Relative density and isobaric expansivity of cold and supercooled heavy water from 254 to 298 K and up to 100 MPa.

A dual-capillary apparatus was developed for highly accurate measurements of density of liquids, including the supercooled liquid region. The device was used to determine the density of supercooled heavy water in the temperature range from 254 K to 298 K at pressures ranging from atmospheric to 100 MPa, relative to density at reference isotherm 298.15 K. The measurements of relative density were reproducible within 10 ppm, and their expanded (k = 2) uncertainty was within 50 ppm. To obtain absolute values of density, thermodynamic integration was performed using recent accurate speed of sound measurements in the stable liquid region. An empirical equation of state (EoS) was developed, giving specific volume as a rational function of pressure and temperature. The new experimental data are represented by EoS within their experimental uncertainty. Gibbs energy was obtained by EoS integration allowing computation of all thermodynamic properties of heavy water using Gibbs energy derivatives. Although based on data in relatively narrow temperature and pressure ranges, the developed EoS shows an excellent agreement with literature data for densities, isothermal compressibilities, and isobaric expansivities of deeply supercooled heavy water. The curvature of the thermodynamic surface steeply increases toward low temperatures and low pressures, thus supporting the existence of the hypothesized liquid-liquid coexistence boundary in a close vicinity of existing experimental data.