Enhancement and maximum in the isobaric specific-heat capacity measurements of deeply supercooled water using ultrafast calorimetry

Abstract

Knowledge of the temperature dependence of the isobaric specific heat (Cp) upon deep supercooling can give insights regarding the anomalous properties of water. If a maximum in Cp 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. Here, we have utilized a methodology of ultrafast calorimetry by determining the temperature jump from femtosecond X-ray pulses after heating with an infrared laser pulse and with a sufficiently long time delay between the pulses to allow measurements at constant pressure. Evaporative cooling of ∼15-µm diameter droplets in vacuum enabled us to reach a temperature down to ∼228 K with a small fraction of the droplets remaining unfrozen. We observed a sharp increase in Cp, from 88 J/mol/K at 244 K to about 218 J/mol/K at 229 K where a maximum is seen. The Cp maximum is at a similar temperature as the maxima of the isothermal compressibility and correlation length. From the Cp measurement, we estimated the excess entropy and self-diffusion coefficient of water and these properties decrease rapidly below 235 K.

Liquid-Liquid Phase Transition in Supercooled H2O and D2O: A Path-Integral Computer Simulation Study

“…In the P-T plane, LDL and HDL are separated by a first-order LLPT line that ends at a liquid-liquid critical point (LLCP) at higher temperatures. Importantly, the LLPT hypothesis explains naturally the maxima in κ T (T ) and C P (T ) observed recently upon cooling water at P = 0.1 MPa [5,10].…”

“…Although many theoretical approaches have been proposed to explain water anomalous behavior, the so-called liquid-liquid phase transition (LLPT) scenario [7,8] is currently the explanation best-supported by experiments [5,[9][10][11][12][13], computer simulations [7,[14][15][16][17][18], and theory [14,[19][20][21][22][23]. In the LLPT scenario, water at low temperatures exists in two distinct liquid states, low-density and high-density liquid (LDL and HDL).…”

From Femtoseconds to Hours—Measuring Dynamics over 18 Orders of Magnitude with Coherent X-rays

“…These observations let to the conclusion that the HDA-LDA transition is actually taking place via high-and low-density liquid forms of water named HDL and LDL, i.e., a HDA-HDL-LDL-LDA transition was observed. This study provided one of the first experimental evidence for the existence of two forms of liquid water, that has been strengthened in subsequent FEL studies on water [111][112][113].…”

Hydrophilic and Hydrophobic Effects on the Structure and Themodynamic Properties of Confined Water: Water in Solutions

“…Figure 5 a, with the specific heats of glycerol [ 74 ], water [ 75 , 76 , 77 , 78 ], and methanol [ 79 , 80 ], shows (vertical dotted lines) their melting temperatures. Regarding water, we must stress that a very recent experiment considered the possible locus and intensity [ 81 ] at ambient pressure of the maximum at the Widom line, suggesting that it is located at about 229 K and has a value of 218 J/molK.…”