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 ", a diameter 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 ", and s. The theory proposes a relation between the critical temperature of a substance and " that is used here to predict effective potentials for the one-carbon Freons: CCl 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.

Effective intermolecular potentials in theoretical thermodynamics of pure substances and solutions

“…The critical volumes V c of the substances so far studied also follow regular tendencies when analysed with respect to δ 3 and s. They have been treated elsewhere [30].…”

“…The application of the theory to fluids at higher densities requires knowledge of the state-dependency of the interaction parameters ε, δ and s, which is mainly due to the presence of many-body forces, as discussed in Section 4, and to the correlation effect that acts through g(r,Ω) as explained in Section 2.1. The latter effect has been analysed in the case of polar molecules [22] and applied to gases of halomethanes [30]. It should also be important for elongated molecules because of the increase in orientational ordering with density.…”

“…We must emphasize that this good representation of the thermodynamic property of gases is achieved with parameters that are independent of temperature, which in some cases runs from below the triple-point to several times T c . A recent publication contains the low-density potential parameters ε 0 , δ 0 and s 0 for these molecules together with the root-mean-square deviation of the ANC model from the B exp (T) data and the temperature range of the latter [30]. Nevertheless, the temperature range over which B ANC (T;s) can be used with confidence is 0.3 ≤ kT/ε ≤ 10, usually wider than the ranges mentioned in that reference.…”

“…6 shows a plot of kT exp c /ε 0 against s for 36 substances that include the eight in Table 2 plus other diatomic, chain and polyatomic molecules dealt with previously. The values of T exp c were taken from the compilation of Ambrose [37] and ε 0 from reference [30]. Indeed all these substances, except the quantum gases He, H 2 and D 2 , exhibit a remarkably systematic and universal behaviour in T exp * 0,c (s) that is well fitted by…”

“…(6) with high degree of approximation. These substances include: the noble gases (He, Ne, Ar, Kr, Xe, Rn) [15], homodiatomic molecules (H 2 , D 2 , N 2 , O 2 , F 2 , Cl 2 ) [27], alkanes (up to C 8 ) and perfluoroalkanes (up to C 7 ) [28], linear (CO 2 , C 2 H 4 ), small dipolar molecules (H 2 O, D 2 O, NH 3 , HCl, NO, CO), hexadecapolar (SF 6 ) [29] and the C 1 -Freons [30]. Indeed, since the potential defined in (5) is constructed especially to satisfy (6), it is no surprise that the results of the one-s ANC model from B exp (T) data are within estimated experimental error.…”