Interaction potentials and thermodynamics of small polar molecules. The case of C₁-Freons (halomethanes)

Abstract

The recently proposed approximate nonconformal (ANC) theory has been very successful in determining effective interaction potentials for a large number of pure substances in the gaseous state: noble gases, homodiatomics, alkanes, perfluoroalkanes and some small polyatomic molecules, as well as many of their binary mixtures. ANC potentials are spherically symmetric Kihara-like functions involving an energy ϵ, a diameter δ and a form parameter s. In this work, we propose an effective-potential theory valid at finite densities. The basic assumption is that ANC potential functions represent effectively the thermodynamics of a substance over the isotropic fluid region given the appropriate state dependencies of ϵ, δ and s. The theory proposes a relation between the critical temperature of a substance and ϵ that is used here to predict effective potentials for the one-carbon Freons: CCl₄, CCl₃F, CCl₂F₂, CClF₃, CHF₃, CH₂F₂, CH₃F, CH₃Cl, CH₂Cl₂, CHCl₃, CHClF₂ and CHCl₂F, a collection of small molecules that are all of similar geometry but varying degrees of polarity. As a test of the reliability of the theory the calculated second virial coefficients B(T ) are compared with experiment. For most of these substances, B(T ) is reproduced within experimental error. The relative contributions of the polar and nonpolar parts of the potential to the thermodynamics are discussed. The approach produces effective interactions and generalized Stockmayer potentials for this set of molecules to be used in predicting other thermodynamic properties.

Acknowledgements

This work was partially supported by the Consejo Nacional de Ciencia y Tecnología (México) Project SEP-2003-43586. J. Chávez acknowledges support from the Sistema Nacional de Investigadores, México.

Notes

^†This fit varies slightly from the one published in 15 due to the incorporation of newer literature values of T_C of some substances.