Ultrafast calorimetry of deeply supercooled water

Knowledge of the temperature dependence of the isobaric specific heat (C_p) upon deep supercooling can give insights regarding the anomalous properties of water. If a maximum in C_p exists at a specific temperature, as in the isothermal compressibility, it would further validate the liquid–liquid critical point model that can explain the anomalous increase in thermodynamic response functions. The challenge is that the relevant temperature range falls in the region where ice crystallization becomes rapid, which has previously excluded experiments.

The pump/probe setup at SwissFEL Bernina was used for ultrafast calorimetry measurements, faster than the crystallisation to ice. A temperature jump was induced in micrometer-sized water droplets by an infrared laser pulse, after supercooling the water by evaporation into vaccuum down to -45°C. At this temperature droplets can remain non-frozen for short time and fingerprints of their liquid structure can be measured in X-ray scattering patterns thanks to ultrashort and bright SwissFEL X-ray pulses.  Sufficient delay between the infrared laser pulse and X-ray pulses allowed measurements at constant pressure in order to determine the C_p. A sharp increase in C_p, from 88 J/mol/K at 244 K towards a maximum of about 218 J/mol/K at 229 K was observed. The C_p maximum is at a similar temperature as the previously measured maxima of the isothermal compressibility and correlation length and therefore support the liquid–liquid critical point model. The excess entropy and self-diffusion coefficient of water was estimated from the C_p measurements and show that  these properties decrease rapidly below 235 K.

The experiment has been the first during the user operation phase at Bernina which started in 2019.  The results of this study have been recently published.