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Equation of state for near critical argon obtained via molecular dynamics
Jakler Nichele, Leonardo S. De B. Alves and Itamar Borges, Jr.
Molecular dynamics (MD) investigations of argon were done in the neighborhood of the thermodynamic critical point. A Lennard-Jones type potential was used to describe the Ar-Ar interactions. We focused on two strongly divergent properties useful to define an equation of state (EOS): the isothermal compressibility, KT, and the thermal expansion coefficient, αP. Both properties already diverge near the critical region in the classical theory. We did not simulate weakly divergent properties like the isochoric heat capacity and the adiabatic compressibility, which are more challenging because they do not diverge in the classical theory. A statistical mechanics analysis of the temporal evolution of 1000 argon atoms during 2 ns was carried out. All results are in good agreement with available experimental data sets compiled by the National Institute of Standards and Technology (NIST). A phase diagram combining MD results and literature data near the critical point was constructed. We could obtain the spinodal curve for subcritical argon, which bounds the metastable region inside the saturation zone of the phase diagram. The computed EOS can be directly used in numerical simulations of compressible fluid mechanics and heat transfer. The approach here employed is of general applicability and can be extended to other atoms and molecules.
Keywords: Supercritical fluid, critical state, argon, differential equation of state, isothermal compressibility, thermal expansion coefficient, molecular dynamics, spinodal curve, phase diagrams, comparison with NIST data.