A Korean research team has proven for the first time in the world the hypothesis of water's "liquid-liquid critical point," a long-standing puzzle in the scientific community.
The Ministry of Science and ICT (MSIT) announced on the 27th that a research team led by Professor Kim Kyung-hwan at Pohang University of Science and Technology (POSTECH) observed water's "liquid-liquid critical point" for the first time through a joint experiment with Professor Anders Nilsson's team in the Department of Physics at Stockholm University in Sweden.
Ordinary liquids become heavier as their temperature drops until just before freezing, but water becomes heaviest at 4 degrees Celsius and then actually becomes lighter as it cools further. This unique characteristic is why only the surface of rivers and lakes freezes in winter while liquid water is maintained below, allowing life to survive underneath.
However, the fundamental reason why water behaves differently from other liquids had remained a long-standing question in the scientific community.
One proposed answer to this question is the "liquid-liquid critical point" hypothesis. The hypothesis assumes that water coexists in two liquid phases — "high-density water" and "low-density water" — and that when a specific temperature (the critical point) is reached, the distinction disappears and becomes the water we see in everyday life.
The scientific community expected this critical point to exist in the ultra-low temperature range between minus 40 and minus 70 degrees Celsius. However, because water freezes extremely rapidly the moment it drops below minus 40 degrees Celsius, no case had ever successfully proven the existence of the critical point.
The POSTECH research team utilized an X-ray free-electron laser (PAL-XFEL) to create water that does not freeze even at ultra-low temperatures and to measure its structure. As a result, they succeeded in confirming that the "liquid-liquid critical point" exists near minus 60 degrees Celsius.
The light source emits light billions of times brighter than the sun and can measure molecular movements in units of one trillionth of a second. Through this, the team captured that water exists in two liquid phases below minus 60 degrees Celsius.
This achievement holds great scientific significance in that it puts an end to the academic debate surrounding water's special properties and serves as a starting point for elucidating the essential role of water in various important natural phenomena going forward.
"With this research, the accuracy of studies on life phenomena and biomolecular research as well as climate change research, which are significantly affected by the properties of water, can be greatly improved," Professor Kim said.
The research, carried out under MSIT's basic research program and Samsung's Future Technology Development Program, was published in Science, one of the world's most prestigious academic journals, on March 27 (March 26 local time). Koo Hyuk-chae, first vice minister of MSIT, said, "We will continue to strengthen our support systems so that early-career researchers can immerse themselves in research in a stable environment and grow into world-class scholars."