Excess properties of Lennard-Jones binary mixtures from computer simulation and theory

Abstract

Monte Carlo simulation and theory are used to calculate the excess thermodynamic properties of binary mixtures of spherical Lennard-Jones molecules. We study the excess functions of three binary mixtures characterized by the following size and dispersive energy ratios: (1) (sigma22/sigma11)^3=2 and (epsilon22/epsilon11)=2; (2) (sigma22/sigma11)^3=1 and (epsilon22/epsilon11)=2; and (3) (sigma22/sigma11)^3=1/2 and (epsilon22/epsilon11)=2. In all cases, the unlike size parameter, 1⁄412, is kept constant and equal to the value given by the Lorentz combining rule sigma12=/sigma11 + sigma22)/2. However, di����� erent unlike dispersive energy parameter values are considered through the following combining rules: (a) epsilon12=(epsilon11*epsilon22)^1/2 (Berthelot rule); (b) epsilon12=epsilon11 (association); and (c) epsilon12=epsilon222 (solvation). The pressure and temperature dependence of the excess volume and excess enthalpy is studied using the NpT Monte Carlo simulation technique for all the systems considered. Additionally, the simplest conformal solution theory is used to check the adequacy of this approach in predicting the excess properties in a wide range of thermodynamic conditions and variety of binary mixtures. In particular, we have applied the van der Waals one-fluid theory to describe Lennard-Jones binary mixtures through the use of the JOHNSON et al. [1993, Molec. Phys., 78, 591] Helmholtz free energy. Agreement between simulation results and theoretical predictions is excellent in all cases and thermodynamic conditions considered. This work confirms the applicability of the van der Waals one-fluid theory in predicting excess thermo- dynamic properties of mixtures of spherical molecules. Furthermore, since binary mixtures of spherical Lennard-Jones molecules constitute the reference fluid to be used in perturbation theories for complex fluids, such as the statistical association fluid theory (SAFT), this work shows clearly the applicability of the conformal solution theory within the framework of SAFT for predicting excess functions.