Nanotechnology Now

Our NanoNews Digest Sponsors

Heifer International

Wikipedia Affiliate Button

Home > Press > Caltech Researchers Find That Disorder Is Key to Nanotube Mystery

This image from the Caltech team’s simulations features a cutaway of a 2.0 nanometer-diameter carbon nanotube, revealing confined water molecules.
[Credit: Caltech/Tod Pascal]
This image from the Caltech team’s simulations features a cutaway of a 2.0 nanometer-diameter carbon nanotube, revealing confined water molecules. [Credit: Caltech/Tod Pascal]

Abstract:
Scientists often find strange and unexpected things when they look at materials at the nanoscale—the level of single atoms and molecules. This holds true even for the most common materials, such as water.

Caltech Researchers Find That Disorder Is Key to Nanotube Mystery

Pasadena, CA | Posted on August 13th, 2011

Case in point: In the last couple of years, researchers have observed that water spontaneously flows into extremely small tubes of graphite or graphene, called carbon nanotubes. This unexpected observation is intriguing because carbon nanotubes hold promise in the emerging fields of nanofluidics and nanofiltration, where nanotubes might be able to help maintain tiny flows or separate impurities from water. However, no one has managed to explain why, at the molecular level, a stable liquid would want to confine itself to such a small area.

Now, using a novel method to calculate the dynamics of water molecules, Caltech researchers believe they have solved the mystery. It turns out that entropy, a measurement of disorder, has been the missing key.

"It's a pretty surprising result," says William Goddard, the Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics at Caltech and director of the Materials and Process Simulation Center. "People normally focus on energy in this problem, not entropy."

That's because water forms an extensive network of hydrogen bonds, which makes it very stable. Breaking those strong interactions requires energy. And since some bonds have to be broken in order for water to flow into small nanotubes, it would seem unlikely that water would do so freely.

"What we found is that it's actually a trade off," Goddard says. "You lose some of that good energy stabilization from the bonding, but in the process you gain in entropy."

Entropy is one of the driving forces that determine whether a process will occur spontaneously. It represents the number of ways a system can exist in a particular state. The more arrangements available to a system, the greater its disorder, and the higher the entropy. And in general, nature proceeds toward disorder.

When water is ideally bonded, all of the hydrogen bonds lock the molecules into place, restricting their freedom and keeping water's entropy low. What Goddard and postdoctoral scholar Tod Pascal found is that in the case of some nanotubes, water gains enough entropy by entering the tubes to outweigh the energy losses incurred by breaking some of its hydrogen bonds. Therefore, water flows spontaneously into the tubes.

Goddard and Pascal explain their findings in a paper recently published in the Proceedings of the National Academy of Sciences (PNAS). They looked at carbon nanotubes with diameters between 0.8 and 2.7 nanometers and found three different reasons why water would flow freely into the tubes, depending on diameter.

For the smallest nanotubes—those between 0.8 and 1.0 nanometers in diameter—the tubes are so minuscule that water molecules line up nearly single file within them and take on a gaslike state. That means the normal bonded structure of liquid water breaks down, giving the molecules greater freedom of motion. This increase in entropy draws water into the tubes.

At the next level, where the nanotubes have diameters between 1.1 and 1.2 nanometers, confined water molecules arrange themselves in stacked, icelike crystals. Goddard and Pascal found such nanotubes to be the perfect size—a kind of Goldilocks match—to accommodate crystallized water. These crystal-bonding interactions, not entropy, make it favorable for water to flow into the tubes.

On the largest scale studied—involving tubes whose diameters are still only 1.4 to 2.7 nanometers wide—the researchers found that the confined water molecules behave more like liquid water. However, once again, some of the normal hydrogen bonds are broken, so the molecules exhibit more freedom of motion within the tubes. And the gains in entropy more than compensate for the loss in hydrogen bonding energy.

Because the insides of the carbon nanotubes are far too small for researchers to examine experimentally, Goddard and Pascal studied the dynamics of the confined water molecules in simulations. Using a new method developed by Goddard's group with a supercomputer, they were able to calculate the entropy for the individual water molecules. In the past, such calculations have been difficult and extremely time-consuming. But the new approach, dubbed the two-phase thermodynamic model, has made the determination of entropy values relatively easy for any system.

"The old methods took eight years of computer processing time to arrive at the same entropies that we're now getting in 36 hours," Goddard says.

The team also ran simulations using an alternative description of water—one where water had its usual properties of energy, density, and viscosity, but lacked its characteristic hydrogen bonding. In that case, water did not want to flow into the nanotubes, providing additional proof that water's naturally occurring low entropy due to extensive hydrogen bonding leads to it spontaneously filling carbon nanotubes when the entropy increases.

