Gaseous speed of sound, ideal-gas heat capacity, and virial coefficients for cis-1,1,1,4,4,4-hexafluoro-2-butene (R1336mzz(Z)) and cis-1,2,3,3,3-pentafluoroprop-1-ene (R1225ye(Z))

The emerging low-global-warming-potential refrigerants cis-1,1,1,4,4,4-Hexafluoro-2-butene (R1336mzz(Z)) and cis-1,2,3,3,3-pentafluoroprop-1-ene (R1225ye(Z)) are showing high application potential in refrigeration and organic Rankine cycle, but the necessary thermophysical property data are too scarce to support their commercial application. Among the thermophysical properties, the speed of sound is essential for engineering design and scientific research. In this study, the speed of sound of gaseous R1336mzz(Z) and R1225ye(Z) was determined experimentally by the acoustic resonance method with the help of a fixed-path cylindrical cavity. The sound speed data of R1336mzz(Z) covers the temperature range of (323 to 363) K and pressure range of (57 to 383) kPa, and that of R1225ye(Z) covers (298 to 363) K and (40 to 1022) kPa. The relative uncertainties of these measurements are within a few parts in 10−4, allowing accurate analysis to derive the ideal-gas heat capacity and the 2nd acoustic density virial coefficients. Further, the parameters of the hard-core square-well intermolecular potentials are fitted to the acoustic data, allowing us to infer an estimate of the 2nd density virial coefficient of these substances. Quantitative calculation of uncertainties is also performed for these derived thermophysical parameters. Comparison of all these results with other data in the existing literature and the predictions of dedicated equations of state shows good agreement, supporting the claimed uncertainty of the experimental results.