Thermodynamics of dipolar square-well fluids

The thermodynamic properties of a dipolar square-well fluid in zero external magnetic field are studied using theory and simulations. The theory is based on the virial expansion of the Helmholtz free energy. The second and third virial coefficients are calculated as functions of the dimensionless temperature T*, the reduced dipolar interaction parameter μ, and the potential well width λ. The formulas are compared to results from Mayer-sampling calculations. The analytical expressions for the virial coefficients are incorporated in to various forms of virial expansion for the Helmholtz free energy and the equation of state. Thermodynamic functions are tested against results from Monte Carlo simulations for subcritical vapor–liquid transition parameters T* ≥ 5 and 10.5; μ ≤ 4; λ ≤ 2 over the range of the particle volume fraction φ ≤ 0.4. Finally, predictions of the critical parameters for the condensation transition are obtained on the basis of the virial expansion of the Helmholtz free energy and compared with computer-simulation results and the theories available in the literature. Although the critical parameter values formally lie beyond the applicability of the theory developed in this work they fall in the range of values previously obtained by other methods. Accurate theoretical prediction of the critical parameters for dipolar square-well fluids remains as a challenge.