The first ever measurements provide evidence that extremely cold liquid water exists in two distinct structures that coexist and vary in proportion with temperature.
Super-cooled water is really two liquids in one. This is the conclusion reached by a research team at the US Department of Energy̵
The discovery, recently published in the magazine Science, provides long-sought experimental data to explain some of the bizarre behaviors that water exhibits at extremely low temperatures found in outer space and at the confines of Earth’s atmosphere. Until now, liquid water at the most extreme temperatures possible has been the subject of competing theories and conjectures. Some scientists have asked whether it is even possible that water really exists as a liquid at temperatures as low as -117.7 F (190 K) or if the odd behavior is just water rearranging its inevitable path to a solid. .
The topic is important because understanding water, which covers 71% of the earth’s surface, is fundamental to understanding how it governs our environment, our bodies and life itself.
“We have shown that liquid water at extremely cold temperatures is not only relatively stable, but exists in two structural reasons,” said Greg Kimmel, a chemical physicist at PNNL. “The results explain a longstanding controversy over whether or not super-cooled water always crystallizes before it can equilibrate. The answer is no.”
Super-cooled water: a story of two liquids
You would think we understand water by now. It is one of the most abundant and most studied substances on the planet. But despite its apparent simplicity: two hydrogen atoms and one atom of oxygen per molecule – H2Or is it deceptively complicated.
It’s surprisingly difficult for water to freeze just below its melting point – water resists freezing unless it has something to start with, like dust or some other solid to cling to. In pure water, it takes an energetic push to push the molecules into the special arrangement needed to freeze. And it expands when it freezes, which is odd behavior compared to other liquids. But that oddity is what sustains life on Earth. If the ice cubes sank or the water vapor in the atmosphere did not hold the heat, life on Earth as we know it would not exist.
The strange behavior of the water has kept chemical physicists Bruce Kay and Greg Kimmel busy for more than 25 years. Now, they and postdoctoral scientists Loni Kringle and Wyatt Thornley have reached a milestone that they hope will expand our understanding of the contortions that liquid water molecules can create.
Various models have been proposed to explain the unusual properties of water. The new data obtained using a kind of stop-motion “snapshot” of super-cooled water shows that it can condense into a high-density liquid-like structure. This higher density form coexists with a lower density structure that is more in line with the typical bond expected for water. The proportion of high-density liquid rapidly decreases as the temperature goes from -18.7 F (245 K) to -117.7 F (190 K), supporting predictions of “mixture” models for supercooled water.
Kringle and Thornley used infrared spectroscopy to spy on water molecules trapped in a kind of stop motion when a thin film of ice was hit with a laser, creating super-cooled liquid water for fleeting nanoseconds.
“A key observation is that all structural changes were reversible and reproducible,” said Kringle, who performed many of the experiments.
Graupel: it’s super cooled water!
This research can help explain graupels, the fluffy pellets that sometimes fall during cold-weather storms. Graupel is formed when a snowflake interacts with supercooled liquid water in the upper atmosphere.
“Liquid water in the upper atmosphere is deeply cooled,” says Kay, a PNNL lab colleague and expert in water physics. “When it encounters a snowflake, it freezes quickly and then, under the right conditions, falls to Earth. It is truly the only time most people will experience the effects of super-cooled water. “
These studies can also help understand how liquid water can exist on very cold planets –Jupiter, Saturn, Uranus, is Neptune—In our solar system and beyond. The superheated water vapor also creates the beautiful tails that move behind comets.
Gymnastics with water molecules
Here on Earth, a better understanding of the contortions water can produce when placed in a difficult situation, such as a single water molecule wedged into a protein, could help scientists design new drugs.
“There isn’t much room for water molecules surrounding individual proteins,” Kringle said. “This research could shed light on how liquid water behaves in very dense environments.”
Thornley noted that “in future studies, we can use this new technique to follow the molecular rearrangements underlying a wide range of chemical reactions.”
There is still a lot to learn and these measurements will help pave the way for a better understanding of the most abundant life-giving liquid on Earth.
Reference: “Reversible Structural Transformations in Super Cooled Liquid Water 135 to 245 K” by Loni Kringle, Wyatt A. Thornley, Bruce D. Kay and Greg A. Kimmel, September 18, 2020, Science.
DOI: 10.1126 / science.abb7542
This work was supported by the United States Department of Energy, Office of Science. Pulsed laser measurements and infrared spectroscopy were performed at EMS, the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility located at PNNL.