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Water’s Heat Capacity Becomes Extreme in Supercooled Conditions

Water’s Heat Capacity Becomes Extreme in Supercooled Conditions

Using x-ray lasers, researchers at Stockholm University have developed a new ultrafast calorimetry technique to measure the heat capacity (Cp) of water down to 228 K. The finding is that Cp increases dramatically below 240 K and reach a maximum at around 230 K indicating an extreme amount of entropy fluctuations. The results are consistent with existence of a critical point at temperatures around 210-215 K and at pressures 800-1000 bar. Their findings are published this week in the Proceedings of the National Academy (US).

We in the Nordic part of Europe can have an enjoyable life due to waters high heat capacity. It allows water to store heat efficiently down in the Caribbean Sea that remains warm when it is transported up to our latitudes with the Gulfstream. If water would have properties as expected from normal liquids our climate would have been similar to Siberia with extremely cold winters. An important question is why water have this high heat capacity.

Figure illustrates a glass containing two water liquids that have different densities surrounded in a cold environment.

One hypothesis is that water at ambient conditions is supercritical. It was recently shown by researchers from Stockholm University (Science 370, 978 (2020)) that two macroscopic liquids of water exist under pressure in the deep supercooled regime. We know that water is only one phase at atmospheric pressure at temperatures from boiling down to 228 K (Nature 510, 381 (2014); Science 358, 1589 (2017). This means that going from two phases to one phase by varying temperature and pressure there must exists a critical point that give rise to fluctuations. The heat capacity corresponds entropy fluctuations and that there is a maximum at around 230 K indicates that the fluctuations between one that is more structurally ordered structure (low-density, low entropy) and the other more disordered (high-density, high entropy) can directly be related to the existence of the critical point. We can thereby attribute the high heat capacity and that we have relatively warm winters in Sweden to that water is supercritical.

This work involved Anders Nilsson, Fivos Perakis, Katrin Amman-Winkel and Marjorie Ladd Parada at Chemical Physics, Fysikum. The study was done in cooperation with Swiss FEL in Switzerland, KTH in Stockholm, the POSTECH University in Korea and SLAC national accelerator laboratory at Stanford, USA. Other people that contributed to the study include previous members of the Chemical Physics group at Stockholm University - Harshad Pathak (first author) and Alexander Späh.

Scientist Contact: Anders Nilsson, andersn@fysik.su.se, +46-73 9946230
Further reading in PNAS: Enhancement and Maximum in the Isobaric Specific Heat Capacity Measurements of Deeply Supercooled Water using Ultrafast Calorimetry by Harshad Pathak et al.: Proceedings of the National Academy (US) 118, 2018379118 (2021)

Last updated: February 8, 2021
Source: Anders Nilsson

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Water’s Heat Capacity Becomes Extreme in Supercooled Conditions