The founder of contemporary thermodynamics and physical chemistry is the American physicist Josiah Willard Gibbs. In the 1870s he derived the phase rule, which describes the maximum number of different phases a substance or mixture of substances can assume simultaneously. For pure substances, the Gibbs Phase Rule predicts a maximum of 3 phases.

Professor Remco Tuinier, of the Institute for Complex Molecular Systems: “At the time, Einstein called Gibbs’ thermodynamics the only theory he really trusted. If we take water as an example, there is one point, with a specific temperature and pressure, where water occurs as gas, liquid and ice at the same time. The so-called triple point.” Assistant professor Mark Vis, from the same research group as Tuinier, adds: “This classic Gibbs phase rule is as solid as a rock and has never been defied.”

SHAPE MATTERS According to this phase rule, the mixture studied by the researchers would also exhibit a maximum of three phases at one specific point at the same time. But Tuinier and his colleagues now show that in this mixture there is a whole series of circumstances in which four phases exist at the same time. There is even one point at which there are five coexisting phases. Two too many, according to Gibbs. At that specific one point, also called a five-phase equilibrium, a gas phase, two liquid crystal phases, and two solid phases with ‘ordinary’ crystals exist simultaneously. And that has never been seen before. “This is the first time that the famous Gibbs rule has been broken,” Vis says enthusiastically.

The crux lies in the shape of the particles in the mixture. Gibbs did not take this into consideration, but the Eindhoven scientists now show that it is precisely the specific length and diameter of the particles that play a major role. Tuinier: “In addition to the known variables of temperature and pressure, you get two additional variables: the length of the particle in relation to its diameter, and the diameter of the particle in relation to the diameter of other particles in the solution.”

RANKED RODS

In their theoretical models, the researchers worked with a mixture of two substances in a background solvent: rods and polymers. This is also called a colloidal system, in which the particles are solid and the medium is liquid. Because the particles cannot occupy exactly the same space, they interact with each other. “This is also called the excluded volume effect; it causes the rods to want to sit together. They are, as it were, pushed towards each other by the polymer chains. In this way, you get a region in the mixture that mainly contains rods, and an area that is rich in polymers,” explains Tuinier.

He continues: “The rods then sink to the bottom, because they’re usually heavier. That’s the beginning of segregation, creating phases.” The lower part, which mainly contains rods, will eventually become so crowded that the rods will interfere with each other. They then take up a preferential position, so that they are less in each other’s way.

With the rods it looks like a neat arrangement next to each other. Eventually you get five different phases, a gas phase with unaligned rods at the top (an isotropic phase), a liquid phase with rods pointing in about the same direction (nematic liquid crystal), a liquid phase with rods lying in different layers (smectic liquid crystal), and two solid phases at the bottom.

MAYONNAISE AND MONITORS

Vis: “Our research contributes to the fundamental knowledge about this kind of phase transition and helps to understand and predict more precisely when these kinds of transition occur.” And that is useful in many areas. Think of pumping complex mixtures around in industrial reactors, making complex products like colloidal mixtures such as mayonnaise and paint, or ice that forms on car windows and black ice on roads.

Even in liquid crystals in monitors, these processes play a role. “Most industries choose to work with a single-phase system, where there is no segregation. But if the exact transitions are clearly described, then the industry can actually use those different phases instead of avoiding them,” says Vis.

CHANCE

It was more or less chance that the researchers arrived at an equilibrium of more than three phases. When simulating and programming plate-shaped particles and polymers, PhD students Álvaro González García and Vincent Peters from Tuinier’s group saw a four-phase equilibrium. Tuinier: “Álvaro came to me one day and asked me what had gone wrong. Because four phases just couldn’t be right.”

Then the researchers tried out multiple shapes, such as cubes and also rods. Tuinier: “With the rods, most phases turned out to be possible, we even found a five-phase equilibrium. That could also mean that even more complicated equilibria are possible, as long as you search long enough for complex different particle shapes.”

Researchers at Rice University’s Laboratory for Nanophotonics (LANP) have long known that a nanoparticle’s shape affects how it interacts with light, and their latest study shows how shape affects a particle’s ability to use light to catalyze important chemical reactions.

In a comparative study, LANP graduate students Lin Yuan and Minhan Lou and their colleagues studied aluminum nanoparticles with identical optical properties but different shapes. The most rounded had 14 sides and 24 blunt points. Another was cube-shaped, with six sides and eight 90-degree corners. The third, which the team dubbed “octopod,” also had six sides, but each of its eight corners ended in a pointed tip.

All three varieties have the ability to capture energy from light and release it periodically in the form of super-energetic hot electrons that can speed up catalytic reactions. Yuan, a chemist in the research group of LANP director Naomi Halas, conducted experiments to see how well each of the particles performed as photocatalysts for hydrogen dissociation reaction. The tests showed octopods had a 10 times higher reaction rate than the 14-sided nanocrystals and five times higher than the nanocubes. Octopods also had a lower apparent activation energy, about 45% lower than nanocubes and 49% lower than nanocrystals.

