garbin said. “If the drop was compressed too much, some particles would fall out of the interface because they didn't fit anymore. This enabled us to measure the amount of particles that were in that falling plume, since the particles are farther apart from each other there. From that measurement, we could work backwards to the number of particles on the interface”

The relationship that exists between the 2-D liquid’s pressure and the packing of the particle provides the basis of fundamental rules that are related to the physics of such systems.

“From this data,” Crocker said, “we can figure out the force versus distance of two nanoparticles. That means we can now make a model of how these particles behave in the 2-D liquid.”

Researchers will be able to develop functional nanoparticles possessing different traits, which include more complex and longer ligands that are suited for performing particular chemical tasks.

“One application is interface catalysis,” Stebe said. “For example, if you have a reagent that’s in the oil phase, but its product is in the water phase, having a particle on the interface that can help move it from one to the other would be perfect.”

Future studies could be supported with better knowledge about the reason for particles getting trapped in liquid-liquid interfaces.

This study has been published in Physical Review Letters.

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