Goddard believes that carbon nanotubes could be used to design supermolecules for water purification. By incorporating pores with the same diameters as carbon nanotubes, he thinks a polymer could be made to suck water out of solution. Such a potential application points to the need for a greater understanding of water transport through carbon nanotubes.

The paper, "Entropy and the driving force for the filling of carbon nanotubes with water," appeared in the July 19 issue of PNAS. Yousung Jung of the Korea Advanced Institute of Science and Technology (KAIST) also contributed to the study. Yousung completed a postdoctoral fellowship at Caltech under Nobel Prize winner Rudy Marcus before joining the faculty at KAIST, where he and Goddard are participating in the World Class University program of Korea. They are developing practical systems as part of the Energy, Environment, Water, and Sustainability Initiative, which provided the supercomputers used in this research.

Written by Kimm Fesenmaier

####

For more information, please click here

Contacts:
Deborah Williams-Hedges
626-395-3227

Copyright © Caltech

If you have a comment, please Contact us.

Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.

Bookmark:
Delicious Digg Newsvine Google Yahoo Reddit Magnoliacom Furl Facebook

Related News Press

News and information

Scientists track chemical and structural evolution of catalytic nanoparticles in 3-D: Up-close, real-time, chemical-sensitive 3-D imaging offers clues for reducing cost/improving performance of catalysts for fuel-cell-powered vehicles and other applications December 8th, 2016

Exotic insulator may hold clue to key mystery of modern physics: Johns Hopkins-led research shows material living between classical and quantum worlds December 8th, 2016

Arrowhead Pharmaceuticals to Webcast Fiscal 2016 Year End Results December 7th, 2016

Journal Nanotechnology Progress International (JONPI), newest edition out December 7th, 2016

Graphene/ Graphite

Physicists decipher electronic properties of materials in work that may change transistors December 6th, 2016

Bumpy surfaces, graphene beat the heat in devices: Rice University theory shows way to enhance heat sinks in future microelectronics November 29th, 2016

Uncovering the secrets of friction on graphene: Sliding on flexible graphene surfaces has been uncharted territory until now November 23rd, 2016

2-D material a brittle surprise: Rice University researchers finds molybdenum diselenide not as strong as they thought November 14th, 2016

Nanotubes/Buckyballs/Fullerenes

Infrared instrumentation leader secures exclusive use of Vantablack coating December 5th, 2016

Novel Electrode Structure Provides New Promise for Lithium-Sulfur Batteries December 3rd, 2016

Cutting-edge nanotechnologies are breaking into industries November 18th, 2016

Hybrid nanostructures hold hydrogen well: Rice University scientists say boron nitride-graphene hybrid may be right for next-gen green cars October 25th, 2016

Discoveries

Scientists track chemical and structural evolution of catalytic nanoparticles in 3-D: Up-close, real-time, chemical-sensitive 3-D imaging offers clues for reducing cost/improving performance of catalysts for fuel-cell-powered vehicles and other applications December 8th, 2016

Exotic insulator may hold clue to key mystery of modern physics: Johns Hopkins-led research shows material living between classical and quantum worlds December 8th, 2016

ANU invention to inspire new night-vision specs December 7th, 2016

Tokyo Institute of Technology research: 3D solutions to energy savings in silicon power transistors December 6th, 2016

Announcements

Scientists track chemical and structural evolution of catalytic nanoparticles in 3-D: Up-close, real-time, chemical-sensitive 3-D imaging offers clues for reducing cost/improving performance of catalysts for fuel-cell-powered vehicles and other applications December 8th, 2016

Exotic insulator may hold clue to key mystery of modern physics: Johns Hopkins-led research shows material living between classical and quantum worlds December 8th, 2016

Arrowhead Pharmaceuticals to Webcast Fiscal 2016 Year End Results December 7th, 2016

Journal Nanotechnology Progress International (JONPI), newest edition out December 7th, 2016

Water

Deep insights from surface reactions: Researchers use Stampede supercomputer to study new chemical sensing methods, desalination and bacterial energy production December 2nd, 2016

Water, water -- the two types of liquid water: Understanding water's behavior could help with Alzheimer's research November 11th, 2016

How nanoscience will improve our health and lives in the coming years: Targeted medicine deliveries and increased energy efficiency are just two of many ways October 26th, 2016

Water vapor sets some oxides aflutter: Newly discovered phenomenon could affect materials in batteries and water-splitting devices October 3rd, 2016

NanoNews-Digest
The latest news from around the world, FREE




  Premium Products
NanoNews-Custom
Only the news you want to read!
 Learn More
NanoTech-Transfer
University Technology Transfer & Patents
 Learn More
NanoStrategies
Full-service, expert consulting
 Learn More











ASP
Nanotechnology Now Featured Books




NNN

The Hunger Project