The size of your pool matters. In a pool at the gym, for example, water is pretty predictable. The depth is clearly marked, and that won’t change as long as the pool stays filled. But water at the huge scale of the ocean follows different rules. Ocean water gets pulled away from shore by the moon’s gravity at low tide, forcing you to move your towel up the beach to avoid getting soaked when high tide rolls back in.
At the almost invisibly tiny scale, it turns out, water behaves totally differently. Go through the trouble of building an infinitesimally small swimming pool, and you’ll find that water has a whole new personality.
That’s what Aniruddha Deb did. Deb, an assistant research scientist in LSA’s Department of Chemistry, built microscopically small “pools” that he calls nanopores. Deb created nanopores with an industrial material called Nafion, which is made of molecules shaped like long threads. These threads formed the walls of the nanopores—the sides of the tiny pools—which Deb filled with water. When he watched the electrons in those tiny swimming pools, Deb noticed something unusual.
When poked, the electrons sped away really, really fast.
Electrons Swimming in Tiny Pools of Water
Deb studied the water at what he calls a “dream institute” in Japan. “It’s a beautiful place,” he says, “at the top of the mountains.” Deb wasn’t there for the view—the facility houses a top-of-the-line magnetic Compton scattering spectrometer, a machine that came in handy for his tiny water experiments.
Using the spectrometer, Deb blasted the electrons in his nanopores with X-rays. When an X-ray crashed into an electron, the electron recoiled from the collision at a speed that Deb could measure. Based on the speed of the recoiling electrons, he could judge how strongly the water molecules in the nanopores interacted with each other. Slower electrons would mean that the water molecules mostly stayed separate, like a group of friends floating in separate kayaks. Faster electrons would mean that the water molecules were strongly bonded in a network, like a group of boaters who had rafted their kayaks together in a flotilla.
When Deb compared the speed of nanopore electrons with the speed of electrons in other contexts, such as water in larger volumes and water at different temperatures, he saw that the water in the nanopores was weird. The speed of electrons in any type of water tends to increase when they’re struck by X-rays, but in nanopores, the recoiling electrons increased their speed to a much greater degree, ranging between a 17-fold and 46-fold difference.
“It behaves as a network!” Deb exclaims about the tiny volumes of water in the nanopores. Faster electron speed means strongly interacting water molecules. Instead of existing independently, as in water at larger scales, water molecules at exceedingly small scales appear to behave in a more coordinated way. This means that we need to handle water differently in extremely small volumes.
Batteries and Biology
We’ve noticed weird things about water before. Unlike other substances, water is less dense as a solid than as a liquid, which is why ice floats. Water expands when it’s cold, unlike most materials, including asphalt, which can expand in the heat of summer to form cracks in roads. Water also has the highest specific heat of any common substance, which is why sweating cools our bodies, using up our body heat in the evaporation process.
This newly discovered quirk of water, that its molecules seem to interact more strongly at the teeny-tiny scale, carries whole new implications, including potential biological applications. Deb’s nanopores span the approximate distance that separates the structures in the cells of organisms. Understanding the properties of water at the cellular scale can allow us to produce new medicines and may reveal clues about how organisms have evolved.
Where else can we apply what we’ve learned about weird water? “I will give you a simple example of a lithium ion battery,” Deb explains. Lithium ion batteries have positive and negative ends that are separated by a material that, like Nafion, contains nanopores. The nanopores are filled with an electrolyte, often a liquid, which allows the battery’s electric charge to flow between the positive and negative ends. As researchers engineer increasingly efficient batteries, they need to know whether electrolytes behave differently within nanopores. “These experiments with water can be applied to understand different electrolytes,” Deb says, “which can help us build a better battery.”