Excess thermodynamic properties for equal size Lennard–Jones mixtures and the effect of the difference of the potential well depth between components

Abstract Monte Carlo simulations are performed for mixtures composed of Lennard–Jones fluids over a wide range of temperatures and pressures. Four types of model mixtures are employed. Each has different combining rules for unlike interactions. The mixtures are composed of two normal liquids at low temperatures and two supercritical fluids at high temperatures. The excess volumes V p E , excess enthalpies H p E , and excess internal energies U p E for the binary mixtures are calculated. The temperature and pressure dependencies of V p E , H p E , and U p E are investigated. The large and positive U p E or H p E values near or in the supercritical region are discussed in terms of the volumetric behavior of the system. Excess molar internal energies at constant volume U V E are evaluated and compared with H p E at an equimolar concentration. Contrary to the complex behavior of H p , x = 0.5 E , U V , x = 0.5 E shows quite a simple density dependence over the whole range of temperatures and pressures. The contributions from the volumetric behavior to H p E and U p E , such as the difference of molar volumes of pure components and V p E , are estimated. The contributions are large and positive, especially at high temperature. In other words, H p E and U p E are mainly affected by the volumetric behavior, particularly in the supercritical region.