“The experiments demonstrated that sharper corners increased efficiencies,” said Yuan, co-lead author of the study, which is published in the American Chemical Society journal ACS Nano. “For the octopods, the angle of the corners is about 60 degrees, compared to 90 degrees for the cubes and more rounded points on the nanocrystals. So the smaller the angle, the greater the increase in reaction efficiencies. But how small the angle can be is limited by chemical synthesis. These are single crystals that prefer certain structures. You cannot make infinitely more sharpness.”

Lou, a physicist and study co-lead author in the research group of LANP’s Peter Nordlander, verified the results of the catalytic experiments by developing a theoretical model of the hot electron energy transfer process between the light-activated aluminum nanoparticles and hydrogen molecules.

“We input the wavelength of light and particle shape,” Lou said. “Using these two aspects, we can accurately predict which shape will produce the best catalyst.”

The work is part of an ongoing green chemistry effort by LANP to develop commercially viable light-activated nanocatalysts that can insert energy into chemical reactions with surgical precision. LANP has previously demonstrated catalysts for ethylene and syngas production, the splitting of ammonia to produce hydrogen fuel and for breaking apart “forever chemicals.”

“This study shows that photocatalyst shape is another design element engineers can use to create photocatalysts with the higher reaction rates and lower activation barriers,” said Halas, Rice’s Stanley C. Moore Professor of Electrical and Computer Engineering, director of Rice’s Smalley-Curl Institute and a professor of chemistry, bioengineering, physics and astronomy, and materials science and nanoengineering.

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In a new paper publishing in Science, the researchers show how they are able to make use of existing seismic monitoring equipment, as well as historic seismic data, to determine how much the temperature of the earth’s oceans has changed and continues changing, even at depths that are normally out of the reach of conventional tools.

They do this by listening for the sounds from the many earthquakes that regularly occur under the ocean, says Jörn Callies, assistant professor of environmental science and engineering at Caltech and study co-author. Callies says these earthquake sounds are powerful and travel long distances through the ocean without significantly weakening, which makes them easy to monitor.

Wenbo Wu, postdoctoral scholar in geophysics and lead author of the paper, explains that when an earthquake happens under the ocean, most of its energy travels through the earth, but a portion of that energy is transmitted into the water as sound. These sound waves propagate outward from the quake’s epicenter just like seismic waves that travel through the ground, but the sound waves move at a much slower speed. As a result, ground waves will arrive at a seismic monitoring station first, followed by the sound waves, which will appear as a secondary signal of the same event. The effect is roughly similar to how you can often see the flash from lightning seconds before you hear its thunder.

“These sound waves in the ocean can be clearly recorded by seismometers at a much longer distance than thunder — from thousands of kilometers away,” Wu says. “Interestingly, they are even ‘louder’ than the vibrations traveling deep in the solid Earth, which are more widely used by seismologists.”

The speed of sound in water increases as the water’s temperature rises, so, the team realized, the length of time it takes a sound to travel a given distance in the ocean can be used to deduce the water’s temperature.

“The key is that we use repeating earthquakes — earthquakes that happen again and again in the same place,” he says. “In this example we’re looking at earthquakes that occur off Sumatra in Indonesia, and we measure when they arrive in the central Indian ocean. It takes about a half hour for them to travel that distance, with water temperature causing about one-tenth-of-a second difference. It’s a very small fractional change, but we can measure it.”

Wu adds that because they are using a seismometer that has been in the same location in the central Indian Ocean since 2004, they can look back at the data it collected each time an earthquake occurred in Sumatra, for example, and thus determine the temperature of the ocean at that same time.

“We are using small earthquakes that are too small to cause any damage or even be felt by humans at all,” Wu says. “But the seismometer can detect them from great distances , thus allowing us to monitor large-scale ocean temperature changes on a particular path in one measurement.”

Callies says the data they have analyzed confirm that the Indian Ocean has been warming, as other data collected through other methods have indicated, but that it might be warming even faster than previously estimated.

“The ocean plays a key role in the rate that the climate is changing,” he says. “The ocean is the main reservoir of energy in the climate system, and the deep ocean in particular is important to monitor. One advantage of our method is that the sound waves sample depths below 2,000 meters, where there are very few conventional measurements.”

Depending on which set of previous data they compare their results to, ocean warming appears to be as much as 69 percent greater than had been believed. However, Callies cautions against drawing any immediate conclusions, as more data need to be collected and analyzed.

Because undersea earthquakes happen all over the world, Callies says it should be possible to expand the system he and his fellow researchers developed so that it can monitor water temperatures in all of the oceans. Wu adds that because the technique makes use of existing infrastructure and equipment, it is relatively low-cost.

“We think we can do this in a lot of other regions,” Callies says. “And by doing this, we hope to contribute to the data about how our oceans are warming.”