Liquid water doesn’t always turn solid when it’s cooled below the freezing point. Under the right conditions, including high purity or being housed in a smooth container, water will remain a liquid well below 0 degrees Celsius.
But supercooled liquid water is a sensitive medium. At temperatures below negative 50 degrees Celsius, a slight disturbance can cause supercooled liquid water to rapidly crystallize into ice, making it very difficult to study experimentally.
In a recent appearing in Proceedings of the National Academy of Sciences, University of California, Davis and Lawrence Livermore National Laboratory (LLNL) researchers use computational modeling to investigate a hypothesized state of supercooled liquid water.
Water is found in many different places in nature, from inside our cells to deep in the Earth. Understanding how water can exist in different states is potentially important for questions ranging from health and disease to earthquakes.
“About 30 years ago, there was a hypothesis that there are actually two different kinds of liquid water and that’s really hard to wrap your head around,” said study author , an associate professor in the Department of Chemistry at 51ԹϺ Davis. “Imagine that liquid water can separate into two different liquids that don’t mix with each other, like oil and water, except they’re both water.”
“If this hypothesis is true, you should be able to observe the phase separation of water into a high-density liquid and a low-density liquid,” he added.
The research team, which included 51ԹϺ Davis Professor of Chemistry and 51ԹϺ Davis alum and LLNL postdoctoral researcher Margaret Berrens, used a physics-based machine-learning model called iAMOEBA to simulate water molecules at this critical point.
The simulations showed evidence of a free energy barrier separating high-density liquid water from low-density liquid water that quickly vanished at a critical point. The research provides evidence for two supercooled liquid water states.
Difficult states to investigate
According to Wang, supercooled water isn’t just difficult to investigate experimentally. It’s also difficult to investigate computationally in large part due to its previously mentioned physical sensitivities.
“In a molecular simulation, what we’re doing is really creating a movie of the water molecules as time proceeds,” said Wang, noting that to do this accurately three requirements must be met.
First, the mathematical equation underlying the model must replicate the quantum mechanical intermolecular interactions as closely as possible. Second, the simulation needs to be big enough, including an adequate number of water molecules to best mimic how liquid water behaves in real life. And third, the simulation needs to run for enough time.
“To simulate supercooled liquid water properly, you really need all three of these ingredients,” Wang said.
By using iAMOEBA, the research team struck a balance between the three requirements. They successfully simulated 8,000 water molecules (50% high-density water molecules and 50% low-density molecules) for several microseconds, keeping the temperature and pressure constant throughout the simulation’s duration. When the simulation’s temperature reached negative 85 degrees Celsius, the barrier between the high-density liquid water and the low-density liquid water vanished.
“That gave us some idea that we might be seeing the liquid-liquid phase transition,” Wang said. “So it presents new evidence of the existence of these two distinct phases of liquid water.”
Why investigate supercooled liquid water?
Wang said that investigating supercooled liquid water states could have wide-ranging applications.
“Water is found in nature in many different environments,” he said. “It’s found inside of the cell, it can be found in the atmosphere, it can be found in the soil and inside of rocks. Many of these microscopic environments could favor the existence of liquid water in one of these forms, even at non-supercooled temperatures.”
“I can imagine medical applications coming from this,” he added. “For example, when people suffer damage from frostbite, it’s due to the crystallization of water inside of the cell. Understanding how you might lower the freezing point of water could be useful for human health.”
Additionally, investigating supercooled liquid water states may even help researchers better understand geological phenomena, such as plate tectonics.
“Water plays a very important role here and it can actually lower the melting temperature of rocks by several hundred degrees,” Wang said. “Why we have such active plate tectonics, earthquakes and why continents are able to drift, a lot of that might have to do with the minute amounts of water that are present in the rocks inside the Earth. We don’t know what forms water could take deep below the Earth’s surface, so simulations of real-life conditions that are difficult to recreate in experiments are very cool.”
The work was supported in part by a grant from the National Science Foundation.
Media Resources
(Proceedings of the National Academy of Sciences)