Thermodynamic perturbation theory for simple polar fluids, I

Abstract

The application of thermodynamic perturbation theory to the computation of the properties of simple polar fluids is considered. The Helmholtz free energy of a fluid of molecules interacting via a Stockmayer potential v S is computed through fourth order in μ, where μ is the dipole moment. Numerical results are obtained on the basis of both the ‘exact’ Monte-Carlo computations for a Lennard-Jones system and the Verlet-Weis perturbation-theoretic computations for that system. The results obtained on the latter basis are then compared with results for a fluid of molecules interacting via a central-force potential v ES that is conformal with the Lennard-Jones 6–12 potential and equivalent to the Stockmayer potential through order μ4. To facilitate the comparison the v ES results are computed according to the Verlet-Weis method. The v ES results and the fourth-order (in μ) v S results constitute two different approximations to the thermodynamics of the Stockmayer potential; the compressibility factors as well as the free energies of the two approximations are compared. It is concluded that the thermodynamic contribution of the dipole term of v S is significant over the whole liquid region when μ2=εσ3, where ε and σ are the usual Lennard-Jones parameters. For this μ, the two approximations we consider give results in close agreement.