Accurate non-asymptotic thermodynamic properties of near-critical N-2 and O-2 computed from molecular dynamics simulations

J Nichele and CRA Abreu and LSD Alves and I Borges, JOURNAL OF SUPERCRITICAL FLUIDS, 135, 225-233 (2018).

DOI: 10.1016/j.supflu.2018.01.011

Four thermodynamic properties of pure N-2 and O-2 fluids were calculated over a continuous temperature range in the near-critical region employing molecular dynamics combined with the multistate Bennett acceptance ratio (MBAR) technique. The non-asymptotic critical anomalies of the isothermal compressibility, the thermal expansion coefficient and the heat capacities at constant pressure and constant volume were determined in the 0.80T(c)-1.20T(c) temperature range along the 1.001P(c) and 1.15P(c) isobars. A single-site united-atom model and a two-site rigid rotor force fields were used in the simulations. The results show that effective force fields developed to describe accurately the behavior of liquids and dense gases in general can be used to obtain near critical anomalies of the thermodynamic properties. Furthermore, the computed values are in very good agreement with NIST data when available. The results fill existing gaps of the NIST database in the near-critical vicinity of the critical temperature along the critical isobar, especially for the heat capacities. The MBAR technique was crucial to describe the continuous and sharp peak of each property as functions of the temperature. The results indicated that even with a simple force field such as the united-atom one-site approach, very good results could be obtained for the non-asymptotic critical anomalies of thermodynamic properties without using renormalization group techniques. The computation of accurate results was possible due to the availability of precise critical constants especially for the simpler one-site force field. The inclusion of the anisotropy in the force field does not necessarily improve the computed thermodynamic properties in the near critical region.

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