Investigation of the transition to liquid-liquid immiscibilitym for Lennard-Jones (12,6) systems, using Gibbs-ensemble molecular simulations

Van Leeuwen M.E., Peters C.J., de Swaan Arons J. and Panagiotopoulos A.Z., 1991. Investigation of the transition to liquid-liquid immiscibility for Lennard-Jones (12,6) systems using Gibbs-ensemble molecular simulations. Fluid Phase Equilibria, 66: 57-75. The transition of liquid-liquid immiscibility in Lennard-Jones systems was studied using Gibbs-ensemble computer simulation. In order to obtain liquid-liquid immiscibility, a binary interaction parameter δ₁₂ within the Lorentz-Berthelot combining rules was used to describe a deviation of the unlike-pair energy interaction parameter ε{lunate}^*₁₂ from the geometric mean. When δ₁₂ is unity, the investigated system approximates to the mixture Ar-Kr at 143.15 K. The binary interaction parameter δ₁₂ was varied over a range of 0.9-0.7, such that the phase behaviour of five hypothetical mixtures was simulated at one temperature. As δ₁₂ decreased, the phase envelope was observed to widen (compared to the phase behaviour of a system with δ₁₂ = 1) through a shift of the location of the liquid branch. A gradual deviation of the gas branch at higher pressures resulted in a positive azeotrope for δ₁₂ = 0.75 and a hetero-azeotrope for δ₁₂ = 0.7. For δ₁₂ ≤ 0.75, liquid-liquid immiscibility was observed. Constant-(total) volume runs turned out to be more stable than constant-pressure runs in situations with large density fluctuations, such as for two coexisting liquids.