Water is a funky molecule. You're probably familiar with some of water's strangeness: It is the only compound that exists naturally in three phases (ice, liquid, gas) on Earth. Solid water floats in liquid water; for most molecules, the solid form would sink. There are dozens of physical properties that separate water from other liquids.
And our knowledge of water's oddities just got a little deeper. Liquid water is not just one liquid but two, reported an international group of scientists Monday in the journal Proceedings of the National Academy of Sciences.
In 1985, a team of Canadian chemists first proposed that water might have this dual nature. Since then it has “heavily been debated,” said Anders Nilsson, a physicist at Stockholm University in Sweden and an author of the study. But Nilsson and his team said they were able to show the transition between the two liquid forms.
They shot X-rays at an unusual type of ice, called amorphous ice, as it transitioned from high to low densities at supercool temperatures (around 130 Kelvin, or minus 226 Fahrenheit). The way that the X-rays scattered off the water revealed that the ice became a viscous liquid and then flipped to a less dense but far more viscous liquid. Water can convert between two types of liquid, in other words.
When asked to imagine what it would be like to pour the different types of liquid waters into a cup, Nilsson said, “If we could hypothetically do such a simple thing there would be a huge effect. The two liquids would separate as oil and water with an interface in between. The difference in density is 25 percent, which is huge.”
Amorphous ice is not the ice you're familiar with. Ice as you know it (as snow or the cubes in a freezer) is a crystal, which means that the water molecules have jiggled themselves into a nice, orderly structure. But in space, for instance, ice is frequently amorphous — it is frozen as a haphazard molecular chain called “glassy water,” not a crystal. By way of comparison, the crystal form of silica makes quartz, whereas amorphous silica is the main component in window glass.
When water cools too quickly to crystallize, it forms amorphous ice. Scientists previously figured out that amorphous ice can exist as low- or high-density types. What was not known, Nilsson said, was how the amorphous ice transitioned between the two types. “The question is then are these transformations occurring in the amorphous phase, or from a liquid state to another amorphous state, or from a liquid state to another liquid state” and back to the other amorphous ice, he said. By beaming X-rays at the very chill ice as it swapped densities, “we could detect diffusion that is only possible in a liquid state,” Nilsson said. That is, the amorphous ice goes through two distinct liquid states before turning into the other type of amorphous ice.
This experiment took place at subzero temperatures to keep the two types of water from mixing too rapidly to detect. But it is possible that, at room temperature, liquid water also exists in two forms. “We have proposed previously that at room temperature there are fluctuations between local structures of high-density liquid (dominating around 80 percent) and low-density liquid,” Nilsson said. But good luck noticing a flip in your cup of tea. The proposed fluctuations take place on a nanoscale, he said, over one trillionth of a second.
“The new results give very strong support to a picture where water at room temperature can't decide in which of the two forms it should be, high or low density, which results in local fluctuations between the two,” said Lars G.M. Pettersson, an author of the study and a theoretical chemical physicist at Stockholm University, in a statement. “In a nutshell: Water is not a complicated liquid, but two simple liquids with a complicated relationship.”
Water is not just complicated but vital. It's crucial for life as we know it, so much so that NASA's first Mars missions centered around the theme “follow the water.” Within our cells, water congeals around DNA and influences its structure. Our organs are fleshy water balloons — our brains are three-quarters water by weight.
Nilsson could not say whether this observed liquid-liquid transition had any impact on water's life-giving ways. “When water is hot, it behaves as a normal liquid,” he said. But cool hot water to around 40 to 50 degrees Celsius, or 104 to 122 degrees Fahrenheit, and local “anomalous fluctuations” of high and low density start to appear, he said. “This is the ambient temperature range where most of life exists. Is this of significant importance for life or a pure coincidence? We don’t know.”
Laura Martínez Maestro, a physicist at Oxford University who was not involved with this new research, said that the scientists offered “quite a good proof” of two types of liquid water. In November, Martínez Maestro and her colleagues discovered the fluctuation phenomenon around 50 degrees Celsius.
“Not many people are studying water right now, which is a pity as this is great and really important material,” she said. Work like this, she added, was “just the beginning.”