In the early 2000s, Ramesh Jagannathan, now associate dean of Engineering and professor of Chemical Engineering at NYU Abu Dhabi, was in Kodak's research labs in Rochester, New York, investigating a common efficiency problem in inkjet printing: the speed of printing was being significantly hindered by the amount of time required for liquid solvents, such as water, to evaporate from a printed surface. Jagannathan found himself in search of a solvent that had a density similar to that of water, allowing enough ink material to be dissolved in it, but that would evaporate by the time the ink was deposited so that no drying would be necessary. As he explored the use of supercritical CO2 — a unique phase of CO2 that at a certain pressure and temperature exhibits both gaseous and liquid properties — he not only found a solution that would advance printing technology, but also realized the unique properties of this solvent that would result in the generation of a new class of materials that could not readily be explained by the current theories.
An Exploration of Cluster Science
Jagannathan began exploring the area of cluster science, which is concerned with clusters of molecules bound by a range of forces, both weak and strong. While covalent bonding, or the sharing of electrons between atoms, is considered to form a stable molecule, other physical forces can also cause weak attractions between molecules. Molecules with a neutral charge, for example, can form weak affinities with others through what is known as Van der Waals forces, established by the movement of electrons within a molecule that create very slight positive or negative charges depending on which side of the molecule the electron is located at a particular moment.
While Jagannathan realized that he was able to create molecular structures that wouldn't ordinarily stick together, the attraction between the molecules was very weak and thermal energy was often enough to bring them apart. "It was a daunting process," he said, "but if I could permanently capture these molecular clusters that demonstrate these unique properties, then I could build unique materials."
The idea persisted with Jagannathan and he considered the use of supercritical CO2, which maintains density similar to that of a liquid — allowing it to dissolve molecules — yet expands to fill up a container much like a gas. He was promptly dissuaded by his peers who pointed out the obvious challenge: Even if he could initially create these molecules, Van der Waals forces were too weak to allow them to hold together. Added to that, while at an industry conference, Jagannathan was posed with an equally perplexing question: Let's assume you can somehow stabilize these molecules, which you can't, how are you going to collect them when they are gas-like?
If I could permanently capture these molecular clusters that demonstrate these unique properties, then I could build unique materials.
A Custom System Leads to Stable, New Molecular Clusters
Using a process known as rapid expansion of supercritical solution (RESS), Jagannathan built a custom system that was able to generate highly mono-dispersed aerosols of solute materials from a supercritical CO2 reactor at a constant pressure and temperature. "When you expand supercritical CO2 through a fine nozzle, it becomes a gas when it expands and goes from a dense state to a non-dense state. When this happens, the velocity of expansion becomes supersonic, resulting in an extremely turbulent environment characterized by large shear forces. Moreover, such expansions are accompanied by a significant amount of local cooling due to a well-known phenomenon called the Joule-Thomson effect." Jagannathan hypothesized that "through the combination of extremely high shear forces and extreme cold temperatures in a supersonic environment, these soft, organic molecular clusters are likely to get locked into configurations that would normally be thermodynamically forbidden." Once formed in an abnormal environment the structures are quickly released to a normal environment, before they have the chance to disentangle themselves. If this happened, Jagannathan explained, they would forever be locked in these unusual structures because, by reverse logic, the energy barriers that made their formation difficult in the first place would provide the same high-energy barrier to prevent their disassembly.
Jagannathan set out to collect and characterize these molecular clusters, and after several trials he successfully used a fine capillary to bubble the aerosols into a liquid vial. As the CO2 gas escaped, it carried with it most of the molecular clusters, but left behind a small amount of clusters as a dispersion. These dispersions, against all odds, were found to be stable even two years after their formation.
These newly formed organic nanoparticles have several unique properties that don't quite adhere to conventional rules of chemistry and thermodynamics. For one, the clusters have unusual phase behavior. For example, the normal melting point for Alq3 — the chemical compound Tris (8-hydroxyquinolinato) aluminium — is greater than 300 degrees Celsius. But, when affected by the supercritical CO2, it is a liquid at room temperature. The "liquid-like" nature of molecular cluster assemblies has been observed in several chemical compounds and suggests that, at these small sizes, the traditional definitions of solids and liquids are no longer valid.
Crystal-like Superlattice Structures That Last
Another original attribute, characterized by X-ray powder diffraction measurements, is that these molecular clusters rapidly self-assemble into highly aligned superlattice structures (an organized 3D layered structure) at room temperature, much like a crystal, while showing complete disorder at the molecular-length scale. Perhaps even more remarkably, testing these same samples after two years has shown that they have maintained this organized structure, a finding that creates possibilities of designing stable mixtures of new materials, which would not have been possible before, Jagannathan said. The optical properties of materials such as Alq3, an organic light-emitting diode (OLED) material, were also found to be tunable by the supercritical CO2 process. For example, the peak fluorescence emission of Alq3 created by the supercritical CO2 process was "blue-shifted" by 40 nanometers. In some cases, peak emission was tunable as a function of the supercritical process parameters and shifts up to 100 nanometers were observed. Functional OLED devices were built with doped Alq3 and their electroluminescence colors were tuned from green to yellow by changing the supercritical CO2 process parameters.
"By changing the process conditions by which I create these clusters, I can change the peak emission to different positions. Our classical understanding says that molecules have very fixed emission characteristics to the position of a tenth of a nanometer, so for me to say I can 'blue-shift' this by 50 to 100 nanometers is difficult to explain."
The molecular cluster assemblies also demonstrate super-hydrophobicity, that is, they repel water. This is important for OLED materials because of their tendency to crystallize in humid ambient conditions. The property of super-hydrophobicity would improve their environmental stability.
Vast Potential for Practical Application
Jagannathan continues to work on building up data and information in this relatively new area in science to help explain and further investigate the universality of these unique properties. His lab at NYUAD is equipped with a custom-built reactor, and he is currently conducting experiments with Teflon, a soft and highly hydrophobic material. He has found through the RESS process that Teflon molecules will congregate into a highly structured interface forming a fine film that acts as a seal against water.
The practical application of these kinds of materials has vast potential, Jagannathan said, from creating self-cleaning glass, to nano-nets that prevent evaporation of water, or to altering the requirements of TVs that use three different chemicals to emit a full color spectrum.
"Using these molecular clusters, we can actually build new materials that you have never heard of. When you take materials like paper, iron, or steel, they have components that are well-defined, but when you design and build materials with these molecular clusters, their properties are so unique that you can't readily explain them."