Thermodynamic properties of perfluorooctane on the liquid-gas coexistence curve: calculation method and tabulated data

The article examines perfluorooctane C₈F₁₈, a technically important substance used in various fields of industry and medicine. It is noted that the existing thermodynamic tables of standard reference data for perfluorooctane are inconsistent with the current data on the density of this substance on the phase equilibrium curve. A procedure was developed to plot the phase equilibrium curve of perfluorooctane in the range from the triple point to the critical point. A model representing the phase equilibrium curve of perfluorooctane is proposed, which includes equations for vapor pressure; saturated liquid density; saturated vapor density; and the “apparent” heat of vaporization. In this case, the vapor pressure equation is consistent with the scaling theory of critical phenomena near the critical point and the Wegner model in the vicinity of the triple point. The coexistence curve model in the vicinity of the critical point satisfies the Yang-Yang model and renormalization group theory for asymmetric systems, while in the region of rarefied gas, it satisfies the linear model of midpoint diameter. Within the temperature range of 246.15–496.99 K, tabulated data including the equilibrium properties of perfluorooctane on the phase equilibrium curve were estimated: pressure and density of saturated vapor; saturated liquid density; heat of vaporization; first and second derivatives of saturated vapor pressure with respect to temperature. Using the latest experimental data, as well as experimental data on the critical pressures of perfluoro-n-alkanes as a function of their molecular weight, the following values of critical parameters were selected: critical pressure—1.548 MPa, critical density—596.4 kg/m³, and critical temperature—496.99 K. A number of statistical characteristics were calculated, including average absolute deviation and standard deviation, characterizing the accuracy of the proposed model of a phase equilibrium curve in describing experimental data that are obtained in recognized international thermophysical laboratories. The obtained results are useful for high-tech companies engaged in the development of innovative technologies in the field of radio electronics, electrical industry, and medicine; as well as manufacturers of magnetic fluid sealers (for the purpose of isolating hazardous substances from the environment and providing seals for devices operated under vacuum or in contact with corrosive substances) and magnetic fluid separators (for separating nonferrous metals by density). The study results can be used in the development of a fundamental equation of state for perfluorooctane.