The Heat Capacity of Bulk Supercooled Water between 185 and 205 K

Recent advances in experimental, computational, and theoretical research on aqueous phases emphasize the need for accurate measurements of the thermodynamic parameters of supercooled bulk-like water at temperatures below its homogeneous ice nucleation limit near 230 K. Using a fast scanning calorimetry technique, we have investigated softening of 25-350 nm thick amorphous solid water films vapor-deposited near 136 K and heated at rates up to a megakelvin per second. The analysis of the relaxation endotherms measured at distinct heating rates shows that the superheated amorphous water films undergo a complete relaxation to a meta-equilibrium liquid state, which heat capacity almost doubles between 185 and 205 K. The inferred heat capacity values are consistent with both the recently reported maximum heat capacity near 229 K and with results of past analyses, which assume a thermodynamic continuity between deeply supercooled and ordinary water.