Water is a strange substance. While most materials shrink and become denser in the cold, water reaches its maximum density at 4 °C, expanding again as it approaches its freezing point.
If it were not the case, ice would not float on water, and the oceans would freeze from the bottom up, making life on Earth all but impossible.
The reason water behaves this way is a fundamental question of chemistry, long studied and much debated. Yet, despite various hypotheses, there have been few conclusive experimental tests.
At low temperatures and high pressures, water’s weirdness becomes especially pronounced. The EU-funded WATER project, supported by the European Research Council, has revealed for the first time the molecular behaviour of water just prior to freezing.
The team was the first to use X-ray lasers for these kinds of experiments. “Most people thought that an experiment to show the liquid-liquid transition was too tricky. But with our X-ray laser beams pulsing at nanoseconds to milliseconds, we managed it,” says project coordinator Anders Nilsson.
The critical point of liquid-liquid transition
Liquid-liquid transition describes the point at which one liquid transforms into another while retaining the same chemical components, through a process called phase transition. The WATER team wanted to investigate liquid-liquid transition of water in the ‘no-man’s land’ realm of low temperatures and high pressure.
The team had already demonstrated that at atmospheric pressure, water has only one phase. But they discovered that when the temperature is lowered and the pressure increases, two phases exist. What especially interested them was the possibility of a critical point marking the boundary between these two regions.
“At this critical point, water is between a one phase region and two and behaves as though it cannot decide which of the two liquids it wants to be – a high-density liquid or low-density liquid. So it constantly shapeshifts between the two,” adds Nilsson.
Superfast X-ray lasers
The challenge in characterising this liquid-liquid transition is that when water supercools within the range of -35 to -70 °C, ice forms so quickly that standard experimental tools are not fast enough to capture the fluctuations along the ice boundary. To compensate, the team altered the pressure from high to low faster than the process of freezing, enabling them to witness the pressure-driven liquid-liquid transition.
To capture the phenomenon, the team used un ultrafast X-ray laser at the Pohang Accelerator Laboratory in South Korea. This is capable of delivering X-ray pulses to samples at intervals measuring quadrillionths of a second.
“Even though X-rays travel at the speed of light, if the electromagnetic radiation is especially intense it can destroy samples before registering anything. Our X-ray laser’s beam is so fast it captures the signal we want before the sample breaks apart,” explains Nilsson.
The measurements achieved by the experiments have led to the creation of a chart tracing the relationship between temperature and pressure, and including a line – known as a Widom line – which plots the liminal zone where water fluctuates between behaving like one liquid and two.
“I suspect that the critical point is not at pressures as high as 2 000 bar as previously thought, but actually around 300-500 bar,” says Nilsson. “This could help explain planetary phenomena such as the evolution of marine life. The water at the bottom of the ocean is typically around 4 °C at its maximum density, thanks to the critical point. If it gets colder, it gets less dense and so rises, causing circulation which transfers heat and nutrients crucial for life.”
No life without water
While a fundamental science project, WATER’s results could have a number of practical implications. As Nilsson notes: “We started by focusing on water in bulk, but now want to understand how water changes in pores, interfaces and biomolecules, which is of value to a range of sectors and applications.”
As increasing global temperatures will likely cause more droughts across southern Europe, better understanding water’s behaviour inside the pores of desalination filtration membranes could help increase the availability of clean water.
Additionally, many energy technologies such as fuel cells, hydrogen-generating water splitting, and artificial photosynthesis take place at water-solid interfaces, where the influence of water fluctuations remains unknown.
A health frontier still to be explored could be better understanding the role water plays inside cells, where it may prove integral to life processes. “Our insights could inform investigations into how fluctuating water behaviours might enable communication between biomolecules. If this is the case we currently don’t know,” concludes Nilsson.
The team are now working to overcome COVID-induced delays to directly observe the critical point itself.