Supercooled Water Exists in Two Different Liquid States

Supercooled Water Exists in Two Different Liquid States


Water shows unusual behavior when it is supercooled, i.e., changes in compressibility, heat capacity, and the thermal expansion coefficient. One explanation for some of this behavior is a liquid-liquid transition between a high-density liquid and a low-density liquid, with a liquid-liquid critical point at supercooled temperatures. However, studying these phenomena experimentally is challenging due to the tendency of supercooled water to crystallize.

Anders Nilsson, Stockholm University, Sweden, and colleagues have found experimental evidence for a liquid-liquid transition in bulk supercooled water using a compression-expansion procedure. The team used an infrared femtosecond laser to heat high-density amorphous ice and create bulk samples of supercooled liquid water under pressure. This first results in a high-density liquid phase, which then expands. The researchers studied the changes in the water’s structure during the expansion using femtosecond X-ray laser pulses. This ultrafast structure determination allowed them to observe structural changes before the water crystallized again.

The team estimates that the initial pressure in the high-density liquid phase is 2.5–3.5 kbar before the rapid expansion. They found that within 20 ns–3 μs, low-density liquid domains appeared and grew, which caused changes in the observed x-ray scattering intensity. Crystallization of the supercooled samples was slower and occurred on time scales of 3–50 μs. According to the researchers, this work supports the existence of a liquid-liquid critical point in water and could help with finding its exact location.



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