ChemInform Abstract: Thermodynamic Properties of Simple Molecular Fluids: Tetrafluoromethane and Trifluoromethane.

Abstract

The thermodynamic properties of CF, and CHF, have been cakuiated from an equation of state of the Strobrldg. type, fitted to extenrlve p-Y-Tdata sets recently publbhed. The thormodynamic properties under saturation condltlons have been compared with previously pubitshed data, the agreement being satisfactory except In the neartrlticai region. The results have been used to dkcusr the performance of a monatomic fluid model, whkh k shown to be unsatisfactory. Introduction The hakcarbon refrigerants are of obvious Industrial impor- tance, and also a class of substances of great theoretical in- terest. Current theories of liquid state highlight the importance of the shape of the molecule and of its polarity. The halo- methane refrigerants are all based on a quasi-spherical non- polar molecuie-methane Rself-and so can be regarded as a series of case studies showing the effect of increasing shape anisotropy and Increasing polarity by the simple substitution of fluorine and/or CMorine for hydrogen atoms. As already polnted out (7-3), lt is dlfficult to appreciate the overall effect of these substitutions without a correlation of the available experimental data, and such correlations, whether in the form of tables or equations, are essential to industrial designers and operators. Two of the simplest halomethanes are CF4 and CHF,. While their grosser features may be modeled by quasi-spherical molecules, the large differences of the behavior of the CF4 + CHF, mixture with respect to that of the ideal mixture are symptomatic of the anisotropies inherent in their microscopic Interactions (4, 5). Rublo et ai. (3, 6) have polnted out that a large number of papers In the literature arrive at contradictory conclusions concerning the intermolecular potential model for these two molecules. One can expect the anisotropies of the potential to be dependent on the packing of the molecules, i.e. the density (especially those arising from the shape of the molecules), and since this can be tuned by varying the tem- perature and the pressure, the availability of thermodynamic propedes In wide ranges of p and Twill be helpful in clarifying the main characteristics of the Intermolecular potential of these simple molecular fluids, a necessary step in our ability to predict their thermodynamic properties. Stewart et ai. (7) summarized the available thermodynamic data for many halocarbons and showed that most of the resutts are concentrated in the low-density region. Recently, extensive experimental studies of the p-V-T surfaces of CF, (3) and CHF, (6) have been publlshed. Although most of our mea- surements were obtained In the high-density region, there is a slgnbnt overtap wtth most of the prevkus lowdensity studies. Figures 1 and 2 show the p-T ranges for which density data are available. In this paper we report calculated thermodynamic properties of CF, In the ranges 90 < T/K < 420 and 0 < pibar < 1000 and of CHF, in the ranges 126 < T/K < 332 and 0 < pibar < 1000, derived from an equation of state of the Strobridge type fitted to the experimental p - V- T data.